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Astronomy Picture of the Day
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Thumbnail image of picture found for this day. APOD: 2024 March 8 - The Tarantula Zone
Explanation: The Tarantula Nebula, also known as 30 Doradus, is more than a thousand light-years in diameter, a giant star forming region within nearby satellite galaxy the Large Magellanic Cloud. About 180 thousand light-years away, it's the largest, most violent star forming region known in the whole Local Group of galaxies. The cosmic arachnid sprawls across this magnificent view, an assembly of image data from large space- and ground-based telescopes. Within the Tarantula (NGC 2070), intense radiation, stellar winds, and supernova shocks from the central young cluster of massive stars cataloged as R136 energize the nebular glow and shape the spidery filaments. Around the Tarantula are other star forming regions with young star clusters, filaments, and blown-out bubble-shaped clouds. In fact, the frame includes the site of the closest supernova in modern times, SN 1987A, at lower right. The rich field of view spans about 2 degrees or 4 full moons in the southern constellation Dorado. But were the Tarantula Nebula closer, say 1,500 light-years distant like the Milky Way's own star forming Orion Nebula, it would take up half the sky.

Thumbnail image of picture found for this day. APOD: 2024 February 15 - NGC 253: Dusty Island Universe
Explanation: Shiny NGC 253 is one of the brightest spiral galaxies visible, and also one of the dustiest. Some call it the Silver Coin Galaxy for its appearance in small telescopes, or just the Sculptor Galaxy for its location within the boundaries of the southern constellation Sculptor. Discovered in 1783 by mathematician and astronomer Caroline Herschel, the dusty island universe lies a mere 10 million light-years away. About 70 thousand light-years across, NGC 253 is the largest member of the Sculptor Group of Galaxies, the nearest to our own Local Group of Galaxies. In addition to its spiral dust lanes, tendrils of dust seem to be rising from its galactic disk laced with young star clusters and star forming regions in this colorful galaxy portrait. The high dust content accompanies frantic star formation, earning NGC 253 the designation of a starburst galaxy. NGC 253 is also known to be a strong source of high-energy x-rays and gamma rays, likely due to massive black holes near the galaxy's center.

Thumbnail image of picture found for this day. APOD: 2023 November 22 - IC 342: Hidden Galaxy in Camelopardalis
Explanation: Similar in size to large, bright spiral galaxies in our neighborhood, IC 342 is a mere 10 million light-years distant in the long-necked, northern constellation Camelopardalis. A sprawling island universe, IC 342 would otherwise be a prominent galaxy in our night sky, but it is hidden from clear view and only glimpsed through the veil of stars, gas and dust clouds along the plane of our own Milky Way galaxy. Even though IC 342's light is dimmed and reddened by intervening cosmic clouds, this sharp telescopic image traces the galaxy's own obscuring dust, young star clusters, and glowing star forming regions along spiral arms that wind far from the galaxy's core. IC 342 has undergone a recent burst of star formation activity and is close enough to have gravitationally influenced the evolution of the local group of galaxies and the Milky Way.

Thumbnail image of picture found for this day. APOD: 2023 October 13 - Hydrogen Clouds of M33
Explanation: Gorgeous spiral galaxy Messier 33 seems to have more than its fair share of glowing hydrogen gas. A prominent member of the local group of galaxies, M33 is also known as the Triangulum Galaxy and lies a mere 3 million light-years away. The galaxy's central 30,000 light-years or so are shown in this sharp galaxy portrait. The portrait features M33's reddish ionized hydrogen clouds or HII regions. Sprawling along loose spiral arms that wind toward the core, M33's giant HII regions are some of the largest known stellar nurseries, sites of the formation of short-lived but very massive stars. Intense ultraviolet radiation from the luminous, massive stars ionizes the surrounding hydrogen gas and ultimately produces the characteristic red glow. In this image, broadband data were combined with narrowband data recorded through a hydrogen-alpha filter. That filter transmits the light of the strongest visible hydrogen emission line.

Thumbnail image of picture found for this day. APOD: 2023 May 26 - Virgo Cluster Galaxies
Explanation: Galaxies of the Virgo Cluster are scattered across this nearly 4 degree wide telescopic field of view. About 50 million light-years distant, the Virgo Cluster is the closest large galaxy cluster to our own local galaxy group. Prominent here are Virgo's bright elliptical galaxies Messier catalog, M87 at bottom center, and M84 and M86 (top to bottom) near top left. M84 and M86 are recognized as part of Markarian's Chain, a visually striking line-up of galaxies on the left side of this frame. Near the middle of the chain lies an intriguing interacting pair of galaxies, NGC 4438 and NGC 4435, known to some as Markarian's Eyes. Of course giant elliptical galaxy M87 dominates the Virgo cluster. It's the home of a super massive black hole, the first black hole ever imaged by planet Earth's Event Horizon Telescope.

Thumbnail image of picture found for this day. APOD: 2023 April 27 - The Tarantula Nebula from SuperBIT
Explanation: The Tarantula Nebula, also known as 30 Doradus, is more than a thousand light-years in diameter, a giant star forming region within nearby satellite galaxy the Large Magellanic Cloud. About 160 thousand light-years away, it's the largest, most violent star forming region known in the whole Local Group of galaxies. The cosmic arachnid is near the center of this spectacular image taken during the flight of SuperBIT (Super Pressure Balloon Imaging Telescope), NASA's balloon-borne 0.5 meter telescope now floating near the edge of space. Within the well-studied Tarantula (NGC 2070), intense radiation, stellar winds and supernova shocks from the central young cluster of massive stars, cataloged as R136, energize the nebular glow and shape the spidery filaments. Around the Tarantula are other star forming regions with young star clusters, filaments, and blown-out bubble-shaped clouds. SuperBIT's field of view spans about 1/3 of a degree in the southern constellation Dorado.

Thumbnail image of picture found for this day. APOD: 2023 March 2 - Unraveling NGC 3169
Explanation: Spiral galaxy NGC 3169 looks to be unraveling like a ball of cosmic yarn. It lies some 70 million light-years away, south of bright star Regulus toward the faint constellation Sextans. Wound up spiral arms are pulled out into sweeping tidal tails as NGC 3169 (left) and neighboring NGC 3166 interact gravitationally. Eventually the galaxies will merge into one, a common fate even for bright galaxies in the local universe. Drawn out stellar arcs and plumes are clear indications of the ongoing gravitational interactions across the deep and colorful galaxy group photo. The telescopic frame spans about 20 arc minutes or about 400,000 light-years at the group's estimated distance, and includes smaller, bluish NGC 3165 at the right. NGC 3169 is also known to shine across the spectrum from radio to X-rays, harboring an active galactic nucleus that is the site of a supermassive black hole.

Thumbnail image of picture found for this day. APOD: 2022 November 3 - M33: The Triangulum Galaxy
Explanation: The small, northern constellation Triangulum harbors this magnificent face-on spiral galaxy, M33. Its popular names include the Pinwheel Galaxy or just the Triangulum Galaxy. M33 is over 50,000 light-years in diameter, third largest in the Local Group of galaxies after the Andromeda Galaxy (M31), and our own Milky Way. About 3 million light-years from the Milky Way, M33 is itself thought to be a satellite of the Andromeda Galaxy and astronomers in these two galaxies would likely have spectacular views of each other's grand spiral star systems. As for the view from the Milky Way, this sharp image combines data from telescopes on and around planet Earth to show off M33's blue star clusters and pinkish star forming regions along the galaxy's loosely wound spiral arms. In fact, the cavernous NGC 604 is the brightest star forming region, seen here at about the 1 o'clock position from the galaxy center. Like M31, M33's population of well-measured variable stars have helped make this nearby spiral a cosmic yardstick for establishing the distance scale of the Universe.

Thumbnail image of picture found for this day. APOD: 2022 September 16 - The Tarantula Zone
Explanation: The Tarantula Nebula, also known as 30 Doradus, is more than a thousand light-years in diameter, a giant star forming region within nearby satellite galaxy the Large Magellanic Cloud. About 180 thousand light-years away, it's the largest, most violent star forming region known in the whole Local Group of galaxies. The cosmic arachnid sprawls across this magnificent view, an assembly of image data from large space- and ground-based telescopes. Within the Tarantula (NGC 2070), intense radiation, stellar winds, and supernova shocks from the central young cluster of massive stars cataloged as R136 energize the nebular glow and shape the spidery filaments. Around the Tarantula are other star forming regions with young star clusters, filaments, and blown-out bubble-shaped clouds. In fact, the frame includes the site of the closest supernova in modern times, SN 1987A, at lower right. The rich field of view spans about 2 degrees or 4 full moons, in the southern constellation Dorado. But were the Tarantula Nebula closer, say 1,500 light-years distant like the Milky Way's own star forming Orion Nebula, it would take up half the sky.

Thumbnail image of picture found for this day. APOD: 2022 June 15 - In the Heart of the Virgo Cluster
Explanation: The Virgo Cluster of Galaxies is the closest cluster of galaxies to our Milky Way Galaxy. The Virgo Cluster is so close that it spans more than 5 degrees on the sky - about 10 times the angle made by a full Moon. With its heart lying about 70 million light years distant, the Virgo Cluster is the nearest cluster of galaxies, contains over 2,000 galaxies, and has a noticeable gravitational pull on the galaxies of the Local Group of Galaxies surrounding our Milky Way Galaxy. The cluster contains not only galaxies filled with stars but also gas so hot it glows in X-rays. Motions of galaxies in and around clusters indicate that they contain more dark matter than any visible matter we can see. Pictured here, the heart of the Virgo Cluster includes bright Messier galaxies such as Markarian's Eyes on the upper left, M86 just to the upper right of center, M84 on the far right, as well as spiral galaxy NGC 4388 at the bottom right.

Thumbnail image of picture found for this day. APOD: 2022 June 6 - Milky Way Galaxy Doomed: Collision with Andromeda Pending
Explanation: Will our Milky Way Galaxy collide one day with its larger neighbor, the Andromeda Galaxy? Most likely, yes. Careful plotting of slight displacements of M31's stars relative to background galaxies on recent Hubble Space Telescope images indicate that the center of M31 could be on a direct collision course with the center of our home galaxy. Still, the errors in sideways velocity appear sufficiently large to admit a good chance that the central parts of the two galaxies will miss, slightly, but will become close enough for their outer halos to become gravitationally entangled. Once that happens, the two galaxies will become bound, dance around, and eventually merge to become one large elliptical galaxy -- over the next few billion years. Pictured here is a combination of images depicting the sky of a world (Earth?) in the distant future when the outer parts of each galaxy begin to collide. The exact future of our Milky Way and the entire surrounding Local Group of Galaxies is likely to remain an active topic of research for years to come.

Thumbnail image of picture found for this day. APOD: 2022 April 12 - N11: Star Clouds of the LMC
Explanation: Massive stars, abrasive winds, mountains of dust, and energetic light sculpt one of the largest and most picturesque regions of star formation in the Local Group of Galaxies. Known as N11, the region is visible on the upper right of many images of its home galaxy, the Milky Way neighbor known as the Large Magellanic Cloud (LMC). The featured image was taken for scientific purposes by the Hubble Space Telescope and reprocessed for artistry. Although the section imaged above is known as NGC 1763, the entire N11 emission nebula is second in LMC size only to the Tarantula Nebula. Compact globules of dark dust housing emerging young stars are also visible around the image. A recent study of variable stars in the LMC with Hubble has helped to recalibrate the distance scale of the observable universe, but resulted in a slightly different scale than found using the pervasive cosmic microwave background.

Thumbnail image of picture found for this day. APOD: 2022 April 3 - CMB Dipole: Speeding Through the Universe
Explanation: Our Earth is not at rest. The Earth moves around the Sun. The Sun orbits the center of the Milky Way Galaxy. The Milky Way Galaxy orbits in the Local Group of Galaxies. The Local Group falls toward the Virgo Cluster of Galaxies. But these speeds are less than the speed that all of these objects together move relative to the cosmic microwave background radiation (CMBR). In the featured all-sky map from the COBE satellite in 1993, microwave light in the Earth's direction of motion appears blueshifted and hence hotter, while microwave light on the opposite side of the sky is redshifted and colder. The map indicates that the Local Group moves at about 600 kilometers per second relative to this primordial radiation. This high speed was initially unexpected and its magnitude is still unexplained. Why are we moving so fast? What is out there?

Thumbnail image of picture found for this day. APOD: 2022 February 11 - IC 342: The Hidden Galaxy in Camelopardalis
Explanation: Similar in size to large, bright spiral galaxies in our neighborhood, IC 342 is a mere 10 million light-years distant in the long-necked, northern constellation Camelopardalis. A sprawling island universe, IC 342 would otherwise be a prominent galaxy in our night sky, but it is hidden from clear view and only glimpsed through the veil of stars, gas and dust clouds along the plane of our own Milky Way galaxy. Even though IC 342's light is dimmed and reddened by intervening cosmic clouds, this sharp telescopic image traces the galaxy's own obscuring dust, young star clusters, and glowing pink star forming regions along spiral arms that wind far from the galaxy's core. IC 342 may have undergone a recent burst of star formation activity and is close enough to have gravitationally influenced the evolution of the local group of galaxies and the Milky Way.

Thumbnail image of picture found for this day. APOD: 2021 December 2 - NGC 6822: Barnard's Galaxy
Explanation: Grand spiral galaxies often seem to get all the glory, flaunting their young, bright, blue star clusters in beautiful, symmetric spiral arms. But small galaxies form stars too, like nearby NGC 6822, also known as Barnard's Galaxy. Beyond the rich starfields in the constellation Sagittarius, NGC 6822 is a mere 1.5 million light-years away, a member of our Local Group of galaxies. A dwarf irregular galaxy similar to the Small Magellanic Cloud, NGC 6822 is about 7,000 light-years across. Brighter foreground stars in our Milky Way have a spiky appearance. Behind them, Barnard's Galaxy is seen to be filled with young blue stars and mottled with the telltale pinkish hydrogen glow of star forming regions in this deep color composite image.

Thumbnail image of picture found for this day. APOD: 2021 November 12 - M33: The Triangulum Galaxy
Explanation: The small, northern constellation Triangulum harbors this magnificent face-on spiral galaxy, M33. Its popular names include the Pinwheel Galaxy or just the Triangulum Galaxy. M33 is over 50,000 light-years in diameter, third largest in the Local Group of galaxies after the Andromeda Galaxy (M31), and our own Milky Way. About 3 million light-years from the Milky Way, M33 is itself thought to be a satellite of the Andromeda Galaxy and astronomers in these two galaxies would likely have spectacular views of each other's grand spiral star systems. As for the view from planet Earth, this sharp image shows off M33's blue star clusters and pinkish star forming regions along the galaxy's loosely wound spiral arms. In fact, the cavernous NGC 604 is the brightest star forming region, seen here at about the 4 o'clock position from the galaxy center. Like M31, M33's population of well-measured variable stars have helped make this nearby spiral a cosmic yardstick for establishing the distance scale of the Universe.

Thumbnail image of picture found for this day. APOD: 2021 November 6 - The Galaxy Between Two Friends
Explanation: On an August night two friends enjoyed this view after a day's hike on the Plateau d'Emparis in the French Alps. At 2400 meters altitude the sky was clear. Light from a setting moon illuminates the foreground captured in the simple vertical panorama of images. Along the plane of our Milky Way galaxy stars of Cassiopeia and Perseus shine along the panorama's left edge. But seen as a faint cloud with a brighter core, the Andromeda galaxy, stands directly above the two friends in the night. The nearest large spiral galaxy, Andromeda is about 2.5 million light-years beyond the stars of the Milky Way. Adding to the evening's shared extragalactic perspective, the fainter fuzzy spot in the sky right between them is M33, also known as the Triangulum galaxy. Third largest in the local galaxy group, after Andromeda and Milky Way, the Triangulum galaxy is about 3 million light-years distant. On that night, the two friends stood about 3 light-nanoseconds apart.

Thumbnail image of picture found for this day. APOD: 2021 September 30 - The Hydrogen Clouds of M33
Explanation: Gorgeous spiral galaxy M33 seems to have more than its fair share of glowing hydrogen gas. A prominent member of the local group of galaxies, M33 is also known as the Triangulum Galaxy and lies a mere 3 million light-years away. Sprawling along loose spiral arms that wind toward the core, M33's giant HII regions are some of the largest known stellar nurseries, sites of the formation of short-lived but very massive stars. Intense ultraviolet radiation from the luminous massive stars ionizes the surrounding hydrogen gas and ultimately produces the characteristic red glow. To highlight the HII regions in this telescopic image, broadband data used to produce a color view of the galaxy were combined with narrowband data recorded through a hydrogen-alpha filter, transmitting the light of the strongest hydrogen emission line. Close-ups of cataloged HII regions appear in the sidebar insets. Use the individual reference number to find their location within the Triangulum Galaxy. For example, giant HII region NGC604 is identified in an inset on the right and appears at position number 15. That's about 4 o'clock from galaxy center in this portrait of M33.

Thumbnail image of picture found for this day. APOD: 2021 September 8 - The Deep Sky Toward Andromeda
Explanation: What surrounds the Andromeda galaxy? Out in space, Andromeda (M31) is closely surrounded by several small satellite galaxies, and further out it is part of the Local Group of Galaxies -- of which our Milky Way galaxy is also a member. On the sky, however, gas clouds local to our Milky Way appear to surround M31 -- not unlike how water clouds in Earth's atmosphere may appear to encompass our Moon. The gas clouds toward Andromeda, however, are usually too faint to see. Enter the featured 45-degree long image -- one of the deeper images yet taken of the broader Andromeda region. This image, sensitive to light specifically emitted by hydrogen gas, shows these faint and unfamiliar clouds in tremendous detail. But the image captures more. At the image top is the Triangulum galaxy (M33), the third largest galaxy in the Local Group and the furthest object that can be seen with the unaided eye. Below M33 is the bright Milky-Way star Mirach. The image is the digital accumulation of several long exposures taken from 2018 to 2021 from Pulsnitz, Germany.

Thumbnail image of picture found for this day. APOD: 2021 July 18 - The Andromeda Galaxy in Ultraviolet
Explanation: What does the Andromeda galaxy look like in ultraviolet light? Young blue stars circling the galactic center dominate. A mere 2.5 million light-years away, the Andromeda Galaxy, also known as M31, really is just next door as large galaxies go. Spanning about 230,000 light-years, it took 11 different image fields from NASA's Galaxy Evolution Explorer (GALEX) satellite telescope to produce this gorgeous portrait of the spiral galaxy in ultraviolet light in 2003. While its spiral arms stand out in visible light images, Andromeda's arms look more like rings in ultraviolet. The rings are sites of intense star formation and have been interpreted as evidence that Andromeda collided with its smaller neighboring elliptical galaxy M32 more than 200 million years ago. The Andromeda galaxy and our own comparable Milky Way galaxy are the most massive members of the Local Group of galaxies and are projected to collide in several billion years -- perhaps around the time that our Sun's atmosphere will expand to engulf the Earth.

Thumbnail image of picture found for this day. APOD: 2021 May 22 - Markarian's Chain
Explanation: Near the heart of the Virgo Galaxy Cluster the string of galaxies known as Markarian's Chain stretches across this deep telescopic field of view. Anchored in the frame at bottom center by prominent lenticular galaxies, M84 (bottom) and M86, you can follow the chain up and to the right. Near center you'll spot the pair of interacting galaxies NGC 4438 and NGC 4435, known to some as Markarian's Eyes. Its center an estimated 50 million light-years distant, the Virgo Cluster itself is the nearest galaxy cluster. With up to about 2,000 member galaxies, it has a noticeable gravitational influence on our own Local Group of Galaxies. Within the Virgo Cluster at least seven galaxies in Markarian's Chain appear to move coherently, although others may appear to be part of the chain by chance.

Thumbnail image of picture found for this day. APOD: 2020 November 28 - NGC 6822: Barnard s Galaxy
Explanation: Grand spiral galaxies often seem to get all the glory, flaunting their young, bright, blue star clusters in beautiful, symmetric spiral arms. But small galaxies form stars too, like nearby NGC 6822, also known as Barnard's Galaxy. Beyond the rich starfields in the constellation Sagittarius, NGC 6822 is a mere 1.5 million light-years away, a member of our Local Group of galaxies. A dwarf irregular galaxy similar to the Small Magellanic Cloud, NGC 6822 is about 7,000 light-years across. Brighter foreground stars in our Milky Way have a spiky appearance. Behind them, Barnard's Galaxy is seen to be filled with young blue stars and mottled with the telltale pinkish hydrogen glow of star forming regions in this deep color composite image.

Thumbnail image of picture found for this day. APOD: 2020 November 13 - The Tarantula Zone
Explanation: The Tarantula Nebula, also known as 30 Doradus, is more than a thousand light-years in diameter, a giant star forming region within nearby satellite galaxy the Large Magellanic Cloud. About 180 thousand light-years away, it's the largest, most violent star forming region known in the whole Local Group of galaxies. The cosmic arachnid sprawls across the top of this spectacular view, composed with narrowband filter data centered on emission from ionized hydrogen and oxygen atoms. Within the Tarantula (NGC 2070), intense radiation, stellar winds and supernova shocks from the central young cluster of massive stars, cataloged as R136, energize the nebular glow and shape the spidery filaments. Around the Tarantula are other star forming regions with young star clusters, filaments, and blown-out bubble-shaped clouds. In fact, the frame includes the site of the closest supernova in modern times, SN 1987A, right of center. The rich field of view spans about 2 degrees or 4 full moons, in the southern constellation Dorado. But were the Tarantula Nebula closer, say 1,500 light-years distant like the local star forming Orion Nebula, it would take up half the sky.

Thumbnail image of picture found for this day. APOD: 2020 October 10 - Virgo Cluster Galaxies
Explanation: Galaxies of the Virgo Cluster are scattered across this deep telescopic field of view. The cosmic scene spans about three Full Moons, captured in dark skies near Jalisco, Mexico, planet Earth. About 50 million light-years distant, the Virgo Cluster is the closest large galaxy cluster to our own local galaxy group. Prominent here are Virgo's bright elliptical galaxies from the Messier catalog, M87 at the top left, and M84 and M86 seen (bottom to top) below and right of center. M84 and M86 are recognized as part of Markarian's Chain, a visually striking line-up of galaxies vertically on the right side of this frame. Near the middle of the chain lies an intriguing interacting pair of galaxies, NGC 4438 and NGC 4435, known to some as Markarian's Eyes. Of course giant elliptical galaxy M87 dominates the Virgo cluster. It's the home of a super massive black hole, the first black hole ever imaged by planet Earth's Event Horizon Telescope.

Thumbnail image of picture found for this day. APOD: 2020 April 14 - NGC 253: The Silver Coin Galaxy
Explanation: NGC 253 is one of the brightest spiral galaxies visible, but also one of the dustiest. Dubbed the Silver Coin for its appearance in smalltelescopes, it is more formally known as the Sculptor Galaxy for its location within the boundaries of the southern constellation Sculptor. Discovered in 1783 by mathematician and astronomer Caroline Herschel, the dusty island universe lies a mere 10 million light-years away. About 70 thousand light-years across, NGC 253, pictured, is the largest member of the Sculptor Group of Galaxies, the nearest to our own Local Group of galaxies. In addition to its spiral dust lanes, tendrils of dust seem to be rising from a galactic disk laced with young star clusters and star forming regions in this sharp color image. The high dust content accompanies frantic star formation, earning NGC 253 the designation of a starburst galaxy. NGC 253 is also known to be a strong source of high-energy x-rays and gamma rays, likely due to massive black holes near the galaxy's center. Take a trip through extragalactic space in this short video flyby of NGC 253.

Thumbnail image of picture found for this day. APOD: 2019 December 31 - M33: The Triangulum Galaxy
Explanation: The small, northern constellation Triangulum harbors this magnificent face-on spiral galaxy, M33. Its popular names include the Pinwheel Galaxy or just the Triangulum Galaxy. M33 is over 50,000 light-years in diameter, third largest in the Local Group of galaxies after the Andromeda Galaxy (M31), and our own Milky Way. About 3 million light-years from the Milky Way, M33 is itself thought to be a satellite of the Andromeda Galaxy and astronomers in these two galaxies would likely have spectacular views of each other's grand spiral star systems. As for the view from planet Earth, this sharp image shows off M33's blue star clusters and pinkish star forming regions along the galaxy's loosely wound spiral arms. In fact, the cavernous NGC 604 is the brightest star forming region, seen here at about the 7 o'clock position from the galaxy center. Like M31, M33's population of well-measured variable stars have helped make this nearby spiral a cosmic yardstick for establishing the distance scale of the Universe.

Thumbnail image of picture found for this day. APOD: 2019 October 3 - The Hydrogen Clouds of M33
Explanation: Gorgeous spiral galaxy M33 seems to have more than its fair share of glowing hydrogen gas. A prominent member of the local group of galaxies, M33 is also known as the Triangulum Galaxy and lies a mere 3 million light-years away. The galaxy's inner 30,000 light-years or so are shown in this magnificent 25 panel telescopic mosaic. Based on image data from space and ground-based telescopes, the portrait of M33 shows off the galaxy's reddish ionized hydrogen clouds or HII regions. Sprawling along loose spiral arms that wind toward the core, M33's giant HII regions are some of the largest known stellar nurseries, sites of the formation of short-lived but very massive stars. Intense ultraviolet radiation from the luminous, massive stars ionizes the surrounding hydrogen gas and ultimately produces the characteristic red glow. To enhance this image, broadband data were used to produce a color view of the galaxy and combined with narrowband data recorded through a hydrogen-alpha filter. That filter transmits the light of the strongest visible hydrogen emission line.

Thumbnail image of picture found for this day. APOD: 2019 April 29 - N11: Star Clouds of the LMC
Explanation: Massive stars, abrasive winds, mountains of dust, and energetic light sculpt one of the largest and most picturesque regions of star formation in the Local Group of Galaxies. Known as N11, the region is visible on the upper right of many images of its home galaxy, the Milky Way neighbor known as the Large Magellanic Clouds (LMC). The featured image was taken for scientific purposes by the Hubble Space Telescope and reprocessed for artistry by an amateur to win a Hubble's Hidden Treasures competition. Although the section imaged above is known as NGC 1763, the entire N11 emission nebula is second in LMC size only to the Tarantula Nebula. Compact globules of dark dust housing emerging young stars are also visible around the image. A new study of variable stars in the LMC with Hubble has helped to recalibrate the distance scale of the observable universe, but resulted in a slightly different scale than found using the pervasive cosmic microwave background.

Thumbnail image of picture found for this day. APOD: 2019 March 31 - Markarian's Chain of Galaxies
Explanation: Across the heart of the Virgo Cluster of Galaxies lies a striking string of galaxies known as Markarian's Chain. The chain, pictured here, is highlighted on the right with two large but featureless lenticular galaxies, M84 and M86. Prominent to their lower left is a pair of interacting galaxies known as The Eyes. The home Virgo Cluster is the nearest cluster of galaxies, contains over 2000 galaxies, and has a noticeable gravitational pull on the galaxies of the Local Group of Galaxies surrounding our Milky Way Galaxy. The center of the Virgo Cluster is located about 70 million light years away toward the constellation of Virgo. At least seven galaxies in the chain appear to move coherently, although others appear to be superposed by chance.

Thumbnail image of picture found for this day. APOD: 2019 February 23 - The Stars of the Triangulum Galaxy
Explanation: Like grains of sand on a cosmic beach, stars of the Triangulum Galaxy are resolved in this sharp mosaic from the Hubble Space Telescope's Advanced Camera for Surveys (ACS). The inner region of the galaxy spanning over 17,000 light-years is covered at extreme resolution, the second largest image ever released by Hubble. At its center is the bright, densely packed galactic core surrounded by a loose array of dark dust lanes mixed with the stars in the galactic plane. Also known as M33, the face-on spiral galaxy lies 3 million light-years away in the small northern constellation Triangulum. Over 50,000 light-years in diameter, the Triangulum Galaxy is the third largest in the Local Group of galaxies after the Andromeda Galaxy (M31), and our own Milky Way. Of course, to fully appreciate the Triangulum's stars, star clusters, and bright nebulae captured in this Hubble mosaic, you'll need to use a zoom tool.

Thumbnail image of picture found for this day. APOD: 2019 January 16 - IC 342: The Hidden Galaxy
Explanation: Similar in size to large, bright spiral galaxies in our neighborhood, IC 342 is a mere 10 million light-years distant in the long-necked, northern constellation Camelopardalis. A sprawling island universe, IC 342 would otherwise be a prominent galaxy in our night sky, but it is hidden from clear view and only glimpsed through the veil of stars, gas and dust clouds along the plane of our own Milky Way galaxy. Even though IC 342's light is dimmed and reddened by intervening cosmic clouds, this sharp telescopic image traces the galaxy's own obscuring dust, young star clusters, and glowing pink star forming regions along spiral arms that wind far from the galaxy's core. IC 342 may have undergone a recent burst of star formation activity and is close enough to have gravitationally influenced the evolution of the local group of galaxies and the Milky Way.

Thumbnail image of picture found for this day. APOD: 2018 December 17 - M31: The Andromeda Galaxy
Explanation: What is the nearest major galaxy to our own Milky Way Galaxy? Andromeda. In fact, our Galaxy is thought to look much like Andromeda. Together these two galaxies dominate the Local Group of galaxies. The diffuse light from Andromeda is caused by the hundreds of billions of stars that compose it. The several distinct stars that surround Andromeda's image are actually stars in our Galaxy that are well in front of the background object. Andromeda is frequently referred to as M31 since it is the 31st object on Messier's list of diffuse sky objects. M31 is so distant it takes about two million years for light to reach us from there. Although visible without aid, the featured image of M31 is a digital mosaic of 20 frames taken with a small telescope. Much about M31 remains unknown, including exactly how long it will be before it collides with our home galaxy.

Thumbnail image of picture found for this day. APOD: 2018 November 17 - The Tarantula Nebula
Explanation: The Tarantula Nebula, also known as 30 Doradus, is more than a thousand light-years in diameter, a giant star forming region within nearby satellite galaxy the Large Magellanic Cloud. About 180 thousand light-years away, it's the largest, most violent star forming region known in the whole Local Group of galaxies. The cosmic arachnid sprawls across this spectacular view, composed with narrowband filter data centered on emission from ionized hydrogen atoms. Within the Tarantula (NGC 2070), intense radiation, stellar winds and supernova shocks from the central young cluster of massive stars, cataloged as R136, energize the nebular glow and shape the spidery filaments. Around the Tarantula are other star forming regions with young star clusters, filaments, and blown-out bubble-shaped clouds. In fact, the frame includes the site of the closest supernova in modern times, SN 1987A, left of center. The rich field of view spans about 1 degree or 2 full moons, in the southern constellation Dorado. But were the Tarantula Nebula closer, say 1,500 light-years distant like the local star forming Orion Nebula, it would take up half the sky.

Thumbnail image of picture found for this day. APOD: 2018 September 27 - M33: Triangulum Galaxy
Explanation: The small, northern constellation Triangulum harbors this magnificent face-on spiral galaxy, M33. Its popular names include the Pinwheel Galaxy or just the Triangulum Galaxy. M33 is over 50,000 light-years in diameter, third largest in the Local Group of galaxies after the Andromeda Galaxy (M31), and our own Milky Way. About 3 million light-years from the Milky Way, M33 is itself thought to be a satellite of the Andromeda Galaxy and astronomers in these two galaxies would likely have spectacular views of each other's grand spiral star systems. As for the view from planet Earth, this sharp image shows off M33's blue star clusters and pinkish star forming regions along the galaxy's loosely wound spiral arms. In fact, the cavernous NGC 604 is the brightest star forming region, seen here at about the 7 o'clock position from the galaxy center. Like M31, M33's population of well-measured variable stars have helped make this nearby spiral a cosmic yardstick for establishing the distance scale of the Universe.

Thumbnail image of picture found for this day. APOD: 2018 March 22 - NGC 253: Dusty Island Universe
Explanation: Shiny NGC 253 is one of the brightest spiral galaxies visible, and also one of the dustiest. Some call it the Silver Dollar Galaxy for its appearance in small telescopes, or just the Sculptor Galaxy for its location within the boundaries of the southern constellation Sculptor. Discovered in 1783 by mathematician and astronomer Caroline Herschel, the dusty island universe lies a mere 10 million light-years away. About 70 thousand light-years across, NGC 253 is the largest member of the Sculptor Group of Galaxies, the nearest to our own Local Group of Galaxies. In addition to its spiral dust lanes, tendrils of dust seem to be rising from a galactic disk laced with young star clusters and star forming regions in this sharp color image. The high dust content accompanies frantic star formation, earning NGC 253 the designation of a starburst galaxy. NGC 253 is also known to be a strong source of high-energy x-rays and gamma rays, likely due to massive black holes near the galaxy's center. Take a trip through extragalactic space in this short video flyby of NGC 253.

Thumbnail image of picture found for this day. APOD: 2017 November 30 - M33: Triangulum Galaxy
Explanation: The small, northern constellation Triangulum harbors this magnificent face-on spiral galaxy, M33. Its popular names include the Pinwheel Galaxy or just the Triangulum Galaxy. M33 is over 50,000 light-years in diameter, third largest in the Local Group of galaxies after the Andromeda Galaxy (M31), and our own Milky Way. About 3 million light-years from the Milky Way, M33 is itself thought to be a satellite of the Andromeda Galaxy and astronomers in these two galaxies would likely have spectacular views of each other's grand spiral star systems. As for the view from planet Earth, this sharp composite image nicely shows off M33's blue star clusters and pinkish star forming regions along the galaxy's loosely wound spiral arms. In fact, the cavernous NGC 604 is the brightest star forming region, seen here at about the 7 o'clock position from the galaxy center. Like M31, M33's population of well-measured variable stars have helped make this nearby spiral a cosmic yardstick for establishing the distance scale of the Universe.

Thumbnail image of picture found for this day. APOD: 2017 November 16 - The Tarantula Nebula
Explanation: The Tarantula Nebula is more than a thousand light-years in diameter, a giant star forming region within nearby satellite galaxy the Large Magellanic Cloud, about 180 thousand light-years away. The largest, most violent star forming region known in the whole Local Group of galaxies, the cosmic arachnid sprawls across this spectacular view composed with narrowband data centered on emission from ionized hydrogen and oxygen atoms. Within the Tarantula (NGC 2070), intense radiation, stellar winds and supernova shocks from the central young cluster of massive stars, cataloged as R136, energize the nebular glow and shape the spidery filaments. Around the Tarantula are other star forming regions with young star clusters, filaments, and blown-out bubble-shaped clouds. In fact, the frame includes the site of the closest supernova in modern times, SN 1987A, right of center. The rich field of view spans about 1 degree or 2 full moons, in the southern constellation Dorado. But were the Tarantula Nebula closer, say 1,500 light-years distant like the local star forming Orion Nebula, it would take up half the sky.

Thumbnail image of picture found for this day. APOD: 2017 August 12 - A Day in the Life of a Human Sundial
Explanation: Have you ever wanted to be a gnomon? Of course, a gnomon is the tall part of a sundial that casts a shadow. The gnomon's shadow moves as the Sun moves across the sky, indicating time by the shadow's position on the dial face. So on July 19th, the Astronomy Group of the Progymnasium Rosenfeld created a human sundial, each participant patiently playing the role of the gnomon for 10 minutes. In this timelapse video of their temporal voyage of discovery, one image was taken every 20 seconds from 8 am until 4 pm Central European Summer Time. Drawn on the ground are the dial hour marks calculated to show the local time for that specific date. Behind, the tower clock offers a time check. Can you find the local time of solar noon? (Hint: At solar noon the Sun is on the meridan.) The persistent group plans a repetition of the human sundial performance next winter to compare the length of the day and the altitude of the Sun.

Thumbnail image of picture found for this day. APOD: 2017 July 8 - Hidden Galaxy IC 342
Explanation: Similar in size to large, bright spiral galaxies in our neighborhood, IC 342 is a mere 10 million light-years distant in the long-necked, northern constellation Camelopardalis. A sprawling island universe, IC 342 would otherwise be a prominent galaxy in our night sky, but it is hidden from clear view and only glimpsed through the veil of stars, gas and dust clouds along the plane of our own Milky Way galaxy. Even though IC 342's light is dimmed by intervening cosmic clouds, this sharp telescopic image traces the galaxy's own obscuring dust, blue star clusters, and glowing pink star forming regions along spiral arms that wind far from the galaxy's core. IC 342 may have undergone a recent burst of star formation activity and is close enough to have gravitationally influenced the evolution of the local group of galaxies and the Milky Way.

Thumbnail image of picture found for this day. APOD: 2017 June 24 - Markarian's Chain to Messier 64
Explanation: Top to bottom, this colorful and broad telescopic mosaic links Markarian's Chain of galaxies across the core of the Virgo Cluster to dusty spiral galaxy Messier 64. Galaxies are scattered through the field of view that spans some 20 full moons across a gorgeous night sky. The cosmic frame is also filled with foreground stars from constellations Virgo and the well-groomed Coma Berenices, and faint, dusty nebulae drifting above the plane of the Milky Way. Look carefully for Markarian's eyes. The famous pair of interacting galaxies is near the top, not far from M87, the Virgo cluster's giant elliptical galaxy. At the bottom, you can stare down Messier 64, also known as the Black Eye Galaxy. The Virgo Cluster is the closest large galaxy cluster to our own local galaxy group. Virgo Cluster galaxies are about 50 million light-years distant, but M64 lies a mere 17 million light-years away.

Thumbnail image of picture found for this day. APOD: 2017 May 19 - Wolf-Lundmark-Melotte
Explanation: Named for the three astronomers instrumental in its discovery and identification, Wolf - Lundmark - Melotte (WLM) is a lonely dwarf galaxy. Seen toward the mostly southern constellation Cetus, about 3 million light-years from the Milky Way, it is one of the most remote members of our local galaxy group. In fact, it may never have interacted with any other local group galaxy. Still, telltale pinkish star forming regions and hot, young, bluish stars speckle the isolated island universe. Older, cool yellowish stars fade into the small galaxy's halo, extending about 8,000 light-years across. This sharp portrait of WLM was captured by the 268-megapixel OmegaCAM widefield imager and survey telescope at ESO's Paranal Observatory.

Thumbnail image of picture found for this day. APOD: 2017 February 24 - NGC 3621: Far Beyond the Local Group
Explanation: Far beyond the local group of galaxies lies NGC 3621, some 22 million light-years away. Found in the multi-headed southern constellation Hydra, the winding spiral arms of this gorgeous island universe are loaded with luminous blue star clusters, pinkish starforming regions, and dark dust lanes. Still, for astronomers NGC 3621 has not been just another pretty face-on spiral galaxy. Some of its brighter stars have been used as standard candles to establish important estimates of extragalactic distances and the scale of the Universe. This beautiful image of NGC 3621, is a composite of space- and ground-based telescope data. It traces the loose spiral arms far from the galaxy's brighter central regions for some 100,000 light-years. Spiky foreground stars in our own Milky Way Galaxy and even more distant background galaxies are scattered across the colorful skyscape.

Thumbnail image of picture found for this day. APOD: 2016 December 27 - M31: The Andromeda Galaxy
Explanation: What is the nearest major galaxy to our own Milky Way Galaxy? Andromeda. In fact, our Galaxy is thought to look much like Andromeda. Together these two galaxies dominate the Local Group of galaxies. The diffuse light from Andromeda is caused by the hundreds of billions of stars that compose it. The several distinct stars that surround Andromeda's image are actually stars in our Galaxy that are well in front of the background object. Andromeda is frequently referred to as M31 since it is the 31st object on Messier's list of diffuse sky objects. M31 is so distant it takes about two million years for light to reach us from there. Although visible without aid, the featured image of M31 is a digital mosaic of several frames taken with a small telescope. Much about M31 remains unknown, including exactly how many billions of years it will before it collides with our home galaxy.

Thumbnail image of picture found for this day. APOD: 2016 November 3 - NGC 253: Dusty Island Universe
Explanation: Shiny NGC 253 is one of the brightest spiral galaxies visible, and also one of the dustiest. Some call it the Silver Dollar Galaxy for its appearance in small telescopes, or just the Sculptor Galaxy for its location within the boundaries of the southern constellation Sculptor. First swept up in 1783 by mathematician and astronomer Caroline Herschel, the dusty island universe lies a mere 10 million light-years away. About 70 thousand light-years across, NGC 253 is the largest member of the Sculptor Group of Galaxies, the nearest to our own Local Group of Galaxies. In addition to its spiral dust lanes, tendrils of dust seem to be rising from a galactic disk laced with young star clusters and star forming regions in this sharp color image. The high dust content accompanies frantic star formation, earning NGC 253 the designation of a starburst galaxy. NGC 253 is also known to be a strong source of high-energy x-rays and gamma rays, likely due to massive black holes near the galaxy's center.

Thumbnail image of picture found for this day. APOD: 2016 October 7 - The Hydrogen Clouds of M33
Explanation: Gorgeous spiral galaxy M33 seems to have more than its fair share of glowing hydrogen gas. A prominent member of the local group of galaxies, M33 is also known as the Triangulum Galaxy and lies about 3 million light-years distant. The galaxy's inner 30,000 light-years or so are shown in this telescopic portrait that enhances its reddish ionized hydrogen clouds or HII regions. Sprawling along loose spiral arms that wind toward the core, M33's giant HII regions are some of the largest known stellar nurseries, sites of the formation of short-lived but very massive stars. Intense ultraviolet radiation from the luminous, massive stars ionizes the surrounding hydrogen gas and ultimately produces the characteristic red glow. To enhance this image, broadband data was used to produce a color view of the galaxy and combined with narrowband data recorded through a hydrogen-alpha filter. That filter transmits the light of the strongest visible hydrogen emission line.

Thumbnail image of picture found for this day. APOD: 2016 September 17 - M33: Triangulum Galaxy
Explanation: The small, northern constellation Triangulum harbors this magnificent face-on spiral galaxy, M33. Its popular names include the Pinwheel Galaxy or just the Triangulum Galaxy. M33 is over 50,000 light-years in diameter, third largest in the Local Group of galaxies after the Andromeda Galaxy (M31), and our own Milky Way. About 3 million light-years from the Milky Way, M33 is itself thought to be a satellite of the Andromeda Galaxy and astronomers in these two galaxies would likely have spectacular views of each other's grand spiral star systems. As for the view from planet Earth, this sharp composite image nicely shows off M33's blue star clusters and pinkish star forming regions along the galaxy's loosely wound spiral arms. In fact, the cavernous NGC 604 is the brightest star forming region, seen here at about the 1 o'clock position from the galaxy center. Like M31, M33's population of well-measured variable stars have helped make this nearby spiral a cosmic yardstick for establishing the distance scale of the Universe.

Thumbnail image of picture found for this day. APOD: 2016 August 26 - The Milky Way Sets
Explanation: Under dark skies the setting of the Milky Way can be a dramatic sight. Stretching nearly parallel to the horizon, this rich, edge-on vista of our galaxy above the dusty Namibian desert stretches from bright, southern Centaurus (left) to Cepheus in the north (right). From early August, the digitally stitched, panoramic night skyscape captures the Milky Way's congeries of stars and rivers of cosmic dust, along with colors of nebulae not readily seen with the eye. Mars, Saturn, and Antares, visible even in more luminous night skies, form the the bright celestial triangle just touching the trees below the galaxy's central bulge. Of course, our own galaxy is not the only galaxy in the scene. Two other major members of our local group, the Andromeda Galaxy and the Triangulum Galaxy, lie near the right edge of the frame, beyond the arc of the setting Milky Way.

Thumbnail image of picture found for this day. APOD: 2016 April 7 - Wolf-Lundmark-Melotte
Explanation: Named for the three astronomers instrumental in its discovery and identification, Wolf - Lundmark - Melotte (WLM) is a lonely dwarf galaxy. Seen toward the mostly southern constellation Cetus, about 3 million light-years from the Milky Way, it is one of the most remote members of our local galaxy group. In fact, it may never have interacted with any other local group galaxy. Still, telltale pinkish star forming regions and hot, young, bluish stars speckle the isolated island universe. Older, cool yellowish stars fade into the small galaxy's halo, extending about 8,000 light-years across. This sharp portrait of WLM was captured by the 268-megapixel OmegaCAM widefield imager and survey telescope at ESO's Paranal Observatory.

Thumbnail image of picture found for this day. APOD: 2016 February 26 - The Tarantula Nebula
Explanation: The Tarantula Nebula is more than a thousand light-years in diameter, a giant star forming region within nearby satellite galaxy the Large Magellanic Cloud, about 180 thousand light-years away. The largest, most violent star forming region known in the whole Local Group of galaxies, the cosmic arachnid sprawls across this spectacular composite view constructed with space- and ground-based image data. Within the Tarantula (NGC 2070), intense radiation, stellar winds and supernova shocks from the central young cluster of massive stars, cataloged as R136, energize the nebular glow and shape the spidery filaments. Around the Tarantula are other star forming regions with young star clusters, filaments, and blown-out bubble-shaped clouds In fact, the frame includes the site of the closest supernova in modern times, SN 1987A, at the lower right. The rich field of view spans about 1 degree or 2 full moons, in the southern constellation Dorado. But were the Tarantula Nebula closer, say 1,500 light-years distant like the local star forming Orion Nebula, it would take up half the sky.

Thumbnail image of picture found for this day. APOD: 2016 February 19 - NGC 2403 in Camelopardalis
Explanation: Magnificent island universe NGC 2403 stands within the boundaries of the long-necked constellation Camelopardalis. Some 10 million light-years distant and about 50,000 light-years across, the spiral galaxy also seems to have more than its fair share of giant star forming HII regions, marked by the telltale reddish glow of atomic hydrogen gas. The giant HII regions are energized by clusters of hot, massive stars that explode as bright supernovae at the end of their short and furious lives. A member of the M81 group of galaxies, NGC 2403 closely resembles another galaxy with an abundance of star forming regions that lies within our own local galaxy group, M33 the Triangulum Galaxy. Spiky in appearance, bright stars in this colorful galaxy portrait of NGC 2403 are in the foreground, within our own Milky Way.

Thumbnail image of picture found for this day. APOD: 2016 January 29 - Hidden Galaxy IC 342
Explanation: Similar in size to large, bright spiral galaxies in our neighborhood, IC 342 is a mere 10 million light-years distant in the long-necked, northern constellation Camelopardalis. A sprawling island universe, IC 342 would otherwise be a prominent galaxy in our night sky, but it is hidden from clear view and only glimpsed through the veil of stars, gas and dust clouds along the plane of our own Milky Way galaxy. Even though IC 342's light is dimmed by intervening cosmic clouds, this deep telescopic image traces the galaxy's obscuring dust, blue star clusters, and glowing pink star forming regions along spiral arms that wind far from the galaxy's core. IC 342 may have undergone a recent burst of star formation activity and is close enough to have gravitationally influenced the evolution of the local group of galaxies and the Milky Way.

Thumbnail image of picture found for this day. APOD: 2016 January 7 - High Energy Andromeda
Explanation: A mere 2.5 million light-years away, the Andromeda Galaxy, also known as M31, really is just next door as large galaxies go. In this (inset) scan, image data from NASA's Nuclear Spectrosopic Telescope Array has yielded the best high-energy X-ray view yet of our large neighboring spiral, revealing some 40 extreme sources of X-rays, X-ray binary star systems that contain a black hole or neutron star orbiting a more normal stellar companion. In fact, larger Andromeda and our own Milky Way are the most massive members of the local galaxy group. Andromeda is close enough that NuSTAR can examine its population of X-ray binaries in detail, comparing them to our own. The background image of Andromeda was taken by NASA's Galaxy Evolution Explorer in energetic ultraviolet light.

Thumbnail image of picture found for this day. APOD: 2015 November 6 - Unraveling NGC 3169
Explanation: Spiral galaxy NGC 3169 appears to be unraveling in this cosmic scene, played out some 70 million light-years away just below bright star Regulus toward the faint constellation Sextans. Its beautiful spiral arms are distorted into sweeping tidal tails as NGC 3169 (top) and neighboring NGC 3166 interact gravitationally, a common fate even for bright galaxies in the local universe. In fact, drawn out stellar arcs and plumes, indications of gravitational interactions, seem rampant in the deep and colorful galaxy group photo. The picture spans 20 arc minutes, or about 400,000 light-years at the group's estimated distance, and includes smaller, dimmer NGC 3165 at bottom right. NGC 3169 is also known to shine across the spectrum from radio to X-rays, harboring an active galactic nucleus that is likely the site of a supermassive black hole.

Thumbnail image of picture found for this day. APOD: 2015 September 26 - M31 versus M33
Explanation: Separated by about 14 degrees (28 Full Moons) in planet Earth's sky, spiral galaxies M31 at left, and M33 are both large members of the Local Group, along with our own Milky Way galaxy. This narrow- and wide-angle, multi-camera composite finds details of spiral structure in both, while the massive neighboring galaxies seem to be balanced in starry fields either side of bright Mirach, beta star in the constellation Andromeda. Mirach is just 200 light-years from the Sun. But M31, the Andromeda Galaxy, is really 2.5 million light-years distant and M33, the Triangulum Galaxy, is also about 3 million light years away. Although they look far apart, M31 and M33 are engaged in a gravitational struggle. In fact, radio astronomers have found indications of a bridge of neutral hydrogen gas that could connect the two, evidence of a closer encounter in the past. Based on measurements, gravitational simulations currently predict that the Milky Way, M31, and M33 will all undergo mutual close encounters and potentially mergers, billions of years in the future.

Thumbnail image of picture found for this day. APOD: 2015 August 30 - M31: The Andromeda Galaxy
Explanation: What is the nearest major galaxy to our own Milky Way Galaxy? Andromeda. In fact, our Galaxy is thought to look much like Andromeda. Together these two galaxies dominate the Local Group of galaxies. The diffuse light from Andromeda is caused by the hundreds of billions of stars that compose it. The several distinct stars that surround Andromeda's image are actually stars in our Galaxy that are well in front of the background object. Andromeda is frequently referred to as M31 since it is the 31st object on Messier's list of diffuse sky objects. M31 is so distant it takes about two million years for light to reach us from there. Although visible without aid, the above image of M31 is a digital mosaic of 20 frames taken with a small telescope. Much about M31 remains unknown, including exactly how long it will before it collides with our home galaxy.

Thumbnail image of picture found for this day. APOD: 2015 August 9 - HCG 87: A Small Group of Galaxies
Explanation: Sometimes galaxies form groups. For example, our own Milky Way Galaxy is part of the Local Group of Galaxies. Small, compact groups, like Hickson Compact Group 87 (HCG 87) shown above, are interesting partly because they slowly self-destruct. Indeed, the galaxies of HCG 87 are gravitationally stretching each other during their 100-million year orbits around a common center. The pulling creates colliding gas that causes bright bursts of star formation and feeds matter into their active galaxy centers. HCG 87 is composed of a large edge-on spiral galaxy visible near the image center, an elliptical galaxy visible to its right, and a spiral galaxy visible near the top. The small spiral near the center might be far in the distance. Several stars from our Galaxy are also visible in the foreground. Studying groups like HCG 87 allows insight into how all galaxies form and evolve.

Thumbnail image of picture found for this day. APOD: 2015 August 4 - Virgo Cluster Galaxies
Explanation: Well over a thousand galaxies are known members of the Virgo Cluster, the closest large cluster of galaxies to our own local group. In fact, the galaxy cluster is difficult to appreciate all at once because it covers such a large area on the sky. This careful wide-field mosaic of telescopic images clearly records the central region of the Virgo Cluster through faint foreground dust clouds lingering above the plane of our own Milky Way galaxy. The cluster's dominant giant elliptical galaxy M87, is just below and to the left of the frame center. To the right of M87 is a string of galaxies known as Markarian's Chain. A closer examination of the image will reveal many Virgo cluster member galaxies as small fuzzy patches. Sliding your cursor over the image will label the larger galaxies using NGC catalog designations. Galaxies are also shown with Messier catalog numbers, including M84, M86, and prominent colorful spirals M88, M90, and M91. On average, Virgo Cluster galaxies are measured to be about 48 million light-years away. The Virgo Cluster distance has been used to give an important determination of the Hubble Constant and the scale of the Universe.

Thumbnail image of picture found for this day. APOD: 2015 July 24 - Ultraviolet Rings of M31
Explanation: A mere 2.5 million light-years away the Andromeda Galaxy, also known as M31, really is just next door as large galaxies go. So close and spanning some 260,000 light-years, it took 11 different image fields from the Galaxy Evolution Explorer (GALEX) satellite's telescope to produce this gorgeous portrait of the spiral galaxy in ultraviolet light. While its spiral arms stand out in visible light images of Andromeda, the arms look more like rings in the GALEX ultraviolet view, a view dominated by the energetic light from hot, young, massive stars. As sites of intense star formation, the rings have been interpreted as evidence Andromeda collided with its smaller neighboring elliptical galaxy M32 more than 200 million years ago. The large Andromeda galaxy and our own Milky Way are the most massive members of the local galaxy group.

Thumbnail image of picture found for this day. APOD: 2015 June 29 - Sunspot Group AR 2339 Crosses the Sun
Explanation: How do sunspots evolve? Large dark sunspots -- and the active regions that contain them -- may last for weeks, but all during that time they are constantly changing. Such variations were particularly apparent a few weeks ago as the active region AR 2339 came around the limb of the Sun and was tracked for the next 12 days by NASA's Solar Dynamic Observatory. In the featured time lapse video, some sunspots drift apart, while others merge. All the while, the dark central umbral regions shift internally and their surrounding lighter penumbras shimmer and wave. The surrounding Sun appears to flicker as the carpet of yellow granules come and go on the time scale of hours. In general, sunspots are relatively cool regions where the local magnetic field pokes through the Sun's surface and inhibits heating. Over the past week, an even more active region -- AR 2371 -- has been crossing the Sun and releasing powerful flares that have resulted in impressive auroras here on Earth.

Thumbnail image of picture found for this day. APOD: 2015 April 7 - In the Heart of the Virgo Cluster
Explanation: The Virgo Cluster of Galaxies is the closest cluster of galaxies to our Milky Way Galaxy. The Virgo Cluster is so close that it spans more than 5 degrees on the sky - about 10 times the angle made by a full Moon. With its heart lying about 70 million light years distant, the Virgo Cluster is the nearest cluster of galaxies, contains over 2,000 galaxies, and has a noticeable gravitational pull on the galaxies of the Local Group of Galaxies surrounding our Milky Way Galaxy. The cluster contains not only galaxies filled with stars but also gas so hot it glows in X-rays. Motions of galaxies in and around clusters indicate that they contain more dark matter than any visible matter we can see. Pictured above, the heart of the Virgo Cluster includes bright Messier galaxies such as Markarian's Eyes on the upper left, M86 just to the upper right of center, M84 on the far right, as well as spiral galaxy NGC 4388 at the bottom right.

Thumbnail image of picture found for this day. APOD: 2015 March 27 - NGC 2403 in Camelopardalis
Explanation: Magnificent island universe NGC 2403 stands within the boundaries of the long-necked constellation Camelopardalis. Some 10 million light-years distant and about 50,000 light-years across, the spiral galaxy also seems to have more than its fair share of giant star forming HII regions, marked by the telltale reddish glow of atomic hydrogen gas. The giant HII regions are energized by clusters of hot, massive stars that explode as bright supernovae at the end of their short and furious lives. A member of the M81 group of galaxies, NGC 2403 closely resembles another galaxy with an abundance of star forming regions that lies within our own local galaxy group, M33 the Triangulum Galaxy. Spiky in appearance, bright stars in this colorful galaxy portrait of NGC 2403 lie in the foreground, within our own Milky Way.

Thumbnail image of picture found for this day. APOD: 2014 September 25 - NGC 206 and the Star Clouds of Andromeda
Explanation: The large stellar association cataloged as NGC 206 is nestled within the dusty arms of the neighboring Andromeda galaxy. Also known as M31, the spiral galaxy is a mere 2.5 million light-years away. NGC 206 is near top center in this gorgeous close-up of the southwestern extent of Andromeda's disk, a remarkable composite of data from space and ground-based observatories. The bright, blue stars of NGC 206 indicate its youth. In fact, its youngest massive stars are less than 10 million years old. Much larger than the open or galactic clusters of young stars in the disk of our Milky Way galaxy, NGC 206 spans about 4,000 light-years. That's comparable in size to the giant stellar nurseries NGC 604 in nearby spiral M33 and the Tarantula Nebula, in the Large Magellanic Cloud. Star forming sites within Andromeda are revealed by the telltale reddish emission from clouds of ionized hydrogen gas.

Thumbnail image of picture found for this day. APOD: 2014 September 10 - Laniakea: Our Home Supercluster of Galaxies
Explanation: It is not only one of the largest structures known -- it is our home. The just-identified Laniakea Supercluster of galaxies contains thousands of galaxies that includes our Milky Way Galaxy, the Local Group of galaxies, and the entire nearby Virgo Cluster of Galaxies. The colossal supercluster is shown in the above computer-generated visualization, where green areas are rich with white-dot galaxies and white lines indicate motion towards the supercluster center. An outline of Laniakea is given in orange, while the blue dot shows our location. Outside the orange line, galaxies flow into other galactic concentrations. The Laniakea Supercluster spans about 500 million light years and contains about 100,000 times the mass of our Milky Way Galaxy. The discoverers of Laniakea gave it a name that means "immense heaven" in Hawaiian.

Thumbnail image of picture found for this day. APOD: 2014 July 30 - M31: The Andromeda Galaxy
Explanation: Andromeda is the nearest major galaxy to our own Milky Way Galaxy. Our Galaxy is thought to look much like Andromeda. Together these two galaxies dominate the Local Group of galaxies. The diffuse light from Andromeda is caused by the hundreds of billions of stars that compose it. The several distinct stars that surround Andromeda's image are actually stars in our Galaxy that are well in front of the background object. Andromeda is frequently referred to as M31 since it is the 31st object on Messier's list of diffuse sky objects. M31 is so distant it takes about two million years for light to reach us from there. Although visible without aid, the above image of M31 was taken with a standard camera through a small telescope. Much about M31 remains unknown, including how it acquired its unusual double-peaked center.

Thumbnail image of picture found for this day. APOD: 2014 July 26 - NGC 253: Dusty Island Universe
Explanation: Shiny NGC 253 is one of the brightest spiral galaxies visible, and also one of the dustiest. Some call it the Silver Dollar Galaxy for its appearance in small telescopes, or just the Sculptor Galaxy for its location within the boundaries of the southern constellation Sculptor. First swept up in 1783 by mathematician and astronomer Caroline Herschel, the dusty island universe lies a mere 10 million light-years away. About 70 thousand light-years across, NGC 253 is the largest member of the Sculptor Group of Galaxies, the nearest to our own Local Group of Galaxies. In addition to its spiral dust lanes, tendrils of dust seem to be rising from a galactic disk laced with young star clusters and star forming regions in this sharp color image. The high dust content accompanies frantic star formation, earning NGC 253 the designation of a starburst galaxy. NGC 253 is also known to be a strong source of high-energy x-rays and gamma rays, likely due to massive black holes near the galaxy's center.

Thumbnail image of picture found for this day. APOD: 2014 June 15 - CMB Dipole: Speeding Through the Universe
Explanation: Our Earth is not at rest. The Earth moves around the Sun. The Sun orbits the center of the Milky Way Galaxy. The Milky Way Galaxy orbits in the Local Group of Galaxies. The Local Group falls toward the Virgo Cluster of Galaxies. But these speeds are less than the speed that all of these objects together move relative to the cosmic microwave background radiation (CMBR). In the above all-sky map from the COBE satellite, radiation in the Earth's direction of motion appears blueshifted and hence hotter, while radiation on the opposite side of the sky is redshifted and colder. The map indicates that the Local Group moves at about 600 kilometers per second relative to this primordial radiation. This high speed was initially unexpected and its magnitude is still unexplained. Why are we moving so fast? What is out there?

Thumbnail image of picture found for this day. APOD: 2013 December 26 - The Hydrogen Clouds of M33
Explanation: Gorgeous spiral galaxy M33 seems to have more than its fair share of glowing hydrogen gas. A prominent member of the local group of galaxies, M33 is also known as the Triangulum Galaxy and lies about 3 million light-years distant. Its inner 30,000 light-years are shown in this telescopic galaxy portrait that enhances the reddish ionized hydrogen clouds or HII regions. Sprawling along loose spiral arms that wind toward the core, M33's giant HII regions are some of the largest known stellar nurseries, sites of the formation of short-lived but very massive stars. Intense ultraviolet radiation from the luminous, massive stars ionizes the surrounding hydrogen gas and ultimately produces the characteristic red glow. To enhance this image, broadband data was used to produce a color view of the galaxy and combined with narrowband data recorded through a hydrogen-alpha filter, transmitting the light of the strongest hydrogen emission line. To see the monochromatic narrowband data alone, move your cursor over the image, or take this video tour of the hydrogen clouds of M33.

Thumbnail image of picture found for this day. APOD: 2013 September 26 - M31 versus M33
Explanation: Separated by about 14 degrees (28 Full Moons) in planet Earth's sky, spiral galaxies M31, left, and M33 are both large members of the Local Group, along with our own Milky Way galaxy. This wide-angle, telescopic mosaic captures colorful details of spiral structure in both, while the massive neighboring galaxies seem to be balanced either side of bright Mirach, beta star in the constellation Andromeda. But M31, the Andromeda Galaxy, is really 2.5 million light-years distant and M33, the Triangulum Galaxy, is also about 3 million light years away. Mirach, just 200 light-years from the Sun, lies well within the Milky Way, along with the dim clouds of dust drifting through the frame only a few hundred light-years above the galactic plane. Although they look far apart, M31 and M33 are locked in a mutual gravitational embrace. Radio astronomers have found indications of a bridge of neutral hydrogen gas that could connect the two, evidence of a closer encounter in the past. Based on measurements, gravitational simulations currently predict that the Milky Way, M31, and M33 will all undergo mutual close encounters and potentially mergers, billions of years in the future.

Thumbnail image of picture found for this day. APOD: 2013 July 18 - Hidden Galaxy IC 342
Explanation: Similar in size to large, bright spiral galaxies in our neighborhood, IC 342 is a mere 10 million light-years distant in the long-necked, northern constellation Camelopardalis. A sprawling island universe, IC 342 would otherwise be a prominent galaxy in our night sky, but it is hidden from clear view and only glimpsed through the veil of stars, gas and dust clouds along the plane of our own Milky Way galaxy. Even though IC 342's light is dimmed by intervening cosmic clouds, this deep telescopic image traces the galaxy's obscuring dust, blue star clusters, and glowing pink star forming regions along spiral arms that wind far from the galaxy's core. IC 342 may have undergone a recent burst of star formation activity and is close enough to have gravitationally influenced the evolution of the local group of galaxies and the Milky Way.

Thumbnail image of picture found for this day. APOD: 2013 June 26 - M31: The Andromeda Galaxy
Explanation: Andromeda is the nearest major galaxy to our own Milky Way Galaxy. Our Galaxy is thought to look much like Andromeda. Together these two galaxies dominate the Local Group of galaxies. The diffuse light from Andromeda is caused by the hundreds of billions of stars that compose it. The several distinct stars that surround Andromeda's image are actually stars in our Galaxy that are well in front of the background object. Andromeda is frequently referred to as M31 since it is the 31st object on Messier's list of diffuse sky objects. M31 is so distant it takes about two million years for light to reach us from there. Although visible without aid, the above image of M31 was taken with a small telescope. Much about M31 remains unknown, including how it acquired its unusual double-peaked center.

Thumbnail image of picture found for this day. APOD: 2013 June 10 - The Large Magellanic Cloud in Ultraviolet
Explanation: Where are the hottest stars in the nearest galaxies? To help find out, NASA commissioned its Earth-orbiting Swift satellite to compile a multi-image mosaic of the neighboring Large Magellanic Cloud (LMC) galaxy in ultraviolet light. The above image shows where recently formed stars occur in the LMC, as the most massive of these young stars shine brightly in blue and ultraviolet. In contrast, visible in an image roll-over, a more familiar view of the LMC in visible light better highlights older stars. On the upper left is one of the largest star forming regions known in the entire Local Group of galaxies: the Tarantula Nebula. The Large Magellanic Cloud and its smaller companion the Small Magellanic Cloud are easily visible with the unaided eye to sky enthusiasts with a view of the southern sky. Detailed inspection of the above image is allowing a better galaxy-comprehensive picture for how star formation occurs.

Thumbnail image of picture found for this day. APOD: 2013 March 28 - Unraveling NGC 3169
Explanation: Bright spiral galaxy NGC 3169 appears to be unraveling in this cosmic scene, played out some 70 million light-years away just below bright star Regulus toward the faint constellation Sextans. Its beautiful spiral arms are distorted into sweeping tidal tails as NGC 3169 (left) and neighboring NGC 3166 interact gravitationally, a common fate even for bright galaxies in the local universe. In fact, drawn out stellar arcs and plumes, indications of gravitational interactions, seem rampant in the deep and colorful galaxy group photo. The picture spans 20 arc minutes, or about 400,000 light-years at the group's estimated distance, and includes smaller, dimmer NGC 3165 at the right. NGC 3169 is also known to shine across the spectrum from radio to X-rays, harboring an active galactic nucleus that is likely the site of a supermassive black hole.

Thumbnail image of picture found for this day. APOD: 2013 February 11 - N11: Star Clouds of the LMC
Explanation: Massive stars, abrasive winds, mountains of dust, and energetic light sculpt one of the largest and most picturesque regions of star formation in the Local Group of Galaxies. Known as N11, the region is visible on the upper right of many images of its home galaxy, the Milky Way neighbor known as the Large Magellanic Clouds (LMC). The above image was taken for scientific purposes by the Hubble Space Telescope and reprocessed for artistry by an amateur to win the Hubble's Hidden Treasures competition. Although the section imaged above is known as NGC 1763, the entire N11 emission nebula is second in LMC size only to 30 Doradus. Studying the stars in N11 has shown that it actually houses three successive generations of star formation. Compact globules of dark dust housing emerging young stars are also visible around the image.

Thumbnail image of picture found for this day. APOD: 2013 February 8 - NGC 6822: Barnard's Galaxy
Explanation: Grand spiral galaxies often seem to get all the glory, flaunting their young, bright, blue star clusters in beautiful, symmetric spiral arms. But small galaxies form stars too, like nearby NGC 6822, also known as Barnard's Galaxy. Beyond the rich starfields in the constellation Sagittarius, NGC 6822 is a mere 1.5 million light-years away, a member of our Local Group of galaxies. About 7,000 light-years across, the dwarf irregular galaxy is seen to be filled with young blue stars and mottled with the telltale pinkish hydrogen glow of star forming regions in the deep color composite image. Contributing to the science of LITTLE THINGS, this portrait of a small galaxy was made as part of the Lowell Amateur Research Initiative (LARI), welcoming collaborations with amateur astronomers.

Thumbnail image of picture found for this day. APOD: 2012 December 20 - M33: Triangulum Galaxy
Explanation: The small, northern constellation Triangulum harbors this magnificent face-on spiral galaxy, M33. Its popular names include the Pinwheel Galaxy or just the Triangulum Galaxy. M33 is over 50,000 light-years in diameter, third largest in the Local Group of galaxies after the Andromeda Galaxy (M31), and our own Milky Way. About 3 million light-years from the Milky Way, M33 is itself thought to be a satellite of the Andromeda Galaxy and astronomers in these two galaxies would likely have spectacular views of each other's grand spiral star systems. As for the view from planet Earth, this sharp composite image, a 25 panel mosaic, nicely shows off M33's blue star clusters and pinkish star forming regions that trace the galaxy's loosely wound spiral arms. In fact, the cavernous NGC 604 is the brightest star forming region, seen here at about the 1 o'clock position from the galaxy center. Like M31, M33's population of well-measured variable stars have helped make this nearby spiral a cosmic yardstick for establishing the distance scale of the Universe.

Thumbnail image of picture found for this day. APOD: 2012 July 6 - The Tidal Tail of NGC 3628
Explanation: A mere 30 million light-years away, large spiral galaxy NGC 3628 (center left) shares its neighborhood in the local Universe with two other large spirals, in a magnificent grouping otherwise known as the Leo Triplet. In fact, fellow trio member M65 is near the center right edge of this deep cosmic group portrait, with M66 just above it and to the left. But, perhaps most intriguing is the spectacular tail stretching down for about 300,000 light-years from NGC 3628's warped, edge-on disk. Known as a tidal tail, the structure has been drawn out of the galaxy by gravitational tides during brief but violent past interactions with its large neighbors. Not often imaged so distinctly, the tidal tail is composed of young bluish star clusters and star-forming regions.

Thumbnail image of picture found for this day. APOD: 2012 June 4 - Milky Way Galaxy Doomed: Collision with Andromeda Pending
Explanation: Will our Milky Way Galaxy collide one day with its larger neighbor, the Andromeda Galaxy? Most likely, yes. Careful plotting of slight displacements of M31's stars relative to background galaxies on recent Hubble Space Telescope images indicate that the center of M31 could be on a direct collision course with the center of our home galaxy. Still, the errors in sideways velocity appear sufficiently large to admit a good chance that the central parts of the two galaxies will miss, slightly, but will become close enough for their outer halos to become gravitationally entangled. Once that happens, the two galaxies will become bound, dance around, and eventually merge to become one large elliptical galaxy -- over the next few billion years. Pictured above is an artist's illustration of the sky of a world in the distant future when the central parts of each galaxy begin to destroy each other. The exact future of our Milky Way and the entire surrounding Local Group of Galaxies is likely to remain an active topic of research for years to come.

Thumbnail image of picture found for this day. APOD: 2012 May 18 - GALEX: The Andromeda Galaxy
Explanation: A mere 2.5 million light-years away, the Andromeda Galaxy really is just next door as large galaxies go. So close, and spanning some 260,000 light-years, it took 11 different image fields from the Galaxy Evolution Explorer (GALEX) satellite's telescope to produce this gorgeous portrait of the spiral galaxy in ultraviolet light. While its spiral arms stand out in visible light images of Andromeda (also known as M31), the arms look more like rings in the GALEX ultraviolet view, dominated by hot, young, massive stars. As sites of intense star formation, the rings have been interpreted as evidence Andromeda collided with its smaller neighboring elliptical galaxy M32 more than 200 million years ago. The large Andromeda galaxy and our own Milky Way are the dominant members of the local galaxy group.

Thumbnail image of picture found for this day. APOD: 2012 May 16 - Star Formation in the Tarantula Nebula
Explanation: The largest, most violent star forming region known in the whole Local Group of galaxies lies in our neighboring galaxy the Large Magellanic Cloud (LMC). Were the Tarantula Nebula at the distance of the Orion Nebula -- a local star forming region -- it would take up fully half the sky. Also called 30 Doradus, the red and pink gas indicates a massive emission nebula, although supernova remnants and dark nebula also exist there. The bright knot of stars left of center is called R136 and contains many of the most massive, hottest, and brightest stars known. The above image is one of the largest mosaics ever created by observations of the Hubble Space Telescope and has revealed unprecedented details of this enigmatic star forming region. The image is being released to celebrate the 22nd anniversary of Hubble's launch.

Thumbnail image of picture found for this day. APOD: 2012 March 13 - The M81 Galaxy Group Through the Integrated Flux Nebula
Explanation: Large galaxies and faint nebulae highlight this deep image of the M81 Group of galaxies. First and foremost in the wide-angle 12-hour exposure is the grand design spiral galaxy M81, the largest galaxy visible in the image. M81 is gravitationally interacting with M82 just below it, a big galaxy with an unusual halo of filamentary red-glowing gas. Around the image many other galaxies from the M81 Group of galaxies can be seen, as well as a lucky satellite glint streaking across the image left. Together with other galaxy congregates including our Local Group of galaxies and the Virgo Cluster of galaxies, the M81 Group is part of the expansive Virgo Supercluster of Galaxies. This whole galaxy menagerie is seen through the faint glow of an Integrated Flux Nebula, a little studied complex of diffuse gas and dust clouds in our Milky Way Galaxy.

Thumbnail image of picture found for this day. APOD: 2012 January 10 - Bright Star Regulus near the Leo I Dwarf Galaxy
Explanation: The star near the top is so bright that it is sometimes hard to notice the galaxy toward the bottom. Pictured above, both the star, Regulus, and the galaxy, Leo I, can be found within one degree of each other toward the constellation of the Lion (Leo). Regulus is part of a multiple star system, with a close companion double star visible to the lower left of the young main sequence star. Leo I is a dwarf spheroidal galaxy in the Local Group of galaxies dominated by our Milky Way Galaxy and M31. Leo I is thought to be the most distant of the several known small satellite galaxies orbiting our Milky Way Galaxy. Regulus is located about 75 light years away, in contrast to Leo 1 which is located about 800,000 light years away.

Thumbnail image of picture found for this day. APOD: 2012 January 4 - Starburst Galaxy IC 10
Explanation: Lurking behind dust and stars near the plane of our Milky Way Galaxy, IC 10 is a mere 2.3 million light-years distant. Even though its light is dimmed by intervening dust, the irregular dwarf galaxy still shows off vigorous star-forming regions that shine with a telltale reddish glow in this colorful skyscape. In fact, also a member of the Local Group of galaxies, IC 10 is the closest known starburst galaxy. Compared to other Local Group galaxies, IC 10 has a large population of newly formed stars that are massive and intrinsically very bright, including a luminous X-ray binary star system thought to contain a black hole. Located within the boundaries of the northern constellation Cassiopeia, IC 10 is about 5,000 light-years across.

Thumbnail image of picture found for this day. APOD: 2011 December 20 - NGC 253: The Sculptor Galaxy
Explanation: NGC 253 is not only one of the brightest spiral galaxies visible, it is also one of the dustiest. Discovered in 1783 by Caroline Herschel in the constellation of Sculptor, NGC 253 lies only about ten million light-years distant. NGC 253 is the largest member of the Sculptor Group of Galaxies, the nearest group to our own Local Group of Galaxies. The dense dark dust accompanies a high star formation rate, giving NGC 253 the designation of starburst galaxy. Visible in the above photograph is the active central nucleus, also known to be a bright source of X-rays and gamma rays.

Thumbnail image of picture found for this day. APOD: 2011 July 23 - NGC 2403 in Camelopardalis
Explanation: Magnificent island universe NGC 2403 stands within the boundaries of the long-necked constellation Camelopardalis. Some 10 million light-years distant and about 50,000 light-years across, the spiral galaxy also seems to have more than its fair share of giant star forming HII regions, marked by the telltale reddish glow of atomic hydrogen gas. In fact, NGC 2403 closely resembles another galaxy with an abundance of star forming regions that lies within our own local galaxy group, M33 the Triangulum Galaxy. Of course, supernova explosions follow close on the heels of the formation of massive, short-lived stars and in 2004 one of the brightest supernovae discovered in recent times was found in NGC 2403. Easy to confuse with a foreground star in our own Milky Way Galaxy, the powerful supernova is seen here as the spiky, bright "star" at the left edge of the field. This stunning cosmic portrait is a composite of space and ground-based image data from the Hubble Legacy Archive and the 8.2 meter Subaru Telescope at the summit of Mauna Kea, Hawaii.

Thumbnail image of picture found for this day. APOD: 2011 May 19 - NGC 253: Close Up
Explanation: This dusty island universe is one of the brightest spiral galaxies in planet Earth's sky. Seen nearly edge-on, NGC 253 is only 13 million light-years away, the largest member of the Sculptor Group of galaxies, neighbor to our own local galaxy group. The detailed close-up view is a five frame mosaic based on data assembled from the Hubble Legacy Archive. Beginning on the left near the galaxy's core, the sharp panorama follows dusty filaments, interstellar gas clouds, and even individual stars toward the galaxy's edge at the right. The magnificent vista spans nearly 50,000 light-years. The frame at the far right has been compressed slightly to bring into view an intriguing pair of background galaxies. Designated a starburst galaxy because of its frantic star forming activity, NGC 253 features tendrils of dust rising from a galactic disk laced with young star clusters and star forming regions. NGC 253 is also known to be a strong source of high-energy x-rays and gamma rays, likely due to massive black holes near the galaxy's center.

Thumbnail image of picture found for this day. APOD: 2011 April 22 - Virgo Cluster Galaxies
Explanation: Well over a thousand galaxies are known members of the Virgo Cluster, the closest large cluster of galaxies to our own local group. In fact, the galaxy cluster is difficult to appreciate all at once because it covers such a large area on the sky. Spanning about 5x3 degrees, this careful mosaic of telescopic images clearly records the central region of the Virgo Cluster through faint foreground dust clouds lingering above the plane of our own Milky Way galaxy. The cluster's dominant giant elliptical galaxy M87, is just below center in the frame. Above M87 is the famous interacting galaxy pair NGC 4438, also known as The Eyes. A closer examination of the image will reveal many Virgo cluster member galaxies as small fuzzy patches. Sliding your cursor over the image will label the larger galaxies using NGC catalog designations. Galaxies are also shown with Messier catalog numbers, including M84, M86, and prominent colorful spirals M88, M90, and M91. On average, Virgo Cluster galaxies are measured to be about 48 million light-years away. The Virgo Cluster distance has been used to give an important determination of the Hubble Constant and the scale of the Universe. (Editor's Note: Labels courtesy of Astrometry.net.)

Thumbnail image of picture found for this day. APOD: 2010 December 22 - Hidden Galaxy IC 342
Explanation: Similar in size to other large, bright spiral galaxies, IC 342 is a mere 7 million light-years distant in the long-necked, northern constellation Camelopardalis. A sprawling island universe, IC 342 would otherwise be a prominent galaxy in our night sky, but it is almost hidden from view behind the veil of stars, gas and dust clouds in the plane of our Milky Way galaxy. Even though IC 342's light is dimmed by intervening cosmic clouds, this remarkably sharp telescopic image traces the galaxy's own obscuring dust, blue star clusters, and glowing pink star forming regions along spiral arms that Wind far from the galaxy's core. IC 342 may have undergone a recent burst of star formation activity and is close enough to have gravitationally influenced the evolution of the local group of galaxies and the Milky Way.

Thumbnail image of picture found for this day. APOD: 2010 December 3 - M33: Triangulum Galaxy
Explanation: The small, northern constellation Triangulum harbors this magnificent face-on spiral galaxy, M33. Its popular names include the Pinwheel Galaxy or just the Triangulum Galaxy. M33 is over 50,000 light-years in diameter, third largest in the Local Group of galaxies after the Andromeda Galaxy (M31), and our own Milky Way. About 3 million light-years from the Milky Way, M33 is itself thought to be a satellite of the Andromeda Galaxy and astronomers in these two galaxies would likely have spectacular views of each other's grand spiral star systems. As for the view from planet Earth, this sharp, detailed image nicely shows off M33's blue star clusters and pinkish star forming regions that trace the galaxy's loosely wound spiral arms. In fact, the cavernous NGC 604 is the brightest star forming region, seen here at about the 4 o'clock position from the galaxy center. Like M31, M33's population of well-measured variable stars have helped make this nearby spiral a cosmic yardstick for establishing the distance scale of the Universe.

Thumbnail image of picture found for this day. APOD: 2010 July 6 - HCG 87: A Small Group of Galaxies
Explanation: Sometimes galaxies form groups. For example, our own Milky Way Galaxy is part of the Local Group of Galaxies. Small, compact groups, like Hickson Compact Group 87 (HCG 87) shown above, are interesting partly because they slowly self-destruct. Indeed, the galaxies of HCG 87 are gravitationally stretching each other during their 100-million year orbits around a common center. The pulling creates colliding gas that causes bright bursts of star formation and feeds matter into their active galaxy centers. HCG 87 is composed of a large edge-on spiral galaxy visible on the lower left, an elliptical galaxy visible on the lower right, and a spiral galaxy visible near the top. The small spiral near the center might be far in the distance. Several stars from our Galaxy are also visible in the foreground. The above picture was taken in 1999 July by the Hubble Space Telescope's Wide Field Planetary Camera 2. Studying groups like HCG 87 allows insight into how all galaxies form and evolve.

Thumbnail image of picture found for this day. APOD: 2010 May 18 - Tentacles of the Tarantula Nebula
Explanation: The largest, most violent star forming region known in the whole Local Group of galaxies lies in our neighboring galaxy the Large Magellanic Cloud (LMC). Were the Tarantula Nebula at the distance of the Orion Nebula -- a local star forming region -- it would take up fully half the sky. Also called 30 Doradus, the red and pink gas indicates a massive emission nebula, although supernova remnants and dark nebula also exist there. The bright knot of stars left of center is called R136 and contains many of the most massive, hottest, and brightest stars known. The above image taken with the European Southern Observatory's (ESO's) Wide Field Imager is one of the most detailed ever of this vast star forming region. A recent Hubble image of part of the nebula has uncovered a very massive star escaping from the region.

Thumbnail image of picture found for this day. APOD: 2010 May 3 - Spiral Galaxy NGC 3190 Almost Sideways
Explanation: Some spiral galaxies are seen almost sideways. NGC 3190, one such galaxy, is the largest member of the Hickson 44 Group, one of the nearer groups of galaxies to our own Local Group of galaxies. Pictured above, finely textured dust lanes surround the brightly glowing center of this picturesque spiral. Gravitational tidal interactions with other members of its group have likely caused the spiral arms of NGC 3190 to appear asymmetric around the center, while the galactic disk also appears warped. NGC 3190 spans about 75,000 light years across and is visible with a small telescope toward the constellation of the Lion (Leo).

Thumbnail image of picture found for this day. APOD: 2010 February 19 - WISE Infrared Andromeda
Explanation: This sharp, wide-field view features infrared light from the spiral Andromeda Galaxy (M31). Dust heated by Andromeda's young stars is shown in yellow and red, while its older population of stars appears as a bluish haze. The false-color skyscape is a mosaic of images from NASA's new Wide-field Infrared Survey Explorer (WISE) satellite. With over twice the diameter of our Milky Way, Andromeda is the largest galaxy in the local group. Andromeda's own satellite galaxies M110 (below) and M32 (above) are also included in the combined fields. Launched in December 2009, WISE began a six month long infrared survey of the entire sky on January 14. Expected to discover near-Earth asteroids as well as explore the distant universe, its sensitive infrared detectors are cooled by frozen hydrogen.

Thumbnail image of picture found for this day. APOD: 2009 November 21 - NGC 253: Dusty Island Universe
Explanation: Shiny NGC 253 Galaxy, is one of the brightest spiral galaxies visible, and also one of the dustiest. Some call it the Silver Dollar Galaxy for its appearance in small telescopes, or just the Sculptor Galaxy for its location within the boundaries of the southern constellation Sculptor. First swept up in 1783 by mathematician and astronomer Caroline Herschel, the dusty island universe lies a mere 10 million light-years away. About 70 thousand light-years across, NGC 253 is the largest member of the Sculptor Group of Galaxies, the nearest to our own Local Group of Galaxies. In addition to its spiral dust lanes, striking tendrils of dust seem to be rising from a galactic disk laced with young star clusters and star forming regions in this processed color image. The high dust content accompanies frantic star formation, giving NGC 253 the designation of a starburst galaxy. NGC 253 is also known to be a strong source of high-energy x-rays and gamma rays, likely due to massive black holes near the galaxy's center.

Thumbnail image of picture found for this day. APOD: 2009 October 17 - Bright Nebulae in M33
Explanation: Gorgeous spiral galaxy M33 seems to have more than its fair share of bright emission nebulae. In fact, narrow-band and broad-band image data are combined in this beautifully detailed composite to trace the reddish emission nebulae, star forming HII regions, sprawling along loose spiral arms that wind toward the galaxy's core. Historically of great interest to astronomers, M33's giant HII regions are some of the largest known stellar nurseries - sites of the formation of short-lived but very massive stars. Intense ultraviolet radiation from the luminous, massive stars ionizes the surrounding gas and ultimately produces the characteristic red glow. Spanning over 50,000 light-years and a prominent member of the local group of galaxies, M33 is also known as the Triangulum Galaxy. It lies about 3 million light-years distant.

Thumbnail image of picture found for this day. APOD: 2009 October 9 - Starburst Galaxy IC 10
Explanation: Lurking behind dust and stars near the plane of our Milky Way Galaxy, IC 10 is a mere 2.3 million light-years distant. Its light dimmed by the intervening dust, the irregular dwarf galaxy still shows off vigorous star-forming regions that shine with a telltale reddish glow in this colorful skyscape. In fact, also a member of the Local Group of galaxies, IC 10 is the closest known starburst galaxy. Compared to other Local Group galaxies, IC 10 has a large population of newly formed stars that are massive and intrinsically very bright, including a luminous X-ray binary star system thought to contain a black hole. Located within the boundaries of the northern constellation Cassiopeia, IC 10 is about 5,000 light-years across.

Thumbnail image of picture found for this day. APOD: 2009 September 19 - NGC 3621: Far Beyond the Local Group
Explanation: Far beyond the local group of galaxies lies NGC 3621, some 22 million light-years away. Found in the multi-headed southern constellation Hydra, the winding spiral arms of this gorgeous island universe are loaded with luminous young star clusters and dark dust lanes. Still, for earthbound astronomers NGC 3621 is not just another pretty face-on spiral galaxy. Some of its brighter stars have been used as standard candles to establish important estimates of extragalactic distances and the scale of the Universe. This beautiful image of NGC 3621 traces the loose spiral arms far from the galaxy's brighter central regions that span some 100,000 light-years. Spiky foreground stars in our own Milky Way Galaxy and even more distant background galaxies are scattered across the colorful skyscape.

Thumbnail image of picture found for this day. APOD: 2009 September 6 - CMBR Dipole: Speeding Through the Universe
Explanation: Our Earth is not at rest. The Earth moves around the Sun. The Sun orbits the center of the Milky Way Galaxy. The Milky Way Galaxy orbits in the Local Group of Galaxies. The Local Group falls toward the Virgo Cluster of Galaxies. But these speeds are less than the speed that all of these objects together move relative to the cosmic microwave background radiation (CMBR). In the above all-sky map from the COBE satellite, radiation in the Earth's direction of motion appears blueshifted and hence hotter, while radiation on the opposite side of the sky is redshifted and colder. The map indicates that the Local Group moves at about 600 kilometers per second relative to this primordial radiation. This high speed was initially unexpected and its magnitude is still unexplained. Why are we moving so fast? What is out there?

Thumbnail image of picture found for this day. APOD: 2009 June 9 - Markarian's Chain of Galaxies
Explanation: Across the heart of the Virgo Cluster of Galaxies lies a striking string of galaxies known as Markarian's Chain. The chain, pictured above, is highlighted on the upper right with two large but featureless lenticular galaxies, M84 and M86. Prominent to their lower left is a pair of interacting galaxies known as The Eyes. The home Virgo Cluster is the nearest cluster of galaxies, contains over 2000 galaxies, and has a noticeable gravitational pull on the galaxies of the Local Group of Galaxies surrounding our Milky Way Galaxy. The center of the Virgo Cluster is located about 70 million light years away toward the constellation of Virgo. At least seven galaxies in the chain appear to move coherently, although others appear to be superposed by chance.

Thumbnail image of picture found for this day. APOD: 2009 May 10 - M31: The Andromeda Galaxy
Explanation: Andromeda is the nearest major galaxy to our own Milky Way Galaxy. Our Galaxy is thought to look much like Andromeda. Together these two galaxies dominate the Local Group of galaxies. The diffuse light from Andromeda is caused by the hundreds of billions of stars that compose it. The several distinct stars that surround Andromeda's image are actually stars in our Galaxy that are well in front of the background object. Andromeda is frequently referred to as M31 since it is the 31st object on Messier's list of diffuse sky objects. M31 is so distant it takes about two million years for light to reach us from there. Although visible without aid, the above image of M31 is a digital mosaic of 20 frames taken with a small telescope. Much about M31 remains unknown, including how it acquired its unusual double-peaked center.

Thumbnail image of picture found for this day. APOD: 2009 February 5 - NGC 604: X-rays from a Giant Stellar Nursery
Explanation: Some 3 million light-years distant in nearby spiral galaxy M33, giant stellar nursery NGC 604 is about 1,300 light-years across, or nearly 100 times the size of the Orion Nebula. In fact, among the star forming regions within the Local Group of galaxies, NGC 604 is second in size only to 30 Doradus, also known as the Tarantula Nebula in the Large Magellanic Cloud. This space-age color composite of X-ray data (in blue hues) from the Chandra Observatory, and Hubble optical data shows that NGC 604's cavernous bubbles and cavities are filled with a hot, tenuous, X-ray emitting gas. Intriguingly, NGC 604 itself is divided by a wall of relatively cool gas. On the western (right) side of the nebula, measurements indicate that material is likely heated to X-ray temperatures by the energetic winds from a cluster of about 200 young, massive stars. On the eastern side the X-ray filled cavities seem to be older, suggesting supernova explosions from the end of massive star evolution contribute to their formation.

Thumbnail image of picture found for this day. APOD: 2008 October 2 - NGC 253 Close Up
Explanation: This dusty island universe is one of the brightest spiral galaxies in the sky. Seen nearly edge-on, NGC 253 lies a mere 13 million light-years away and is the largest member of the Sculptor Group of galaxies, neighbor to our own local galaxy group. The remarkably sharp, close-up view is based on data from the Hubble's Advanced Camera for Surveys (ACS). Beginning on the left near the galaxy's core, the panorama follows filaments of dust, interstellar gas, and even individual stars toward the galaxy's edge at the far right, a magnificent vista spanning nearly 50,000 light-years. The image data are part of ANGST, the ACS Nearby Galaxy Survey Treasury, a program to explore our cosmic backyard.

Thumbnail image of picture found for this day. APOD: 2008 September 13 - M33: Triangulum Galaxy
Explanation: The small, northern constellation Triangulum harbors this magnificent face-on spiral galaxy, M33. Its popular names include the Pinwheel Galaxy or just the Triangulum Galaxy. M33 is over 50,000 light-years in diameter, third largest in the Local Group of galaxies after the Andromeda Galaxy (M31), and our own Milky Way. About 3 million light-years from the Milky Way, M33 is itself thought to be a satellite of the Andromeda Galaxy and astronomers in these two galaxies would likely have spectacular views of each other's grand spiral star systems. As for the view from planet Earth, this sharp, detailed image nicely shows off M33's blue star clusters and pinkish star forming regions that trace the galaxy's loosely wound spiral arms. In fact, the cavernous NGC 604 is the brightest star forming region, seen here at about the 1 o'clock position from the galaxy center. Like M31, M33's population of well-measured variable stars have helped make this nearby spiral a cosmic yardstick for establishing the distance scale of the Universe.

Thumbnail image of picture found for this day. APOD: 2008 September 9 - M110: Satellite of the Andromeda Galaxy
Explanation: Our Milky Way Galaxy is not alone. It is part of a gathering of about 25 galaxies known as the Local Group. Members include the Great Andromeda Galaxy (M31), M32, M33, the Large Magellanic Cloud, the Small Magellanic Cloud, Dwingeloo 1, several small irregular galaxies, and many dwarf elliptical and dwarf spheroidal galaxies. Pictured on the lower right is one of the dwarf ellipticals: NGC 205. Like M32, NGC 205 is a companion to the large M31, and can sometimes be seen to the south of M31's center in photographs. The image shows NGC 205 to be unusual for an elliptical galaxy in that it contains at least two dust clouds (at 9 and 2 o'clock - they are visible but hard to spot) and signs of recent star formation. This galaxy is sometimes known as M110, although it was actually not part of Messier's original catalog.

Thumbnail image of picture found for this day. APOD: 2008 July 8 - In the Heart of the Virgo Cluster
Explanation: The Virgo Cluster of Galaxies is the closest cluster of galaxies to our Milky Way Galaxy. The Virgo Cluster is so close that it spans more than 5 degrees on the sky - about 10 times the angle made by a full Moon. With its heart lying about 70 million light years distant, the Virgo Cluster is the nearest cluster of galaxies, contains over 2,000 galaxies, and has a noticeable gravitational pull on the galaxies of the Local Group of Galaxies surrounding our Milky Way Galaxy. The cluster contains not only galaxies filled with stars but also gas so hot it glows in X-rays. Motions of galaxies in and around clusters indicate that they contain more dark matter than any visible matter we can see. Pictured above, the heart of the Virgo Cluster includes bright Messier galaxies such as Markarian's Eyes on the upper left, M86 just to the upper right of center, M84 on the far right, as well as spiral galaxy NGC 4388 at the bottom right.

Thumbnail image of picture found for this day. APOD: 2008 May 12 - The M81 Galaxy Group Through the Integrated Flux Nebula
Explanation: Large galaxies and faint nebulae highlight this deep image of the M81 Group of galaxies. First and foremost in the wide-angle 12-hour exposure is the grand design spiral galaxy M81, the largest galaxy visible in the image. M81 is gravitationally interacting with M82 just below it, a big galaxy with an unusual halo of filamentary red-glowing gas. Around the image many other galaxies from the M81 Group of galaxies can be seen. Together with other galaxy congregates including our Local Group of galaxies and the Virgo Cluster of galaxies, the M81 Group is part of the expansive Virgo Supercluster of Galaxies. This whole galaxy menagerie is seen through the faint glow of an Integrated Flux Nebula, a little studied complex of diffuse gas and dust clouds in our Milky Way Galaxy.

Thumbnail image of picture found for this day. APOD: 2008 March 9 - CMBR Dipole: Speeding Through the Universe
Explanation: Our Earth is not at rest. The Earth moves around the Sun. The Sun orbits the center of the Milky Way Galaxy. The Milky Way Galaxy orbits in the Local Group of Galaxies. The Local Group falls toward the Virgo Cluster of Galaxies. But these speeds are less than the speed that all of these objects together move relative to the cosmic microwave background radiation (CMBR). In the above all-sky map from the COBE satellite, radiation in the Earth's direction of motion appears blueshifted and hence hotter, while radiation on the opposite side of the sky is redshifted and colder. The map indicates that the Local Group moves at about 600 kilometers per second relative to this primordial radiation. This high speed was initially unexpected and its magnitude is still unexplained. Why are we moving so fast? What is out there?

Thumbnail image of picture found for this day. APOD: 2008 January 9 - Hidden Galaxy IC 342 from Kitt Peak
Explanation: Beautiful nearby spiral galaxy IC 342 could be more famous if it wasn't so hidden. A sprawling island universe, IC 342 would be a prominent galaxy in our night sky, but it is almost hidden from view behind the veil of stars, gas and dust clouds in the plane of our Milky Way galaxy. Similar in size to other large, bright spiral galaxies IC 342 is a mere 7 million light-years distant in the long-necked, northern constellation of the Giraffe (Camelopardalis). Even though IC 342's light is dimmed by intervening cosmic clouds, this remarkably sharp telescopic image traces the galaxy's own obscuring dust, blue star clusters, and glowing pink star forming regions along spiral arms that wind far from the galaxy's core. IC 342 may have undergone a recent burst of star formation activity and is close enough to have gravitationally influenced the evolution of the local group of galaxies and the Milky Way.

Thumbnail image of picture found for this day. APOD: 2007 August 22 - Tentacles of the Tarantula Nebula
Explanation: The largest, most violent star forming region known in the whole Local Group of galaxies lies in our neighboring galaxy the Large Magellanic Cloud (LMC). Were the Tarantula Nebula at the distance of the Orion Nebula -- a local star forming region -- it would take up fully half the sky. Also called 30 Doradus, the red and pink gas indicates a massive emission nebula, although supernova remnants and dark nebula also exist there. The bright knot of stars left of center is called R136 and contains many of the most massive, hottest, and brightest stars known. The above image taken with the European Southern Observatory's (ESO's) Wide Field Imager is one of the most detailed ever of this vast star forming region. ESO has made it possible to fly around and into this detailed image by clicking here.

Thumbnail image of picture found for this day. APOD: 2007 July 27 - The Tidal Tail of NGC 3628
Explanation: A mere 30 million light-years away, large spiral galaxy NGC 3628 (center) shares its neighborhood in the local Universe with two other large spirals, in a magnificent grouping otherwise known as the Leo Triplett. In fact, fellow trio member M65 is near the bottom edge of this deep cosmic group portrait, with M66, just above it and to the left. But, perhaps most intriguing is the spectacular tail stretching up and to the left for about 300,000 light-years from NGC 3628's warped, edge-on disk. Known as a tidal tail, the structure has been drawn out of the galaxy by gravitational tides during brief but violent past interactions with its large neighbors. Not often imaged so distinctly, the tidal tail is composed of young bluish star clusters and star-forming regions.

Thumbnail image of picture found for this day. APOD: 2007 March 19 - Galaxy Group Hickson 44
Explanation: Galaxies, like stars, frequently form groups. A group of galaxies is a system containing more than two galaxies but less than the tens or hundreds typically found in a cluster of galaxies. A most notable example is the Local Group of Galaxies, which houses over 30 galaxies including our Milky Way, Andromeda, and the Magellanic Clouds. Pictured above is nearby compact group Hickson 44. This group is located about 60 million light-years away toward the constellation of Leo. Also known as the NGC 3190 Group, Hickson 44 contains several bright spiral galaxies and one bright elliptical galaxy on the upper left. The bright source on the upper right is a foreground star. Many galaxies in Hickson 44 and other compact groups are either slowly merging or gravitationally pulling each other apart.

Thumbnail image of picture found for this day. APOD: 2006 November 26 - M31: The Andromeda Galaxy
Explanation: Andromeda is the nearest major galaxy to our own Milky Way Galaxy. Our Galaxy is thought to look much like Andromeda. Together these two galaxies dominate the Local Group of galaxies. The diffuse light from Andromeda is caused by the hundreds of billions of stars that compose it. The several distinct stars that surround Andromeda's image are actually stars in our Galaxy that are well in front of the background object. Andromeda is frequently referred to as M31 since it is the 31st object on Messier's list of diffuse sky objects. M31 is so distant it takes about two million years for light to reach us from there. Although visible without aid, the above image of M31 is a digital mosaic of 20 frames taken with a small telescope. Much about M31 remains unknown, including how the center acquired two nuclei.

Thumbnail image of picture found for this day. APOD: 2006 November 23 - Hydrogen in M33
Explanation: Gorgeous spiral galaxy M33 seems to have more than its fair share of hydrogen. Its inner 30,000 light-years are shown here in an image processed to fully reveal the reddish glow of ionized hydrogen regions (HII regions) sprawling along loose spiral arms that wind toward the galaxy's core. Historically of great interest to astronomers, M33's giant HII regions are some of the largest known stellar nurseries - sites of the formation of short-lived but very massive stars. Intense ultraviolet radiation from the luminous, massive stars ionizes the surrounding hydrogen gas and ultimately produces the characteristic red glow. A prominent member of the local group of galaxies, M33 is also known as the Triangulum Galaxy and lies about 3 million light-years distant.

Thumbnail image of picture found for this day. APOD: 2006 October 11 - Markarian's Chain of Galaxies
Explanation: Across the heart of the Virgo Cluster of Galaxies lies a striking string of galaxies known as Markarian's Chain. The chain, pictured above, is highlighted on the lower right with two large but featureless lenticular galaxies, M84 and M86, and connects through several large spiral to the upper left, including M88. The home Virgo Cluster is the nearest cluster of galaxies, contains over 2,000 galaxies, and has a noticeable gravitational pull on the galaxies of the Local Group of Galaxies surrounding our Milky Way Galaxy. The center of the Virgo Cluster is located about 70 million light years away toward the constellation of Virgo. At least seven galaxies in the chain appear to move coherently, although others appear to be superposed by chance. The above image is just a small part of a mosaic dubbed the Big Picture taken by the Samuel Oschin Telescope at Palomar Observatory, in California, USA. A mural of the Big Picture will be displayed at the newly renovated Griffith Observatory near Los Angeles, California.

Thumbnail image of picture found for this day. APOD: 2006 October 8 - CMBR Dipole: Speeding Through the Universe
Explanation: Our Earth is not at rest. The Earth moves around the Sun. The Sun orbits the center of the Milky Way Galaxy. The Milky Way Galaxy orbits in the Local Group of Galaxies. The Local Group falls toward the Virgo Cluster of Galaxies. But these speeds are less than the speed that all of these objects together move relative to the cosmic microwave background radiation (CMBR). In the above all-sky map from the COBE satellite, radiation in the Earth's direction of motion appears blueshifted and hence hotter, while radiation on the opposite side of the sky is redshifted and colder. The map indicates that the Local Group moves at about 600 kilometers per second relative to this primordial radiation. This high speed was initially unexpected and its magnitude is still unexplained. Why are we moving so fast? What is out there?

Thumbnail image of picture found for this day. APOD: 2006 October 5 - Hidden Galaxy IC 342
Explanation: Similar in size to other large, bright spiral galaxies IC 342 is a mere 7 million light-years distant in the long-necked, northern constellation Camelopardalis. A sprawling island universe, IC 342 would otherwise be a prominent galaxy in our night sky, but it is almost hidden from view behind the veil of stars, gas and dust clouds in the plane of our Milky Way galaxy. Even though IC 342's light is dimmed by intervening cosmic clouds, this remarkably sharp telescopic image traces the galaxy's own obscuring dust, blue star clusters, and glowing pink star forming regions along spiral arms that wind far from the galaxy's core. IC 342 may have undergone a recent burst of star formation activity and is close enough to have gravitationally influenced the evolution of the local group of galaxies and the Milky Way.

Thumbnail image of picture found for this day. APOD: 2006 September 14 - M33: Spiral Galaxy in Triangulum
Explanation: The small, northern constellation Triangulum harbors this magnificent face-on spiral galaxy, M33. Its popular names include the Pinwheel Galaxy or just the Triangulum Galaxy. M33 is over 50,000 light-years in diameter, third largest in the Local Group of galaxies after the Andromeda Galaxy (M31), and our own Milky Way. About 3 million light-years from the Milky Way, M33 is itself thought to be a satellite of the Andromeda Galaxy and astronomers in these two galaxies would likely have spectacular views of each other's grand spiral star systems. As for the view from planet Earth, this detailed, wide field image nicely shows off M33's blue star clusters and pinkish star forming regions which trace the galaxy's loosely wound spiral arms. In fact, the cavernous NGC 604 is the brightest star forming region, seen here at about the 1 o'clock position from the galaxy center. Like M31, M33's population of well-measured variable stars have helped make this nearby spiral a cosmic yardstick for establishing the distance scale of the Universe.

Thumbnail image of picture found for this day. APOD: 2006 September 8 - Messier 110
Explanation: This very sharp telescopic vista features the last object in the modern version of Charles Messier's catalog of bright clusters and nebulae - Messier 110. A dwarf elliptical galaxy, M110 (aka NGC 205) is actually a bright satellite of the large spiral galaxy Andromeda, making M110 a fellow member of the local group of galaxies. Seen through a foreground of nearby stars, M110 is about 15,000 light-years across. That makes it comparable in size to satellite galaxies of our own Milky Way, the Large and Small Magellanic Clouds. Though elliptical galaxies are normally thought to be lacking in gas and dust to form new stars, M110 is known to contain young stars, and faint dust clouds can easily be seen in this detailed image at about the 7 and 11 o'clock positions relative to the galaxy center.

Thumbnail image of picture found for this day. APOD: 2006 July 5 - Spiral Galaxy NGC 2403 from Subaru
Explanation: Sprawling spiral arms dotted with bright red emission nebulas highlight this new and detailed image of nearby spiral galaxy NGC 2403. Also visible in the photogenic spiral galaxy are blue open clusters, dark dust lanes, and a bright but relatively small central nucleus. NGC 2403 is located just beyond the Local Group of Galaxies, at a relatively close 10 million light years away toward the constellation of the Giraffe (Camelopardalis). NGC 2403 has a designated Hubble type of Sc. In 2004, NGC 2403 was home to one of the brightest supernovas of modern times. The above image, the highest resolution complete image of NGC 2403 ever completed, was taken by the Japan's 8.3-meter Subaru telescope located on Mauna Kea, Hawaii, USA.

Thumbnail image of picture found for this day. APOD: 2006 June 19 - Bright Star Regulus near the Leo 1 Dwarf Galaxy
Explanation: The star on the upper left is so bright it is sometimes hard to notice the galaxy on the lower right. Both the star, Regulus, and the galaxy, Leo I, can be found within one degree of each other toward the constellation of Leo. Regulus is part of a multiple star system, with a close companion double star visible to the upper right of the young main sequence star. Leo I is a dwarf spheroidal galaxy in the Local Group of galaxies dominated by our Milky Way Galaxy and M31. Leo I is thought to be the most distant of the several known small satellite galaxies orbiting our Milky Way Galaxy. Regulus is located about 75 light years away, in contrast to Leo 1 which is located about 800,000 light years away.

Thumbnail image of picture found for this day. APOD: 2006 April 21 - NGC 253: Dusty Island Universe
Explanation: Shiny NGC 253, sometimes called the Silver Dollar Galaxy, is one of the brightest spiral galaxies visible - and also one of the dustiest. First swept up in 1783 by mathematician and astronomer Caroline Herschel, the dusty island universe lies a mere 10 million light-years away in the southern constellation Sculptor. About 70 thousand light-years across, NGC 253 is the largest member of the Sculptor Group of Galaxies, the nearest to our own Local Group of Galaxies. In addition to its spiral dust lanes, striking tendrils of dust seem to be rising from the galactic disk in this gorgeous view. The high dust content accompanies frantic star formation, giving NGC 253 the designation of a starburst galaxy. NGC 253 is also known to be a strong source of high-energy x-rays and gamma rays, likely due to massive black holes near the galaxy's center.

Thumbnail image of picture found for this day. APOD: 2005 May 8 - CMBR Dipole: Speeding Through the Universe
Explanation: Our Earth is not at rest. The Earth moves around the Sun. The Sun orbits the center of the Milky Way Galaxy. The Milky Way Galaxy orbits in the Local Group of Galaxies. The Local Group falls toward the Virgo Cluster of Galaxies. But these speeds are less than the speed that all of these objects together move relative to the cosmic microwave background radiation (CMBR). In the above all-sky map, radiation in the Earth's direction of motion appears blueshifted and hence hotter, while radiation on the opposite side of the sky is redshifted and colder. The map indicates that the Local Group moves at about 600 kilometers per second relative to this primordial radiation. This high speed was initially unexpected and its magnitude is still unexplained. Why are we moving so fast? What is out there?

Thumbnail image of picture found for this day. APOD: 2005 March 16 - Markarian's Chain of Galaxies
Explanation: Across the heart of the Virgo Cluster of Galaxies lies a striking string of galaxies known as Markarian's Chain. The chain, pictured above, is highlighted on the upper right with two large but featureless lenticular galaxies, M84 and M86, and connects to the large spiral on the lower left, M88. Prominent on the lower right but not part of Markarian's Chain is the giant elliptical galaxy M87. The home Virgo Cluster is the nearest cluster of galaxies, contains over 2000 galaxies, and has a noticeable gravitational pull on the galaxies of the Local Group of Galaxies surrounding our Milky Way Galaxy. The center of the Virgo Cluster is located about 70 million light years away toward the constellation of Virgo. At least seven galaxies in the chain appear to move coherently, although others appear to be superposed by chance.

Thumbnail image of picture found for this day. APOD: 2004 December 14 - Nearby Spiral M33
Explanation: Spiral galaxy M33 is a mid-sized member of our Local Group of Galaxies. M33 is also called the Triangulum Galaxy for the constellation in which it resides. About four times smaller (in radius) than our Milky Way Galaxy and the Andromeda Galaxy (M31), it is much larger than the many of the local dwarf spheroidal galaxies. M33's proximity to M31 causes it to be thought by some to be a satellite galaxy of this more massive galaxy. M33's proximity to our Milky Way Galaxy causes it to appear more than twice the angular size of the Full Moon, and be visible with a good pair of binoculars.

Thumbnail image of picture found for this day. APOD: 2004 November 16 - Sagittarius Dwarf Irregular Galaxy
Explanation: How old is this galaxy? The nearby Local Group galaxy dubbed the Sagittarius Dwarf Irregular Galaxy (SagDIG) is not only very small but also has relatively few elements more massive than helium. Now the lack of heavy elements might mean that SagDIG is very young, so that component stars had little time to create and disperse massive elements. Conversely, SagDIG's diminutive size could indicate that it formed in the early universe, being a surviving building block of modern large galaxies. The above detailed image from the Hubble Space Telescope has now resolved enough stars to solve this mystery: SagDIG is ancient. Although SagDIG does have some groups of young stars, many stars are very old, and the galaxy as a whole helps astronomers to understand how the universe evolved, and show that at least one metal-poor galaxy is almost as old as the universe. Pictured above, SagDIG spans about 1,500 light years and lies about 3.5 million light years away toward the constellation of Sagittarius.

Thumbnail image of picture found for this day. APOD: 2004 November 10 - Leo A: Nearby Dwarf Irregular Galaxy
Explanation: Why isn't this small galaxy simple? The above image and contemporary observations of small nearby galaxy Leo A were supposed to show it has a simple structure. Now Leo A is known to be a dwarf irregular galaxy - one of the most common types of galaxies in the universe and a type that is likely a building block of more massive galaxy like our Milky Way Galaxy. In general, larger galaxies have recently been shown to continually eat, and be primarily composed of, many of the smaller satellite galaxies that have surrounded them. Leo A's surprising complexity indicates that that it, and possibly many small galaxies, have formation histories nearly as complex as large galaxies. Leo A spans about 10,000 light years and lies about 2.5 million light years away toward the constellation of Leo.

Thumbnail image of picture found for this day. APOD: 2004 July 18 - M31: The Andromeda Galaxy
Explanation: Andromeda is the nearest major galaxy to our own Milky Way Galaxy. Our Galaxy is thought to look much like Andromeda. Together these two galaxies dominate the Local Group of galaxies. The diffuse light from Andromeda is caused by the hundreds of billions of stars that compose it. The several distinct stars that surround Andromeda's image are actually stars in our Galaxy that are well in front of the background object. Andromeda is frequently referred to as M31 since it is the 31st object on Messier's list of diffuse sky objects. M31 is so distant it takes about two million years for light to reach us from there. Although visible without aid, the above image of M31 is a digital mosaic of 20 frames taken with a small telescope. Much about M31 remains unknown, including how the center acquired two nuclei.

Thumbnail image of picture found for this day. APOD: 2004 July 7 - N11B: Star Cloud of the LMC
Explanation: Massive stars, abrasive winds, mountains of dust, and energetic light sculpt one of the largest and most picturesque regions of star formation in the Local Group of Galaxies. Known as N11, the region is visible on the upper right of many images of its home galaxy, the Milky Way neighbor known as the Large Magellanic Clouds (LMC). The above image actually highlights N11B, part of the nebula that spans about 100 light years and is particularly active. The entire emission nebula N11 is second in LMC size only to 30 Doradus. Studying the stars in N11B has shown that it actually houses three successive generations of star formation. Compact globules of dark dust housing emerging young stars are also visible on the upper right.

Thumbnail image of picture found for this day. APOD: 2003 September 24 - M33: Spiral Galaxy in Triangulum
Explanation: The small constellation Triangulum in the northern sky harbors this magnificent face-on spiral galaxy, M33. Its popular names include the Pinwheel Galaxy or just the Triangulum Galaxy. M33's diameter spans over 50,000 light-years, making it third largest in the Local Group of galaxies after the Andromeda Galaxy (M31), and our own Milky Way. About 3 million light-years from the Milky Way, M33 lies very close to the Andromeda Galaxy and observers in these two galaxies would likely have spectacular views of each other's grand spiral star systems. As for the view from planet Earth, this sharp 27 frame mosaic of M33 nicely shows off blue star clusters and pinkish star forming regions which trace the galaxy's loosely wound spiral arms. In fact, the cavernous NGC 604 is the brightest star forming region seen here, visible along an arm arcing above and to the right of the galaxy center. Like M31, M33's population of well-measured variable stars have helped make this nearby spiral a cosmic yardstick for establishing the distance scale of the Universe.

Thumbnail image of picture found for this day. APOD: 2003 July 27 -The Aquarius Dwarf
Explanation: Our Milky Way Galaxy is not alone. It is part of a gathering of about 50 galaxies known as the Local Group. Members include the Great Andromeda Galaxy (M31), M32, M33, the Large Magellanic Cloud, the Small Magellanic Cloud, Dwingeloo 1, several small irregular galaxies, and many dwarf elliptical and dwarf spheroidal galaxies. Pictured above is the Aquarius Dwarf, a faint dwarf irregular galaxy over 3 million light years away. An earlier APOD erroneously identified the above image as the Sagittarius Dwarf.

Thumbnail image of picture found for this day. APOD: 2003 July 19 - NGC 3621: Far Beyond the Local Group
Explanation: Far beyond the local group of galaxies lies NGC 3621, some 22 million light-years away. Found in the serpentine southern constellation Hydra, the loose spiral arms of this gorgeous island universe are loaded with luminous young star clusters and dark dust lanes. Still, for earthbound astronomers NGC 3621 is not just another pretty face-on spiral galaxy. Some of its brighter stars have been used as standard candles to establish important estimates of extragalactic distances and the scale of the Universe. This color picture was constructed from astronomical image data recorded with the Very Large Telescope Antu, at Paranal Observatory in Chile. At the original resolution, individual, hot supergiant stars can be identified and studied across NGC 3621.

Thumbnail image of picture found for this day. APOD: 2003 May 25 - Spiral Galaxy NGC 253 Almost Sideways
Explanation: NGC 253 is a normal spiral galaxy seen here almost sideways. It is the largest member of the Sculptor Group of Galaxies, the nearest group to our own Local Group of Galaxies. NGC 253, pictured above, appears visually as one of the brightest spirals on the sky, and is easily visible in southern hemisphere with a good pair of binoculars. The type Sc galaxy is about 10 million light years distant. NGC 253 is considered a starburst galaxy because of high star formation rates and dense dust clouds in its nucleus. The energetic nuclear region is seen to glow in X-ray and gamma-ray light.

Thumbnail image of picture found for this day. APOD: 2003 March 16 - NGC 253: The Sculptor Galaxy
Explanation: NGC 253 is not only one of the brightest spiral galaxies visible, it is also one of the dustiest. Discovered in 1783 by Caroline Herschel in the constellation of Sculptor, NGC 253 lies only about ten million light-years distant. NGC 253 is the largest member of the Sculptor Group of Galaxies, the nearest group to our own Local Group of Galaxies. The dense dark dust accompanies a high star formation rate, giving NGC 253 the designation of starburst galaxy. Visible in the above photograph from the Hubble Space Telescope is the active central nucleus, also known to be a bright source of X-rays and gamma rays.

Thumbnail image of picture found for this day. APOD: 2003 February 9 - COBE Dipole: Speeding Through the Universe
Explanation: Our Earth is not at rest. The Earth moves around the Sun. The Sun orbits the center of the Milky Way Galaxy. The Milky Way Galaxy orbits in the Local Group of Galaxies. The Local Group falls toward the Virgo Cluster of Galaxies. But these speeds are less than the speed that all of these objects together move relative to the cosmic microwave background (CMB). In the above all-sky map, radiation in the Earth's direction of motion appears blueshifted and hence hotter, while radiation on the opposite side of the sky is redshifted and colder. The map indicates that the Local Group moves at about 600 kilometers per second relative to this primordial radiation. This high speed was initially unexpected and its magnitude is still unexplained. Why are we moving so fast? What is out there?

Thumbnail image of picture found for this day. APOD: 2002 December 2 - Nearby Spiral M33
Explanation: Spiral galaxy M33 is a mid-sized member of our Local Group of Galaxies. M33 is also called the Triangulum Galaxy for the constellation in which it resides. About four times smaller (in radius) than our Milky Way Galaxy and the Andromeda Galaxy (M31), it is much larger than the many of the local dwarf spheroidal galaxies. M33's proximity to M31 causes it to be thought by some to be a satellite galaxy of this more massive galaxy. M33's proximity to our Milky Way Galaxy causes it to appear more than twice the angular size of the Full Moon, and be visible with a good pair of binoculars. The above high-resolution image from the 0.90-m telescope at Kitt Peak National Observatory is a four-color composite.

Thumbnail image of picture found for this day. APOD: 2002 October 21 - M31: The Andromeda Galaxy
Explanation: Andromeda is the nearest major galaxy to our own Milky Way Galaxy. Our Galaxy is thought to look much like Andromeda. Together these two galaxies dominate the Local Group of galaxies. The diffuse light from Andromeda is caused by the hundreds of billions of stars that compose it. The several distinct stars that surround Andromeda's image are actually stars in our Galaxy that are well in front of the background object. Andromeda is frequently referred to as M31 since it is the 31st object on Messier's list of diffuse sky objects. M31 is so distant it takes about two million years for light to reach us from there. Although visible without aid, the above image of M31 is a digital mosaic of 20 frames taken with a small telescope. Much about M31 remains unknown, including how the center acquired two nuclei.

Thumbnail image of picture found for this day. APOD: 2002 June 5 - NGC 3621: Far Beyond the Local Group
Explanation: Far beyond the local group of galaxies lies NGC 3621, some 22 million light-years away. Found in the serpentine southern constellation Hydra, the loose spiral arms of this gorgeous island universe are loaded with luminous young star clusters and dark dust lanes. Still, for earthbound astronomers NGC 3621 is not just another pretty face-on spiral galaxy. Some of its brighter stars have been used as standard candles to establish important estimates of extragalactic distances and the scale of the Universe. This color picture was constructed from astronomical image data recorded with the Very Large Telescope Antu, at Paranal Observatory in Chile. At the original resolution, individual, hot supergiant stars can be identified and studied across NGC 3621.

Thumbnail image of picture found for this day. APOD: 2002 January 23 - Local Group Galaxy NGC 6822
Explanation: Nearby galaxy NGC 6822 is irregular in several ways. First, the galaxy's star distribution merits a formal classification of dwarf irregular, and from our vantage-point the small galaxy appears nearly rectangular. What strikes astronomers as more peculiar, however, is NGC 6822's unusually high abundance of HII regions, locales of ionized hydrogen that surround young stars. Large HII regions, also known as emission nebulas, are visible surrounding the small galaxy, particularly toward the upper right. Toward the lower left are bright stars that are loosely grouped into an arm. Pictured above, NGC 6822, also known as Barnard's Galaxy, is located only about 1.5 million light years away and so is a member of our Local Group of Galaxies. The galaxy, home to famous nebulas including Hubble V, is visible with a small telescope toward the constellation of Sagittarius.

Thumbnail image of picture found for this day. APOD: 2001 September 27 - Elements of Nearby Spiral M33
Explanation: Spiral galaxy M33 is a mid-sized member of our Local Group of Galaxies. M33 is also called the Triangulum Galaxy for the constellation in which it resides. About four times smaller (in radius) than our Milky Way Galaxy and the Andromeda Galaxy (M31), it is much larger than the many of the local dwarf spheroidal galaxies. M33's proximity to M31 causes it to be thought by some to be a satellite galaxy of this more massive galaxy. M33's proximity to our Milky Way Galaxy causes it to appear more than twice the angular size of the Full Moon, and be visible with a good pair of binoculars. The above high-resolution image highlights light emitted by hydrogen in red and oxygen in blue. It was taken to help separate stars from emission nebulae, and therefore help study how galaxies form stars.

Thumbnail image of picture found for this day. APOD: 2001 September 17 - Southwest Andromeda
Explanation: This new image composite of the southwest region of M31 from the Subaru Telescope shows many stars, nebulae, and star clusters never before resolved. An older population of stars near Andromeda's center causes the yellow hue visible on the upper right. Young blue stars stand out in the spiral arms on the lower left. Red emission nebula, blue open clusters of stars, and sweeping lanes of dark dust punctuate the swirling giant. Andromeda, at about 2.5 million light years distant, and our Milky Way are the largest galaxies in the Local Group of Galaxies. Understanding M31 helps astronomers to understand our own Milky Way Galaxy, since the two are so similar.

Thumbnail image of picture found for this day. APOD: 2001 August 4 - Neighboring Galaxy: The Large Magellanic Cloud
Explanation: The brightest galaxy visible from our own Milky Way Galaxy is the Large Magellanic Cloud (LMC). Visible predominantly from Earth's Southern Hemisphere, the LMC is the second closest galaxy, neighbor to the Small Magellanic Cloud, and one of eleven known dwarf galaxies that orbit our Milky Way Galaxy. The LMC is an irregular galaxy composed of a bar of older red stars, clouds of younger blue stars, and a bright red star forming region visible near the top of the above image called the Tarantula Nebula. The brightest supernova of modern times, SN1987A, occurred in the LMC.

Thumbnail image of picture found for this day. APOD: 2001 April 11 - Large Sunspot Group AR 9393
Explanation: The largest sunspot group of the past ten years crossed the surface of the Sun late last month and early this month. The group was designated Active Region 9393 as it was the 9393rd region identified since counting officially began in 1973. The number of active regions on the Sun is high recently because the Sun is reaching the maximum of its current 11-year cycle of magnetic activity. The above time-lapse sequence shows AR 9393 as it evolved from 27 March to April 2 to become over 10 times larger than our Earth. Just after the end of the movie, on April 2, AR 9393 unleashed the largest solar flare of the last 25 years. Luckily, the flare was not pointed toward the Earth, or flare particles might have damaged satellites or even caused local electrical blackouts. Yesterday morning, however, a less powerful flare was ejected from a different sunspot group (AR 9415) toward Earth that has already caused radio interference. This and solar activity from Monday should cause significant aurorae over the next two nights. Will the above sunspot group remain as its region rotates back into view in a few days, or will it break up on the far side of the Sun? Currently, no one knows for sure.

Thumbnail image of picture found for this day. APOD: 2001 January 28 - CMB Dipole: Speeding Through the Universe
Explanation: Our Earth is not at rest. The Earth moves around the Sun. The Sun orbits the center of the Milky Way Galaxy. The Milky Way Galaxy orbits in the Local Group of Galaxies. The Local Group falls toward the Virgo Cluster of Galaxies. But these speeds are less than the speed that all of these objects together move relative to the cosmic microwave background (CMB). In the above all-sky map, radiation in the Earth's direction of motion appears blueshifted and hence hotter, while radiation on the opposite side of the sky is redshifted and colder. The map indicates that the Local Group moves at about 600 kilometers per second relative to this primordial radiation. This high speed was initially unexpected and its magnitude is still unexplained. Why are we moving so fast? What is out there?

Thumbnail image of picture found for this day. APOD: 2001 January 26 - Galaxies Of The Virgo Cluster
Explanation: Well over a thousand galaxies are known members of the Virgo Cluster, the closest large cluster of galaxies to our own local group. The galaxy cluster is difficult to see all at once because it covers such a large area on the sky. Still, this excellent telescopic view records the region of the Virgo Cluster around its dominant giant elliptical galaxy M87. M87 can be seen as a fuzzy patch near the picture's bottom center. In fact, a close examination of the image will reveal that many of the "stars" are actually surrounded by a telltale fuzz, indicating that they are Virgo Cluster galaxies. How many galaxies can you pick out? Click on the image for an uncropped, labeled version which includes the NGC catalog numbers for most of the visible galaxies. On average, Virgo Cluster galaxies are measured to be about 48 million light-years away. The Virgo Cluster distance has been used to give an important determination of the Hubble Constant and the scale of the Universe.

Thumbnail image of picture found for this day. APOD: 2000 December 31 - The Millennium that Defined Earth
Explanation: When the second millennium began, people generally knew that the Earth was round, but few saw much of it beyond their local village. As the millennium progressed, humans mapped the continents, circumnavigated the globe, and determined the composition of the Earth. The Earth started as the center of everything, but became a planet placed in the Solar System, which became placed in a Galaxy, which became placed in the Local Group of Galaxies, which became placed in an expanse so vast we call it just the Universe. As millennium two ends people generally know what Earth looks like from afar, and how it is that all of humanity is confined to the surface of this fragile and watery globe.

Thumbnail image of picture found for this day. APOD: 2000 October 23 - Dwarf Elliptical Galaxy NGC 205 in the Local Group
Explanation: Our Milky Way Galaxy is not alone. It is part of a gathering of about 25 galaxies known as the Local Group. Members include the Great Andromeda Galaxy (M31), M32, M33, the Large Magellanic Cloud, the Small Magellanic Cloud, Dwingeloo 1, several small irregular galaxies, and many dwarf elliptical and dwarf spheroidal galaxies. Pictured on the lower left is one of the many dwarf ellipticals: NGC 205. Like M32, NGC 205 is a companion to the large M31, and can sometimes be seen to the south of M31's center in photographs. The above image shows NGC 205 to be unusual for an elliptical galaxy in that it contains at least two dust clouds (at 1 and 4 o'clock - they are visible but hard to spot) and signs of recent star formation. This galaxy is sometimes known as M110, although it was actually not part of Messier's original catalog.

Thumbnail image of picture found for this day. APOD: 2000 April 30 - The Small Cloud of Magellan (SMC)
Explanation: Almost unknown to casual observers in the northern hemisphere, the southern sky contains two diffuse wonders known as the Magellanic Clouds. The Magellanic Clouds are small irregular galaxies orbiting our own larger Milky Way spiral galaxy. The Small Magellanic Cloud (SMC), pictured here, is about 250,000 light years away and contains a preponderance of young, hot, blue stars indicating it has undergone a recent period of star formation. There is evidence that the SMC is not gravitationally bound to the LMC.

Thumbnail image of picture found for this day. APOD: 2000 February 22 - Neighboring Galaxy: The Large Magellanic Cloud
Explanation: The brightest galaxy visible from our own Milky Way Galaxy is the Large Magellanic Cloud (LMC). Visible predominantly from Earth's Southern Hemisphere, the LMC is the second closest galaxy, neighbor to the Small Magellanic Cloud, and one of eleven known dwarf galaxies that orbit our Milky Way Galaxy. The LMC is an irregular galaxy composed of a bar of older red stars, clouds of younger blue stars, and a bright red star forming region visible near the top of the above image called the Tarantula Nebula. The brightest supernova of modern times, SN1987A, occurred in the LMC.

Thumbnail image of picture found for this day. APOD: December 31, 1999 - The Millennium that Defined Earth
Explanation: When the second millennium began, people generally knew that the Earth was round, but few saw much of it beyond their local village. As the millennium progressed, humans mapped the continents, circumnavigated the globe, and determined the composition of the Earth. The Earth started as the center of everything, but became a planet placed in the Solar System, which became placed in a Galaxy, which became placed in the Local Group of Galaxies, which became placed in an expanse so vast we call it just the Universe. As millennium two ends people generally know what Earth looks like from afar, and how it is that all of humanity is confined to the surface of this fragile and watery globe.

Thumbnail image of picture found for this day. APOD: December 18, 1999 - Irregular Galaxy Sextans A
Explanation: Grand spiral galaxies often seem to get all the glory. Their newly formed, bright, blue star clusters found along beautiful, symmetric spiral arms are guaranteed to attract attention. But small irregular galaxies form stars too, like this lovely, gumdrop-shaped galaxy, Sextans A. A member of the local group of galaxies which includes the massive spirals Andromeda and our own Milky Way, Sextans A is about 10 million light years distant. The bright Milky Way foreground stars appear yellowish in this view. Beyond them lie the stars of Sextans A with tantalizing young blue clusters clearly visible.

Thumbnail image of picture found for this day. APOD: November 14, 1999 - M31: The Andromeda Galaxy
Explanation: Andromeda is the nearest major galaxy to our own Milky Way Galaxy. Our Galaxy is thought to look much like Andromeda. Together these two galaxies dominate the Local Group of galaxies. The diffuse light from Andromeda is caused by the hundreds of billions of stars that compose it. The several distinct stars that surround Andromeda's image are actually stars in our Galaxy that are well in front of the background object. Andromeda is frequently referred to as M31 since it is the 31st object on Messier's list of diffuse sky objects. M31 is so distant it takes about two million years for light to reach us from there. Much about M31 remains unknown, including why the center contains two nuclei.

Thumbnail image of picture found for this day. APOD: October 3, 1999 - Nearby Dwarf Galaxy Leo I
Explanation: Leo I is a dwarf spheroidal galaxy in the Local Group of galaxies dominated by our Milky Way Galaxy and M31. Leo I is thought to be the most distant of the eleven known small satellite galaxies orbiting our Milky Way Galaxy. Besides the LMC and the SMC, all Milky Way satellite galaxies are small, dim, dwarf spheroidals, including the closest galaxy - the Sagittarius Dwarf. Leo I is more distant than most of them, thought to be about 250 kpc away. Although very little star-forming gas is visible in Leo I, analysis of star ages shows that stars have formed as recently as a billion years ago.

Thumbnail image of picture found for this day. APOD: September 6, 1999 - HCG 87: A Small Group of Galaxies
Explanation: Sometimes galaxies form groups. For example, our own Milky Way Galaxy is part of the Local Group of Galaxies. Small, compact groups, like Hickson Compact Group 87 (HCG 87) shown above, are interesting partly because they slowly self-destruct. Indeed, the galaxies of HCG 87 are gravitationally stretching each other during their 100-million year orbits around a common center. The pulling creates colliding gas that causes bright bursts of star formation and feeds matter into their active galaxy centers. HCG 87 is composed of a large edge-on spiral galaxy visible on the lower left, an elliptical galaxy visible on the lower right, and a spiral galaxy visible near the top. The small spiral near the center might be far in the distance. Several stars from our Galaxy are also visible in the foreground. The above picture was taken in July by the Hubble Space Telescope's Wide Field Planetary Camera 2. Studying groups like HCG 87 allows insight into how all galaxies form and evolve.

Thumbnail image of picture found for this day. APOD: June 27, 1999 - COBE Dipole: Speeding Through the Universe
Explanation: Our Earth is not at rest. The Earth moves around the Sun. The Sun orbits the center of the Milky Way Galaxy. The Milky Way Galaxy orbits in the Local Group. The Local Group falls toward the Virgo Cluster of Galaxies. But these speeds are less than the speed that all of these objects together move relative to the microwave background. In the above all-sky map, radiation in the Earth's direction of motion appears blueshifted and hence hotter, while radiation on the opposite side of the sky is redshifted and colder. The map indicates that the Local Group moves at about 600 kilometers per second relative to this primordial radiation. This high speed was initially unexpected and its magnitude is still unexplained. Why are we moving so fast? What is out there?

Thumbnail image of picture found for this day. APOD: April 2, 1999 - Stars of NGC 206
Explanation: Nestled within the dusty arms of the large spiral galaxy Andromeda (M31), the star cluster NGC 206 is one of the largest star forming regions known in our local group of galaxies. The beautiful bright blue stars of NGC 206 betray its youth - but close, systematic studies of variable stars in and around NGC 206 will also accurately reveal its distance. Astronomers are searching for variable stars in NGC 206, particularly pulsating stars known as Cepheids and eclipsing binary star systems. Distances for these types of stars can be effectively determined by following the periodic changes in their brightness and spectra. About 3 million light-years away, an accurately known distance to NGC 206 and thus M31 is critical to the larger understanding of galaxy formation, galaxy evolution, and ultimately the distance scale of the Universe.

Thumbnail image of picture found for this day. APOD: February 17, 1999 - Hickson Compact Group 40
Explanation: Galaxies, like stars, frequently form groups. A group of galaxies is a system containing more than two galaxies but less than the tens or hundreds typically found in a cluster of galaxies. A most notable example is the Local Group of Galaxies, which houses over 30 galaxies including our Milky Way, Andromeda, and the Magellanic Clouds. Pictured above is nearby compact group Hickson 40. This group is located about 300 million light-years away toward the constellation of Hydra. Of the five prominent galaxies in Hickson 40, three are spirals, one is an elliptical and one is a lenticular. Many galaxies in compact groups are either slowly merging or gravitationally pulling each other apart.

Thumbnail image of picture found for this day. APOD: January 22, 1999 - Pegasus dSph: Little Galaxy of the Local Group
Explanation: The Pegasus dwarf spheroidal galaxy (Peg dSph) is a small, newly recognized member of the Local Group of Galaxies. Likely a satellite companion of the Local Group's dominant player, the large spiral Andromeda (M31), the Pegasus dwarf galaxy is almost hidden in the glare of relatively bright foreground stars in our own Milkyway. Still, this dramatic Keck telescope 3-color image reveals Peg dSph as a clump of fainter, bluer stars 2,000 or so light-years across. Excitement over discoveries of Peg dSph and other nearby dwarf galaxies reflects the fact that little galaxies may loom large in the process of galaxy evolution. They are thought to be the building blocks from which larger galaxies are constructed.

Thumbnail image of picture found for this day. APOD: December 9, 1998 - NGC 253: The Sculptor Galaxy
Explanation: NGC 253 is not only one of the brightest spiral galaxies visible, it is also one of the dustiest. Discovered in 1783 by Caroline Herschel in the constellation of Sculptor, NGC 253 lies only about ten million light-years distant. NGC 253 is the largest member of the Sculptor Group of Galaxies, the nearest group to our own Local Group of Galaxies. The dense dark dust accompanies a high star formation rate, giving NGC 253 the designation of starburst galaxy. Visible in the above photograph from the Hubble Space Telescope is the active central nucleus, also known to be a bright source of X-rays and gamma rays.

Thumbnail image of picture found for this day. APOD: December 1, 1998 - Cepheus 1: Nearby Galaxy Hiding
Explanation: Some galaxies are hard to find. Besides being hidden behind the dust and bright foreground stars of our Milky Way Galaxy, recently discovered Cepheus 1 was missed previously because it is so uniformly dim. In fact, the first indication that any galaxy was there at all came in the radio band from the Dwingeloo radio telescope in the Netherlands. Close inspections of optical photographs like the one above then revealed Cepheus 1 as the low-surface brightness galaxy in the center. Cepheus 1 turns out to be only about 20 million light years distant, and so is one of the few spiral galaxies that live close by. Low-surface brightness (LSB) galaxies like Cepheus 1 are still being studied, but are known to have relatively large separations between bright stars, and to be more commonly found away from other galaxies.

Thumbnail image of picture found for this day. APOD: November 3, 1998 - Sextans A: A Seemingly Square Galaxy
Explanation: What's bothering local galaxy Sextans A? A small dwarf irregular galaxy spanning 5 thousand light years across, Sextans A is located only 5 million light-years away. Named for its home constellation of Sextans, the "diamond in the rough" structure relates to an ancient unknown event. 100 million years ago, something mysterious started a new wave of star formation in Sextans A's center. Massive short-lived stars exploded in supernovae that caused more star formation and yet more supernovae, ultimately resulting in an expanding shell. Today, young blue stars highlight areas and shell edges high in current star formation, a shell that from our perspective appears roughly square. In the above picture, a bright orange star in our own Milky Way Galaxy appears superposed in the foreground.

Thumbnail image of picture found for this day. APOD: October 17, 1998 - A Giant Globular Cluster in M31
Explanation: This cluster of stars, known as G1, is the brightest globular cluster in the whole Local Group of galaxies. Also called Mayall II, it orbits the center of the largest nearby galaxy: M31. G1 contains over 300,000 stars and is almost as old as the entire universe. In fact, observations of this globular star cluster show it to be as old as the oldest of the roughly 250 known globular clusters in our own Milky Way Galaxy. Two bright foreground stars appear in this image of G1 taken with the orbiting Hubble Space Telescope in July of 1994. It shows detail in the distant cluster comparable to ground-based telescopic views of globular star clusters in our own Galaxy.

Thumbnail image of picture found for this day. APOD: July 21, 1998 - Nearby Spiral M33
Explanation: M33 is a prominent nearby spiral galaxy. Nicknamed the Triangulum, M33 is one of the larger members of the Local Group of Galaxies. Two massive spiral galaxies dominate the Local Group: M31 and our Milky Way Galaxy. M33 is the only other spiral galaxy known in the Local Group. At 3 million light-years, M33 is the second closest spiral galaxy. M33 is thought by some to be a satellite galaxy to massive M31. M33 is close enough to appear twice the angular size of the full moon, when viewed with binoculars. Globular clusters in M33's halo appear unusual and might be much younger than globular clusters in our Galaxy's halo.

Thumbnail image of picture found for this day. APOD: July 15, 1998 - Ghost Galaxy NGC 2915
Explanation: How do you find a nearly invisible galaxy? Pictured above is the blue compact dwarf (BCD) galaxy NGC 2915. In visible light, shown above in yellow, this galaxy appears to be a normal dwarf galaxy, as indicated by the yellow smudge in the image center. Yet when imaged in a very specific color, shown in blue, a whole spiral galaxy appears. This specific color is in the radio band and is preferentially emitted by neutral hydrogen atoms. Much about BCD galaxies remains mysterious, such as how the neutral hydrogen obtained its shape, what drives current star formation, and why there is so much dark matter. NGC 2915 is located at the relatively nearby distance of 15 million light-years - just outside our Local Group of Galaxies.

Thumbnail image of picture found for this day. APOD: February 16, 1998 - Sagittarius Dwarf to Collide with Milky Way
Explanation: Our Galaxy is being invaded. Recent observations indicate that in the next 100 million years, the Sagittarius Dwarf galaxy will move though the disk of our own Milky Way Galaxy yet again . The Sagittarius Dwarf (Sgr), shown as the extended irregular shape below the Galactic Center, is the closest of 9 known small dwarf spheroidal galaxies that orbit our Galaxy. Don't worry, our Galaxy is not in danger, but no such assurances are issued for the Sagittarius Dwarf: the intense gravitational tidal forces might pull it apart. Oddly, however, Sgr's orbit indicates that is has been through our Galaxy several times before, and survived! One possibility is that Sgr contains a great deal of low-density dark matter that hold it together gravitationally during these collisions.

Thumbnail image of picture found for this day. APOD: February 3, 1998 - A Magellanic Mural
Explanation: Two galaxies stand out to casual observers in Earth's Southern Hemisphere: the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). These irregular galaxies are two of the closest galaxies to our Milky Way Galaxy. Recent observations of the LMC (on the left) have determined that it is on a nearly circular orbit around our Galaxy, and have even helped in the determination of the composition of dark matter in our Galaxy. The above photograph spans 40 degrees. Visible on the lower left of the LMC is the Tarantula Nebula (in red). In the foreground to the right of the SMC is globular cluster 47 Tucanae, appearing here as a bright point of light.

Thumbnail image of picture found for this day. APOD: January 25, 1998 - The Small Cloud of Magellan (SMC)
Explanation: Almost unknown to casual observers in the northern hemisphere, the southern sky contains two diffuse wonders known as the Magellanic Clouds. The Magellanic Clouds are small irregular galaxies orbiting our own larger Milky Way spiral galaxy. The Small Magellanic Cloud (SMC), pictured here, is about 250,000 light years away and contains a preponderance of young, hot, blue stars indicating it has undergone a recent period of star formation. There is evidence that the SMC is not gravitationally bound to the LMC.

Thumbnail image of picture found for this day. APOD: January 24, 1998 - The Large Cloud Of Magellan (LMC)
Explanation: Ferdinand Magellan and his crew had plenty of time to study the southern sky during their famous voyage around the world. As a result, two fuzzy cloud-like objects, not visible to northern hemisphere dwellers, are now known as the Clouds of Magellan. These star clouds are small irregular galaxies, satellites of our larger Milky Way spiral galaxy. The Large Magellanic Cloud (LMC) pictured above is only about 180,000 light-years distant - the only known galaxy closer is the Sagittarius Dwarf. Both the LMC and the Small Magellanic Cloud (SMC) are joined to the Milky Way by a stream of cold hydrogen gas. An unusual effect called gravitational lensing has recently been detected in a few LMC stars, and there is hope this could tell us important information about the true composition of our universe.

Thumbnail image of picture found for this day. APOD: November 14, 1997 - Irregular Galaxy Sextans A
Explanation: Grand spiral galaxies often seem to get all the glory. Their newly formed, bright, blue star clusters found along beautiful, symmetric spiral arms are guaranteed to attract attention. But small irregular galaxies form stars too, like this lovely, gumdrop-shaped galaxy, Sextans A. A member of the local group of galaxies which includes the massive spirals Andromeda and our own Milky Way, Sextans A is about 10 million light years distant. The bright Milky Way foreground stars appear yellowish in this view. Beyond them lie the stars of Sextans A with tantalizing young blue clusters clearly visible.

Thumbnail image of picture found for this day. APOD: November 1, 1997 - M31: The Andromeda Galaxy
Explanation: Andromeda is the nearest major galaxy to our own Milky Way Galaxy. Our Galaxy is thought to look much like Andromeda. Together these two galaxies dominate the Local Group of galaxies. The diffuse light from Andromeda is caused by the hundreds of billions of stars that compose it. The several distinct stars that surround Andromeda's image are actually stars in our Galaxy that are well in front of the background object. Andromeda is frequently referred to as M31 since it is the 31st object on Messier's list of diffuse sky objects. M31 is so distant it takes about 2 million years for light to reach us from there. Much about M31 remains unknown, including why the center contains two nuclei.

Thumbnail image of picture found for this day. APOD: August 9, 1997 - The Hydra Cluster of Galaxies
Explanation: You are flying through space and come to ... the Hydra Cluster of Galaxies. Listed as Abell 1060, the Hydra Cluster contains well over 100 bright galaxies. Clusters of galaxies are the largest gravitationally-bound objects in the universe. All of the bright extended images in the above picture are galaxies in the Hydra Cluster with the exception of unrelated diffraction crosses centered on bright stars. Several proximate clusters and galaxy groups might together create an even larger entity - a supercluster - but these clumps of matter are not (yet) falling toward each other. In fact, the Hydra cluster is thought to be part of the Hydra-Centaurus Supercluster of galaxies. Similarly, our own Milky Way Galaxy is part of the Local Group of Galaxies which is part of the Virgo Supercluster of Galaxies.

Thumbnail image of picture found for this day. APOD: July 26, 1997 - M81 in True Color
Explanation: Here's is a spiral galaxy in true colors. Previously, M81 was shown in two colors only, but M81's real colors are just as dramatic. In the above picture, note how blue the spiral arms are - this indicates the presence of hot young stars and on-going star formation. Also note the yellow hue of the nucleus, indicating am ancient population of stars many billions of years old. M81 is actually a dominant member of a group of galaxies which includes M82 and several other galaxies. Unlike our Local Group of galaxies, large galaxies in the M81 group are actually colliding. It is possible that M81's interaction with M82 create the density waves which generate M81's spiral structure.

Thumbnail image of picture found for this day. APOD: April 23, 1997 - Antlia: A New Galactic Neighbor
Explanation: Meet Antlia, a newly discovered neighbor of our Milky Way Galaxy. Announced just last week, this faint member of our Local Group of Galaxies has only about one million stars. It is late in being noticed because it is so hard to see against a bright sky. This faint galaxy is seen here in the background - the bright stars visible are in our own galaxy. The Antlia Dwarf Galaxy is actually a dwarf spheroidal galaxy, similar to nine known dwarf spheroidals that orbit our Milky Way Galaxy. Antlia, named for the constellation in which it resides, is unusual because it does not hover near either of the two major galaxies of the Local Group: our Milky Way Galaxy and M31.

Thumbnail image of picture found for this day. APOD: March 8, 1997 - COBE Dipole: Speeding Through the Universe
Explanation: Our Earth is not at rest. The Earth moves around the Sun. The Sun orbits the center of the Milky Way Galaxy. The Milky Way Galaxy orbits in the Local Group. The Local Group falls toward the Virgo Cluster of Galaxies. But these speeds are less than the speed that all of these objects together move relative to the microwave background. In the above all-sky map, radiation in the Earth's direction of motion appears blueshifted and hence hotter, while radiation on the opposite side of the sky is redshifted and colder. The map indicates that the Local Group moves at about 600 kilometers per second relative to this primordial radiation. This high speed was initially unexpected and its magnitude is still unexplained. Why are we moving so fast? What is out there?

Thumbnail image of picture found for this day. APOD: November 2, 1996 - Spiral Galaxy NGC 253 Almost Sideways
Explanation: NGC 253 is a normal spiral galaxy seen here almost sideways. It is the largest member of the Sculptor Group of Galaxies, the nearest group to our own Local Group of Galaxies. NGC 253 appears visually as one of the brightest spirals on the sky, and is easily visible in southern hemisphere with a good pair of binoculars. The type "Sc" galaxy is about 10 million light years distant. NGC 253 is considered a "starburst" galaxy because of high star formation rates and dense dust clouds in its nucleus. The energetic nuclear region is seen to glow in X-ray and gamma-ray light.

Thumbnail image of picture found for this day. APOD: October 9, 1996 - M31: The Andromeda Galaxy
Explanation: Andromeda is the nearest major galaxy to our own Milky Way Galaxy. Our Galaxy is thought to look much like Andromeda. Together these two galaxies dominate the Local Group of galaxies. The diffuse light from Andromeda is caused by the hundreds of billions of stars that compose it. The several distinct stars that surround Andromeda's image are actually stars in our Galaxy that are well in front of the background object. Andromeda is frequently referred to as M31 since it is the 31st object on Messier's list of diffuse sky objects. M31 is so distant it takes about 2 million years for light to reach us from there. Much about M31 remains unknown, including why the center contains two nuclei.

Thumbnail image of picture found for this day. APOD: August 1, 1996 - The Hydra Cluster of Galaxies
Explanation: You are flying through space and come to ... the Hydra Cluster of Galaxies. Listed as Abell 1060, the Hydra Cluster contains well over 100 bright galaxies. Clusters of galaxies are the largest gravitationally-bound objects in the universe. All of the bright extended images in the above picture are galaxies in the Hydra Cluster with the exception of unrelated diffraction crosses centered on bright stars. Several proximate clusters and galaxy groups might together create an even larger entity - a supercluster - but these clumps of matter are not (yet) falling toward each other. In fact, the Hydra cluster is thought to be part of the Hydra-Centaurus Supercluster of galaxies. Similarly, our own Milky Way Galaxy is part of the Local Group of Galaxies which is part of the Virgo Supercluster of Galaxies.

Thumbnail image of picture found for this day. APOD: July 14, 1996 - M81 in True Color
Explanation: Here's what a spiral galaxy REALLY looks like. Yesterday, M81 was shown in two colors only, but here we see M81 at its most colorful. In the above picture, note how blue the spiral arms are - this indicates the presence of hot young stars and on-going star formation. Also note the yellow hue of the nucleus, possibly designating a population of older stars many billions of years old. M81 is actually a dominant member of a group of galaxies which includes M82 and several other galaxies. Unlike our Local Group of galaxies, large galaxies in the M81 group are actually colliding. It is possible that M81's interaction with M82 create the density waves which generate M81's spiral structure.

Thumbnail image of picture found for this day. APOD: July 8, 1996 - M33: The Triangulum Galaxy
Explanation: The spiral galaxy M33 is a mid-sized member of our Local Group of galaxies. M33 is also called the Triangulum Galaxy for the constellation in which it resides. About four times smaller (in radius) than our Milky Way Galaxy and the Andromeda Galaxy (M31), it is much larger than the many of the local dwarf spheroidal galaxies. M33's proximity to M31 causes it to be thought by some to be a satellite galaxy of this more massive galaxy. M33's proximity to our Milky Way galaxy causes it to appear more than twice the angular size of the full moon, and visible with a good pair of binoculars. In the above picture, visible light is shown in red and ultraviolet light superposed in blue. Stars in M33 are the most distant ever to be studied spectroscopically.

Thumbnail image of picture found for this day. APOD: May 23, 1996 - The Violent Star Cluster 30 Doradus
Explanation: The largest, most violent star forming region known in the whole Local Group of galaxies lies in our neighboring galaxy the LMC. Were 30 Doradus at the distance of the Orion Nebula -- a local star forming region -- it would take up fully half the sky. Also called the Tarantula Nebula, the red gas indicates a massive emission nebula, although supernova remnants and dark nebula also exist in 30 Doradus. The bright knot of stars just below center is called R136 and contains many of the most massive, hottest, and brightest stars known.

Thumbnail image of picture found for this day. APOD: May 19, 1996 - Nearby Dwarf Galaxy Leo I
Explanation: Leo I is a dwarf spheroidal galaxy in the Local Group of galaxies dominated by our Milky Way Galaxy and M31. Leo I is thought to be the most distant of the eleven known small satellite galaxies orbiting our Milky Way Galaxy. Besides the LMC and the SMC, all Milky Way satellite galaxies are small, dim, dwarf spheroidals, including the closest galaxy - the Sagittarius Dwarf. Leo I is most distant than most of them, thought to be about 250 kpc away. Analysis of stars in Leo I show it contains many stars only about 3 billion years old - much younger than in most galaxies.

Thumbnail image of picture found for this day. APOD: April 26, 1996 - A Giant Globular Cluster in M31
Explanation: G1, pictured above, is the brightest known globular cluster in the whole Local Group of galaxies. Also called Mayall II, it orbits the center of the largest nearby galaxy: M31. G1 contains over 300,000 stars and is almost as old as the entire universe. In fact, observations of this globular cluster show stars as old as the oldest of the roughly 250 known globular clusters in our own Milky Way Galaxy. This image was taken with the Hubble Space Telescope in July of 1994. It shows, for the first time, the same fine detail in a distant globular cluster as can be discerned from a ground-based telescope of a globular cluster in our own Galaxy.

Thumbnail image of picture found for this day. APOD: February 5, 1996 - COBE Dipole: Speeding Through the Universe
Explanation: Our Earth is not at rest. The Earth moves around the Sun. The Sun orbits the center of the Milky Way Galaxy. The Milky Way Galaxy orbits in the Local Group. The Local Group falls toward the Virgo Cluster of Galaxies. But these speeds are less than the speed that all of these objects together move relative to the microwave background. In the above all-sky map, radiation in the Earth's direction of motion appears blueshifted and hence hotter, while radiation on the opposite side of the sky is redshifted and colder. The map indicates that the Local Group moves at about 600 kilometers per second relative to this primordial radiation. This high speed was initially unexpected and its magnitude is still unexplained. Why are we moving so fast? What is out there?

Thumbnail image of picture found for this day. APOD: January 8, 1996 - Local Group Galaxy NGC 205
Explanation: The Milky Way Galaxy is not alone. It is part of a gathering of about 25 galaxies known as the Local Group. Members include the Great Andromeda Galaxy (M31), M32, M33, the Large Magellanic Clouds, the Small Magellanic Clouds, Dwingeloo 1, several small irregular galaxies, and many dwarf elliptical galaxies. Pictured is one of the many dwarf ellipticals: NGC 205. Like M32, NGC 205 is a companion to the large M31, and can sometimes be seen to the south of M31's center in photographs. The above image shows this galaxy to be unusual for an elliptical galaxy in that it contains at least two dust clouds (at 7 and 11 o'clock - they are visible but hard to spot) and signs of recent star formation. This galaxy is sometimes known as M110, although it was actually not part of Messier's original catalog.

Thumbnail image of picture found for this day. APOD: January 6, 1996 - Dwarf Elliptical Galaxy M32
Explanation: Being the largest galaxy around can sometimes make you popular. Pictured is M31's companion galaxy M32. M31, the Andromeda galaxy, is the largest galaxy in our Local Group of galaxies - even our tremendous Milky Way Galaxy is smaller. Little M32 is visible in most pictures of M31 - it is the small circular spot north of M31's center. M32 is a dwarf elliptical galaxy. Elliptical galaxies have little or no measurable gas or dust - they are composed completely of stars and typically appear more red than spiral galaxies. Elliptical galaxies do not have disks - they generally have oblong shapes and therefore show elliptical profiles on the sky.

Thumbnail image of picture found for this day. APOD: July 24, 1995 - M31: The Andromeda Galaxy
Explanation: Andromeda is the nearest major galaxy to our own Milky Way Galaxy. Our Galaxy is thought to look much like Andromeda. Together these two galaxies dominate the Local Group of galaxies. The diffuse light from Andromeda is caused by the hundreds of billions of stars that compose it. The several distinct stars that surround Andromeda's image are actually stars in our Galaxy that are well in front of the background object. Andromeda is frequently referred to as M31 since it is the 31st object on Messier's list of diffuse sky objects. M31 is so distant it takes about 2 million years for light to reach us from there.

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