Astronomy Picture of the Day Search Results for "irregular AND galaxy"

APOD: 2023 August 27 – Three Galaxies and a Comet
Explanation: Diffuse starlight and dark nebulae along the southern Milky Way arc over the horizon and sprawl diagonally through this gorgeous nightscape. The breath-taking mosaic spans a wide 100 degrees, with the rugged terrain of the Patagonia, Argentina region in the foreground. Along with the insider's view of our own galaxy, the image features our outside perspective on two irregular satellite galaxies - the Large and Small Magellanic Clouds. The scene also captures the broad tail and bright coma of Comet McNaught, the Great Comet of 2007.

APOD: 2023 March 7 – Deep Field: The Large Magellanic Cloud
Explanation: Is this a spiral galaxy? No. Actually, it is the Large Magellanic Cloud (LMC), the largest satellite galaxy of our own Milky Way Galaxy. The LMC is classified as a dwarf irregular galaxy because of its normally chaotic appearance. In this deep and wide exposure, however, the full extent of the LMC becomes visible. Surprisingly, during longer exposures, the LMC begins to resemble a barred spiral galaxy. The Large Magellanic Cloud lies only about 180,000 light-years distant towards the constellation of the Dolphinfish (Dorado). Spanning about 15,000 light-years, the LMC was the site of SN1987A, the brightest and closest supernova in modern times. Together with the Small Magellanic Cloud (SMC), the LMC can be seen in Earth's southern hemisphere with the unaided eye.

APOD: 2023 February 11 - Magellanic Clouds over Chile
Explanation: The two prominent clouds in this Chilean Atacama Desert skyscape captured on January 21 actually lie beyond our Milky Way galaxy. Known as the Large and the Small Magellanic Clouds they are so named for the 16th century Portuguese explorer Ferdinand Magellan, leader of the first circumnavigation of planet Earth. Famous jewels of southern hemisphere skies, they are the brightest satellite galaxies of the Milky Way. The larger cloud is some 160,000 light-years, and the smaller 210,000 light-years distant. While both are irregular dwarf galaxies in their own right, they exhibit central barred structures in the deep wide-angle view. Wide and deep exposures also reveal faint dusty galactic cirrus nebulae and the imprints of gravitational tidal interactions between the Large and Small Magellanic Clouds.

APOD: 2023 January 20 - Galaxy Wars: M81 and M82
Explanation: The two dominant galaxies near center are far far away, 12 million light-years distant toward the northern constellation of the Great Bear. On the right, with grand spiral arms and bright yellow core is spiral galaxy M81. Also known as Bode's galaxy, M81 spans some 100,000 light-years. On the left is cigar-shaped irregular galaxy M82. The pair have been locked in gravitational combat for a billion years. Gravity from each galaxy has profoundly affected the other during a series of cosmic close encounters. Their last go-round lasted about 100 million years and likely raised density waves rippling around M81, resulting in the richness of M81's spiral arms. M82 was left with violent star forming regions and colliding gas clouds so energetic that the galaxy glows in X-rays. In the next few billion years, their continuing gravitational encounters will result in a merger, and a single galaxy will remain. This extragalactic scenario also includes other members of the interacting M81 galaxy group with NGC 3077 below and right of the large spiral, and NGC 2976 at upper right in the frame. Captured under dark night skies in the Austrian Alps, the foreground of the wide-field image is filled with integrated flux nebulae. Those faint, dusty interstellar clouds reflect starlight above the plane of our own Milky Way galaxy.

APOD: 2022 May 5 - NGC 3521: Galaxy in a Bubble
Explanation: Gorgeous spiral galaxy NGC 3521 is a mere 35 million light-years away, toward the northern springtime constellation Leo. Relatively bright in planet Earth's sky, NGC 3521 is easily visible in small telescopes but often overlooked by amateur imagers in favor of other Leo spiral galaxies, like M66 and M65. It's hard to overlook in this colorful cosmic portrait though. Spanning some 50,000 light-years the galaxy sports characteristic patchy, irregular spiral arms laced with dust, pink star forming regions, and clusters of young, blue stars. This deep image also finds NGC 3521 embedded in fainter, gigantic, bubble-like shells. The shells are likely tidal debris, streams of stars torn from satellite galaxies that have undergone mergers with NGC 3521 in the distant past.

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.

APOD: 2021 April 2 - NGC 3521: Galaxy in a Bubble
Explanation: Gorgeous spiral galaxy NGC 3521 is a mere 35 million light-years away, toward the constellation Leo. Relatively bright in planet Earth's sky, NGC 3521 is easily visible in small telescopes but often overlooked by amateur imagers in favor of other Leo spiral galaxies, like M66 and M65. It's hard to overlook in this colorful cosmic portrait, though. Spanning some 50,000 light-years the galaxy sports characteristic patchy, irregular spiral arms laced with dust, pink star forming regions, and clusters of young, blue stars. Remarkably, this deep image also finds NGC 3521 embedded in gigantic bubble-like shells. The shells are likely tidal debris, streams of stars torn from satellite galaxies that have undergone mergers with NGC 3521 in the distant past.

APOD: 2021 March 12 - Messier 81
Explanation: One of the brightest galaxies in planet Earth's sky is similar in size to our Milky Way Galaxy: big, beautiful Messier 81. Also known as NGC 3031 or Bode's galaxy for its 18th century discoverer, this grand spiral can be found toward the northern constellation of Ursa Major, the Great Bear. The sharp, detailed telescopic view reveals M81's bright yellow nucleus, blue spiral arms, pinkish starforming regions, and sweeping cosmic dust lanes. Some dust lanes actually run through the galactic disk (left of center), contrary to other prominent spiral features though. The errant dust lanes may be the lingering result of a close encounter between M81 and the nearby galaxy M82 lurking outside of this frame. M81's faint, dwarf irregular satellite galaxy, Holmberg IX, can be seen just below the large spiral. Scrutiny of variable stars in M81 has yielded a well-determined distance for an external galaxy -- 11.8 million light-years.

APOD: 2021 January 5 - The Small Cloud of Magellan
Explanation: What is the Small Magellanic Cloud? It has turned out to be a galaxy. People who have wondered about this little fuzzy patch in the southern sky included Portuguese navigator Ferdinand Magellan and his crew, who had plenty of time to study the unfamiliar night sky of the south during the first circumnavigation of planet Earth in the early 1500s. As a result, two celestial wonders easily visible for southern hemisphere skygazers are now known in Western culture as the Clouds of Magellan. Within the past 100 years, research has shown that these cosmic clouds are dwarf irregular galaxies, satellites of our larger spiral Milky Way Galaxy. The Small Magellanic Cloud actually spans 15,000 light-years or so and contains several hundred million stars. About 210,000 light-years away in the constellation of the Tucan (Tucana), it is more distant than other known Milky Way satellite galaxies, including the Sagittarius Dwarf galaxy and the Large Magellanic Cloud. This sharp image also includes the foreground globular star cluster 47 Tucanae on the right.

APOD: 2020 December 1 - NGC 346: Star Forming Cluster in the SMC
Explanation: Are stars still forming in the Milky Way's satellite galaxies? Found among the Small Magellanic Cloud's (SMC's) clusters and nebulas, NGC 346 is a star forming region about 200 light-years across, pictured here in the center of a Hubble Space Telescope image. A satellite galaxy of the Milky Way, the Small Magellanic Cloud (SMC) is a wonder of the southern sky, a mere 210,000 light-years distant in the constellation of the Toucan (Tucana). Exploring NGC 346, astronomers have identified a population of embryonic stars strung along the dark, intersecting dust lanes visible here on the right. Still collapsing within their natal clouds, the stellar infants' light is reddened by the intervening dust. Toward the top of the frame is another star cluster with intrinsically older and redder stars. A small, irregular galaxy, the SMC itself represents a type of galaxy more common in the early Universe. These small galaxies, though, are thought to be building blocks for the larger galaxies present today.

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.

APOD: 2020 May 15 - Galaxy Wars: M81 and M82
Explanation: These two galaxies are far far away, 12 million light-years distant toward the northern constellation of the Great Bear. On the left, with grand spiral arms and bright yellow core is spiral galaxy M81, some 100,000 light-years across. On the right marked by red gas and dust clouds, is irregular galaxy M82. The pair have been locked in gravitational combat for a billion years. Gravity from each galaxy has profoundly affected the other during a series of cosmic close encounters. Their last go-round lasted about 100 million years and likely raised density waves rippling around M81, resulting in the richness of M81's spiral arms. M82 was left with violent star forming regions and colliding gas clouds so energetic the galaxy glows in X-rays. In the next few billion years, their continuing gravitational encounters will result in a merger, and a single galaxy will remain.

APOD: 2019 July 12 - Magellanic Galaxy NGC 55
Explanation: Irregular galaxy NGC 55 is thought to be similar to the Large Magellanic Cloud (LMC). But while the LMC is about 180,000 light-years away and a well-known satellite of our own Milky Way Galaxy, NGC 55 is more like 6 million light-years distant, a member of the Sculptor Galaxy Group. Classified as an irregular galaxy, in deep exposures the LMC itself resembles a barred disk galaxy. Spanning about 50,000 light-years, NGC 55 is seen nearly edge-on though, presenting a flattened, narrow profile in contrast with our face-on view of the LMC. Just as large star forming regions create emission nebulae in the LMC, NGC 55 is also seen to be producing new stars. This highly detailed galaxy portrait highlights a bright core crossed with dust clouds, telltale pinkish star forming regions, and young blue star clusters in NGC 55.

APOD: 2018 September 21 - Irregular Galaxy NGC 55
Explanation: Irregular galaxy NGC 55 is thought to be similar to the Large Magellanic Cloud (LMC). But while the LMC is about 180,000 light-years away and is a well known satellite of our own Milky Way Galaxy, NGC 55 is more like 6 million light-years distant and is a member of the Sculptor Galaxy Group. Classified as an irregular galaxy, in deep exposures the LMC itself resembles a barred disk galaxy. Spanning about 50,000 light-years, NGC 55 is seen nearly edge-on though, presenting a flattened, narrow profile in contrast with our face-on view of the LMC. Just as large star forming regions create emission nebulae in the LMC, NGC 55 is also seen to be producing new stars. This highly detailed galaxy portrait highlights a bright core crossed with dust clouds, telltale pinkish star forming regions, and young blue star clusters in NGC 55.

APOD: 2018 January 14 - Three Galaxies and a Comet
Explanation: Diffuse starlight and dark nebulae along the southern Milky Way arc over the horizon and sprawl diagonally through this gorgeous nightscape. The breath-taking mosaic spans a wide 100 degrees, with the rugged terrain of the Patagonia, Argentina region in the foreground. Along with the insider's view of our own galaxy, the image features our outside perspective on two irregular satellite galaxies - the Large and Small Magellanic Clouds. The scene also captures the broad tail and bright coma of Comet McNaught, the Great Comet of 2007.

APOD: 2017 July 14 - NGC 4449: Close up of a Small Galaxy
Explanation: Grand spiral galaxies often seem to get all the glory. Their young, blue star clusters and pink star forming regions along sweeping spiral arms are guaranteed to attract attention. But small irregular galaxies form stars too, like NGC 4449, about 12 million light-years distant. Less than 20,000 light-years across, the small island universe is similar in size, and often compared to our Milky Way's satellite galaxy, the Large Magellanic Cloud (LMC). This remarkable Hubble Space Telescope close-up of the well-studied galaxy was reprocessed to highlight the telltale reddish glow of hydrogen gas. The glow traces NGC 4449's widespread star forming regions, some even larger than those in the LMC, with enormous interstellar arcs and bubbles blown by short-lived, massive stars. NGC 4449 is a member of a group of galaxies found in the constellation Canes Venatici. It also holds the distinction of being the first dwarf galaxy with an identified tidal star stream.

APOD: 2016 February 21 - M82: Galaxy with a Supergalactic Wind
Explanation: What's lighting up the Cigar Galaxy? M82, as this irregular galaxy is also known, was stirred up by a recent pass near large spiral galaxy M81. This doesn't fully explain the source of the red-glowing outwardly expanding gas, however. Evidence indicates that this gas is being driven out by the combined emerging particle winds of many stars, together creating a galactic superwind. The featured photographic mosaic highlights a specific color of red light strongly emitted by ionized hydrogen gas, showing detailed filaments of this gas. The filaments extend for over 10,000 light years. The 12-million light-year distant Cigar Galaxy is the brightest galaxy in the sky in infrared light, and can be seen in visible light with a small telescope towards the constellation of the Great Bear (Ursa Major).

APOD: 2016 February 3 - Galaxy Wars: M81 versus M82
Explanation: In the lower left corner, surrounded by blue spiral arms, is spiral galaxy M81. In the upper right corner, marked by red gas and dust clouds, is irregular galaxy M82. This stunning vista shows these two mammoth galaxies locked in gravitational combat, as they have been for the past billion years. The gravity from each galaxy dramatically affects the other during each hundred million-year pass. Last go-round, M82's gravity likely raised density waves rippling around M81, resulting in the richness of M81's spiral arms. But M81 left M82 with violent star forming regions and colliding gas clouds so energetic the galaxy glows in X-rays. This big battle is seen from Earth through the faint glow of an Integrated Flux Nebula, a little studied complex of diffuse gas and dust clouds in our Milky Way Galaxy. In a few billion years only one galaxy will remain.

APOD: 2015 November 30 - In the Center of Spiral Galaxy NGC 3521
Explanation: This huge swirling mass of stars, gas, and dust occurs near the center of a nearby spiral galaxy. Gorgeous spiral NGC 3521 is a mere 35 million light-years distant, toward the constellation Leo. Spanning some 50,000 light-years, its central region is shown in this dramatic image, constructed from data from the Hubble Space Telescope. The close-up view highlights this galaxy's characteristic multiple, patchy, irregular spiral arms laced with dust and clusters of young, blue stars. In contrast, many other spirals exhibit grand, sweeping arms. A relatively bright galaxy in planet Earth's sky, NGC 3521 is easily visible in small telescopes, but often overlooked by amateur imagers in favor of other Leo spiral galaxies, like M65 and M66.

APOD: 2015 September 24 - LDN 988 and Friends
Explanation: Stars are forming in dark, dusty molecular cloud LDN 988. Seen near picture center some 2,000 light-years distant, LDN 988 and other nearby dark nebulae were cataloged by Beverly T. Lynds in 1962 using Palomar Observatory Sky Survey plates. Narrowband and near-infrared explorations of the dark nebula reveal energetic shocks and outflows light-years across associated with dozens of newborn stars. But in this sharp optical telescopic view, the irregular outlines of LDN 988 and friends look like dancing stick figures eclipsing the rich starfields of the constellation Cygnus. From dark sky sites the region can be identified by eye alone. It's part of the Great Rift of dark nebulae along the plane of the Milky Way galaxy known as the Northern Coalsack.

APOD: 2013 October 20 - Three Galaxies and a Comet
Explanation: Diffuse starlight and dark nebulae along the southern Milky Way arc over the horizon and sprawl diagonally through this gorgeous nightscape. The breath-taking mosaic spans a wide 100 degrees, with the rugged terrain of the Patagonia, Argentina region in the foreground. Along with the insider's view of our own galaxy, the image features our outside perspective on two irregular satellite galaxies - the Large and Small Magellanic Clouds. The scene also captures the broad tail and bright coma of Comet McNaught, the Great Comet of 2007. Currently, many sky enthusiasts are following the development of Comet ISON, a comet which might become the Great Comet of 2013.

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.

APOD: 2012 June 22 - IC 2574: Coddington's Nebula
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, irregular galaxies form stars too. In fact dwarf galaxy IC 2574 shows clear evidence of intense star forming activity in its telltale pinkish regions of glowing hydrogen gas. Just as in spiral galaxies, the turbulent star-forming regions in IC 2574 are churned by stellar winds and supernova explosions spewing material into the galaxy's interstellar medium and triggering further star formation. A mere 12 million light-years distant, IC 2574 is part of the M81 group of galaxies, seen toward the northern constellation Ursa Major. Also known as Coddington's Nebula, the lovely island universe is about 50,000 light-years across, discovered by American astronomer Edwin Coddington in 1898.

APOD: 2012 March 26 - M82: Galaxy with a Supergalactic Wind
Explanation: What's lighting up the Cigar Galaxy? M82, as this irregular galaxy is also known, was stirred up by a recent pass near large spiral galaxy M81. This doesn't fully explain the source of the red-glowing outwardly expanding gas, however. Recent evidence indicates that this gas is being driven out by the combined emerging particle winds of many stars, together creating a galactic superwind.. The above photographic mosaic highlights a specific color of red light strongly emitted by ionized hydrogen gas, showing detailed filaments of this gas. The filaments extend for over 10,000 light years. The 12-million light-year distant Cigar Galaxy is the brightest galaxy in the sky in infrared light, and can be seen in visible light with a small telescope towards the constellation of the Great Bear (Ursa Major).

APOD: 2012 January 27 - NGC 3239 and SN 2012A
Explanation: About 40,000 light-years across, pretty, irregular galaxy NGC 3239 lies near the center of this lovely field of galaxies in the galaxy rich constellation Leo. At a distance of only 25 million light-years it dominates the frame, sporting a peculiar arrangement of structures, young blue star clusters and star forming regions, suggesting that NGC 3239 (aka Arp 263) is the result of a galaxy merger. Appearing nearly on top of the pretty galaxy is a bright, spiky, foreground star, a nearby member of our own Milky Way galaxy almost directly along our line-of-sight to NGC 3239. Still, NGC 3239 is notable for hosting this year's first confirmed supernova, designated SN 2012A. It was discovered early this month by supernova hunters Bob Moore, Jack Newton, and Tim Puckett. Indicated in a cropped version of the wider image, SN 2012A is just below and right of the bright foreground star. Of course, based on the light-travel time to NGC 3239, the supernova explosion itself occurred 25 million years ago, triggered by the core collapse of a massive star.

APOD: 2012 January 26 - NGC 4449: Star Stream for a Dwarf Galaxy
Explanation: A mere 12.5 million light-years from Earth, irregular dwarf galaxy NGC 4449 lies within the confines of Canes Venatici, the constellation of the Hunting Dogs. About the size of our Milky Way's satellite galaxy the Large Magellanic Cloud, NGC 4449 is undergoing an intense episode of star formation, evidenced by its wealth of young blue star clusters, pinkish star forming regions, and obscuring dust clouds in this deep color portrait. It also holds the distinction of being the first dwarf galaxy with an identified tidal star stream, faintly seen at the lower right. Placing your cursor over the image reveals an inset of the stream resolved into red giant stars. The star stream represents the remains of a still smaller infalling satellite galaxy, disrupted by gravitational forces and destined to merge with NGC 4449. With relatively few stars, small galaxies are thought to possess extensive dark matter halos. But since dark matter interacts gravitationally, these observations offer a chance to examine the significant role of dark matter in galactic merger events. The interaction is likely responsible for NGC 4449's burst of star formation and offers a tantalizing insight into how even small galaxies are assembled over time.

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.

APOD: 2011 September 15 - NGC 3521: Galaxy in a Bubble
Explanation: Gorgeous spiral galaxy NGC 3521 is a mere 35 million light-years away, toward the constellation Leo. Relatively bright in planet Earth's sky, NGC 3521 is easily visible in small telescopes but often overlooked by amateur imagers in favor of other Leo spiral galaxies, like M66 and M65. It's hard to overlook in this colorful cosmic portrait, though. Spanning some 50,000 light-years the galaxy sports characteristic patchy, irregular spiral arms laced with dust, pink star forming regions, and clusters of young, blue stars. Remarkably, this deep image also finds NGC 3521 embedded in gigantic bubble-like shells. The shells are likely tidal debris, streams of stars torn from satellite galaxies that have undergone mergers with NGC 3521 in the distant past.

APOD: 2011 February 25 - NGC 4449: Close up of a Small Galaxy
Explanation: Grand spiral galaxies often seem to get all the glory. Their young, blue star clusters and pink star forming regions along sweeping spiral arms are guaranteed to attract attention. But small irregular galaxies form stars too, like NGC 4449, about 12 million light-years distant. Less than 20,000 light-years across, the small island universe is similar in size, and often compared to our Milky Way's satellite galaxy, the Large Magellanic Cloud (LMC). This remarkable Hubble Space Telescope close-up of the well-studied galaxy was reprocessed to highlight the telltale reddish glow of hydrogen gas. The glow traces NGC 4449's widespread star forming regions, some even larger than those in the LMC, with enormous interstellar arcs and bubbles blown by short-lived, massive stars. NGC 4449 is a member of a group of galaxies found in the constellation Canes Venatici. Interactions with the nearby galaxies are thought to have influenced star formation in NGC 4449.

APOD: 2011 January 13 - NGC 3521 Close Up
Explanation: Gorgeous spiral galaxy NGC 3521 is a mere 35 million light-years distant, toward the constellation Leo. Spanning some 50,000 light-years, its central region is shown in this dramatic image, constructed from data drawn from the Hubble Legacy Archive. The close-up view highlights this galaxy's characteristic multiple, patchy, irregular spiral arms laced with dust and clusters of young, blue stars. In contrast, many other spirals exhibit grand, sweeping arms. A relatively bright galaxy in planet Earth's sky, NGC 3521 is easily visible in small telescopes, but often overlooked by amateur imagers in favor of other Leo spiral galaxies, like M66 and M65.

APOD: 2010 December 19 - M82: Galaxy with a Supergalactic Wind
Explanation: What's lighting up the Cigar Galaxy? M82, as this irregular galaxy is also known, was stirred up by a recent pass near large spiral galaxy M81. This doesn't fully explain the source of the red-glowing outwardly expanding gas, however. Recent evidence indicates that this gas is being driven out by the combined emerging particle winds of many stars, together creating a galactic superwind.. The above photographic mosaic highlights a specific color of red light strongly emitted by ionized hydrogen gas, showing detailed filaments of this gas. The filaments extend for over 10,000 light years. The 12-million light-year distant Cigar Galaxy is the brightest galaxy in the sky in infrared light, and can be seen in visible light with a small telescope towards the constellation of the Great Bear (Ursa Major).

APOD: 2010 October 17 - NGC 346 in the Small Magellanic Cloud
Explanation: How and why are all these stars forming? Found among the Small Magellanic Cloud's (SMC's) clusters and nebulae NGC 346 is a star forming region about 200 light-years across, pictured above by the Hubble Space Telescope. A satellite galaxy of the Milky Way, the Small Magellanic Cloud (SMC) is a wonder of the southern sky, a mere 210,000 light-years distant in the constellation of the Toucan (Tucana). Exploring NGC 346, astronomers have identified a population of embryonic stars strung along the dark, intersecting dust lanes visible here on the right. Still collapsing within their natal clouds, the stellar infants' light is reddened by the intervening dust. A small, irregular galaxy, the SMC itself represents a type of galaxy more common in the early Universe. But these small galaxies are thought to be a building blocks for the larger galaxies present today. Within the SMC, stellar nurseries like NGC 346 are also thought to be similar to those found in the early Universe.

APOD: 2010 September 3 - The Small Cloud of Magellan
Explanation: Portuguese navigator Ferdinand Magellan and his crew had plenty of time to study the southern sky during the first circumnavigation of planet Earth. As a result, two celestial wonders easily visible for southern hemisphere skygazers are known as the Clouds of Magellan. These cosmic clouds are now understood to be dwarf irregular galaxies, satellites of our larger spiral Milky Way Galaxy. The Small Magellanic Cloud actually spans 15,000 light-years or so and contains several hundred million stars. About 210,000 light-years away in the constellation Tucana, it is more distant than other known Milky Way satellite galaxies, including the Canis Major and Sagittarius Dwarf galaxies and the Large Magellanic Cloud. This sharp image also includes two foreground globular star clusters NGC 362 (bottom right) and 47 Tucanae. Spectacular 47 Tucanae is a mere 13,000 light-years away and seen here to the left of the Small Magellanic Cloud.

APOD: 2010 April 29 - Virgo Cluster Galaxy NGC 4731
Explanation: Barred spiral galaxy NGC 4731 lies some 65 million light-years away. The lovely island universe resides in the large Virgo cluster of galaxies. Colors in this well-composed, cosmic portrait, highlight plentiful, young, bluish star clusters along the galaxy's sweeping spiral arms. Its broad arms are distorted by gravitational interaction with a fellow Virgo cluster member, giant elliptical galaxy NGC 4697. NGC 4697 is beyond this frame above and to the left, but a smaller irregular galaxy NGC 4731A can be seen near the bottom in impressive detail with its own young blue star clusters. Of course, the individual, colorful, spiky stars in the scene are much closer, within our own Milky Way galaxy. NGC 4731 itself is well over 100,000 light-years across.

APOD: 2010 March 24 - Galaxy Wars: M81 versus M82
Explanation: On the right, surrounded by blue spiral arms, is spiral galaxy M81. On the left, marked by red gas and dust clouds, is irregular galaxy M82. This stunning vista shows these two mammoth galaxies locked in gravitational combat, as they have been for the past billion years. The gravity from each galaxy dramatically affects the other during each hundred million-year pass. Last go-round, M82's gravity likely raised density waves rippling around M81, resulting in the richness of M81's spiral arms. But M81 left M82 with violent star forming regions and colliding gas clouds so energetic the galaxy glows in X-rays. In a few billion years only one galaxy will remain.

APOD: 2010 January 7 - The Tail of the Small Magellanic Cloud
Explanation: A satellite galaxy of our Milky Way, the Small Magellanic Cloud is wonder of the southern sky, named for 16th century Portuguese circumnavigator Ferdinand Magellan. Some 200,000 light-years distant in the constellation Tucana, the small irregular galaxy's stars, gas, and dust that lie along a bar and extended "wing", are familiar in images from optical telescopes. But the galaxy also has a tail. Explored in this false-color, infrared mosaic from the Spitzer Space Telescope, the tail extends to the right of the more familiar bar and wing. Likely stripped from the galaxy by gravitational tides, the tail contains mostly gas, dust, and newly formed stars. Two clusters of newly formed stars, warming their surrounding natal dust clouds, are seen in the tail as red spots.

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.

APOD: 2009 August 12 - Irregular Galaxy NGC 55
Explanation: Irregular galaxy NGC 55 is thought to be similar to the Large Magellanic Cloud (LMC). But while the LMC is about 180,000 light-years away and is a well known satellite of our own Milky Way Galaxy, NGC 55 is more like 6 million light-years distant and is a member of the Sculptor Galaxy Group. Classified as an irregular galaxy, in deep exposures the LMC itself resembles a barred disk galaxy. However, spanning about 50,000 light-years, NGC 55 is seen nearly edge-on, presenting a flattened, narrow profile in contrast with our face-on view of the LMC. Just as large star forming regions create emission nebulae in the LMC, NGC 55 is also seen to be producing new stars. This highly detailed galaxy portrait highlights a bright core crossed with dust clouds, telltale pinkish star forming regions, and young blue star clusters in NGC 55.

APOD: 2008 December 29 - NGC 1569: Starburst in a Dwarf Irregular 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, irregular galaxies form stars too. In fact, as pictured here, dwarf galaxy NGC 1569 is apparently undergoing a burst of star forming activity, thought to have begun over 25 million years ago. The resulting turbulent environment is fed by supernova explosions as the cosmic detonations spew out material and trigger further star formation. Two massive star clusters - youthful counterparts to globular star clusters in our own spiral Milky Way galaxy - are seen left of center in the gorgeous Hubble Space Telescope image. The above picture spans about 8,000 light-years across NGC 1569. A mere 11 million light-years distant, this relatively close starburst galaxy offers astronomers an excellent opportunity to study stellar populations in rapidly evolving galaxies. NGC 1569 lies in the long-necked constellation Camelopardalis.

APOD: 2008 October 10 - Irregular Galaxy NGC 55
Explanation: Irregular galaxy NGC 55 is thought to be similar to the Large Magellanic Cloud (LMC). But while the LMC is about 180,000 light-years away and is a well known satellite of our own Milky Way Galaxy, NGC 55 is more like 6 million light-years distant and is a member of the Sculptor Galaxy Group. Classified as an irregular galaxy, in deep exposures the LMC itself resembles a barred disk galaxy. However, spanning about 50,000 light-years, NGC 55 is seen nearly edge-on, presenting a flattened, narrow profile in contrast with our face-on view of the LMC. Just as large star forming regions create emission nebulae in the LMC, NGC 55 is also seen to be producing new stars. This gorgeous galaxy portrait highlights a bright core, telltale pinkish emission regions, and young blue star clusters in NGC 55.

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.

APOD: 2008 April 9 - A Large Magellanic Cloud Deep Field
Explanation: Is this a spiral galaxy? No. Actually, it is the Large Magellanic Cloud (LMC), the largest satellite galaxy of our own Milky Way Galaxy. The LMC is classified as a dwarf irregular galaxy because of its normally chaotic appearance. In this deep and wide exposure, however, the full extent of the LMC becomes visible. Surprisingly, during longer exposures, the LMC begins to resemble a barred spiral galaxy. The Large Magellanic Cloud lies only about 180,000 light-years distant towards the constellation of Dorado. Spanning about 15,000 light-years, the LMC was the site of SN1987A, the brightest and closest supernova in modern times. Together with the Small Magellanic Cloud (SMC), the LMC can be seen in Earth's southern hemisphere with the unaided eye.

APOD: 2008 March 25 - Galaxy Wars: M81 versus M82
Explanation: On the left, surrounded by blue spiral arms, is spiral galaxy M81. On the right marked by red gas and dust clouds, is irregular galaxy M82. This stunning vista shows these two mammoth galaxies locked in gravitational combat, as they have been for the past billion years. The gravity from each galaxy dramatically affects the other during each hundred million-year pass. Last go-round, M82's gravity likely raised density waves rippling around M81, resulting in the richness of M81's spiral arms. But M81 left M82 with violent star forming regions and colliding gas clouds so energetic the galaxy glows in X-rays. In a few billion years only one galaxy will remain.

APOD: 2007 October 1- The Small Cloud of Magellan
Explanation: Portuguese navigator Ferdinand Magellan and his crew had plenty of time to study the southern sky during the first circumnavigation of planet Earth. As a result, two celestial wonders easily visible for southern hemisphere skygazers are known as the Clouds of Magellan. These cosmic clouds are now understood to be dwarf irregular galaxies, satellites of our larger spiral Milky Way Galaxy. The Small Magellanic Cloud pictured above actually spans 15,000 light-years or so and contains several hundred million stars. About 210,000 light-years distant in the constellation Tucana, it is the fourth closest of the Milky Way's known satellite galaxies, after the Canis Major and Sagittarius Dwarf galaxies and the Large Magellanic Cloud. This gorgeous view also includes two foreground globular star clusters NGC 362 (bottom right) and 47 Tucanae. Spectacular 47 Tucanae is a mere 13,000 light-years away and seen here to the left of the Small Magellanic Cloud.

APOD: 2007 July 10 - NGC 4449: Close-Up of a Small Galaxy
Explanation: Grand spiral galaxies often seem to get all the glory. Their newly formed, bright, blue star clusters along beautiful, symmetric spiral arms are guaranteed to attract attention. But small irregular galaxies form stars too, like NGC 4449, located about 12 million light-years away. In fact, this sharp Hubble Space Telescope close-up of the well-studied galaxy clearly demonstrates that reddish star forming regions and young blue star clusters are widespread. Less than 20,000 light-years across, the small island universe is similar in size, and often compared to our Milky Way's satellite galaxy, the Large Magellanic Cloud. NGC 4449 is a member of a group of galaxies found in the constellation Canes Venatici.

APOD: 2007 May 15 - Bright Spiral Galaxy M81 in Ultraviolet from Galex
Explanation: Where are the hot stars in M81, one of the closest major spiral galaxies? To help find out, astronomers took a deep image in ultraviolet light of the sprawling spiral with the Earth-orbiting Galex telescope. Hot stars emit more ultraviolet than cool stars, and are frequently associated with young open clusters of stars and energetic star forming regions. Magnificent spiral galaxy M81, slightly smaller in size to our own Milky Way Galaxy, shows off its young stars in its winding spiral arms in the above image. Less than 100 million years old, the young stars are blue in the above false-color Galex image and seen to be well separated from the older yellowish stars of the galactic core. Visible above M81 is a satellite galaxy dubbed Holmberg IX. Studying the unexpectedly bright ultraviolet glow of this small irregular galaxy may help astronomers understand how the many satellites of our own Milky Way Galaxy developed. M81, visible through a small telescope, spans about 70,000 light years and lies about 12 million light years away toward the constellation of the Great Bear (Ursa Major).

APOD: 2007 May 3 - Small Galaxy NGC 4449
Explanation: Grand spiral galaxies often seem to get all the glory. Their newly formed, bright, blue star clusters along beautiful, symmetric spiral arms are guaranteed to attract attention. But small irregular galaxies form stars too, like NGC 4449, located about 12 million light-years away. The well-studied galaxy is similar in size, and often compared to our Milky Way's satellite galaxy, the Large Magellanic Cloud (LMC). This lovely color image shows NGC 4449's general bar shape, also characteristic of the LMC, with scattered young blue star clusters. Near the bottom is the pinkish glow of atomic hydrogen gas, the telltale tracer of massive star forming regions. NGC 4449 is a member of a group of galaxies found in the constellation Canes Venatici. In fact, interactions with nearby galaxies are thought to have influenced star formation in NGC 4449.

APOD: 2007 March 30 - Three Galaxies and a Comet
Explanation: Diffuse starlight and dark nebulae along the southern Milky Way arc over the horizon and sprawl diagonally through this gorgeous nightscape. The breath-taking mosaic spans a wide 100 degrees, with the rugged terrain of the Patagonia, Argentina region in the foreground. Along with the insider's view of our own galaxy, the image features our outside perspective on two irregular satellite galaxies - the Large and Small Magellanic Clouds. Recorded on January 28, the scene also captures the broad tail and bright coma of Comet McNaught, The Great Comet of 2007.

APOD: 2006 August 9 - Magellanic Morning
Explanation: This early morning skyscape recorded near Winton, Queensland, Australia, looks toward the southeast. Low clouds are seen in silhouette against the first hints of sunlight, while two famous cosmic clouds, the Clouds of Magellan, also hover in the brightening sky. The Small Magellanic Cloud (SMC, upper right), and the Large Magellanic Cloud (LMC) are prominent wonders of the southern sky, named for the 16th century Portuguese explorer Ferdinand Magellan. They are small, irregular galaxies in their own right, satellites of our much larger, spiral Milky Way galaxy. The SMC is about 210,000 light-years and the LMC about 180,000 light-years away. At lower left, bright star Canopus (Alpha Carinae), denizen of the Milky Way, is a mere 310 light-years distant.

APOD: 2006 May 10 - The Large Cloud of Magellan
Explanation: Portuguese navigator Fernando de Magellan and his crew had plenty of time to study the southern sky during the first circumnavigation of planet Earth. As a result, two fuzzy cloud-like objects easily visible for southern hemisphere skygazers are known as the Clouds of Magellan. Of course, these star clouds are now understood to be dwarf irregular galaxies, satellites of our larger spiral Milky Way galaxy. The Large Magellanic Cloud (LMC) pictured above is only about 180,000 light-years distant in the constellation Dorado. Spanning about 15,000 light-years or so, it is the most massive of the Milky Way's satellite galaxies and is the site of the closest supernova in modern times. The prominent red knot on the left is 30 Doradus, or the Tarantula Nebula, a giant star-forming region in the Large Magellanic Cloud.

APOD: 2006 April 25 - M82: Galaxy with a Supergalactic Wind
Explanation: What's lighting up the Cigar Galaxy? M82, as this irregular galaxy is also known, was stirred up by a recent pass near large spiral galaxy M81. This doesn't fully explain the source of the red-glowing outwardly expanding gas, however. Recent evidence indicates that this gas is being driven out by the combined emerging particle winds of many stars, together creating a galactic "superwind." The above photographic mosaic, released yesterday to commemorate the sixteenth anniversary of the Hubble Space Telescope, highlights a specific color of red light strongly emitted by ionized hydrogen gas, showing detailed filaments of this gas. The filaments extend for over 10,000 light years. The 12-million light-year distant Cigar Galaxy is the brightest galaxy in the sky in infrared light, and can be seen in visible light with a small telescope towards the constellation of Ursa Major.

APOD: 2005 November 22 - A Galactic Collision in Cluster Abell 1185
Explanation: What is a guitar doing in a cluster of galaxies? Colliding. Clusters of galaxies are sometimes packed so tight that the galaxies that compose them collide. A prominent example occurs on the left of the above image of the rich cluster of galaxies Abell 1185. There at least two galaxies, cataloged as Arp 105 and dubbed The Guitar for their familiar appearance, are pulling each other apart gravitationally. Most of Abell 1185's hundreds of galaxies are elliptical galaxies, although spiral, lenticular, and irregular galaxies are all clearly evident. Many of the spots on the above image are fully galaxies themselves containing billions of stars, but some spots are foreground stars in our own Milky Way Galaxy. Recent observations of Abell 1185 have found unusual globular clusters of stars that appear to belong only to the galaxy cluster and not to any individual galaxy. Abell 1185 spans about one million light years and lies 400 million light years distant.

APOD: 2005 June 18 - Visitors Galaxy Gallery
Explanation: A tantalizing assortment of island universes is assembled here. From top left to bottom right are the lovely but distant galaxies M61, NGC 4449, NGC 4725, NGC 5068, NGC 5247, and NGC 5775/5774. Most are spiral galaxies more or less like our own Milky Way. The color images reveal distinct pink patches marking the glowing hydrogen gas clouds in star forming regions along the graceful spiral arms. While Virgo cluster galaxy M61 is perhaps the most striking of these spirals, the interesting galaxy pair NGC 5775/5774 neatly contrasts the characteristic spiral edge-on and face-on appearance. The one exception to this parade of photogenic spiral galaxies is the small and relatively close irregular galaxy NGC 4449 (top middle). Similar to the Large Magellanic Cloud, companion galaxy to the Milky Way, NGC 4449 also sports young blue star clusters and pink star forming regions. All the galaxies in this gallery were imaged with a small (16 inch diameter) reflecting telescope and digital camera by public participants in the Kitt Peak National Observatory Visitor Center's Advanced Observing Program.

APOD: 2005 June 17 - The Small Cloud of Magellan
Explanation: Portuguese navigator Ferdinand Magellan and his crew had plenty of time to study the southern sky during the first circumnavigation of planet Earth. As a result, two celestial wonders easily visible for southern hemisphere skygazers are known as the Clouds of Magellan. These cosmic clouds are now understood to be dwarf irregular galaxies, satellites of our larger spiral Milky Way Galaxy. The Small Magellanic Cloud pictured above actually spans 15,000 light-years or so and contains several hundred million stars. About 210,000 light-years distant in the constellation Tucana, it is the fourth closest of the Milky Way's known satellite galaxies, after the Canis Major and Sagittarius Dwarf galaxies and the Large Magellanic Cloud. This gorgeous view also includes two foreground globular star clusters NGC 362 (top left) and 47 Tucanae. Spectacular 47 Tucanae is a mere 13,000 light-years away and seen here to the right of the Small Magellanic Cloud.

APOD: 2005 March 4 - NGC 1427A: Galaxy in Motion
Explanation: In this tantalizing image, young blue star clusters and pink star-forming regions abound in NGC 1427A, a galaxy in motion. The small irregular galaxy's swept back outline points toward the top of this picture from the Hubble Space Telescope - and that is indeed the direction NGC 1427A is moving as it travels toward the center of the Fornax cluster of galaxies, some 62 million light-years away. Over 20,000 light-years long and similar to the nearby Large Magellanic Cloud, NGC 1427A is speeding through the Fornax cluster's intergalactic gas at around 600 kilometers per second. The resulting pressure is giving the galaxy its arrowhead outline and triggering the beautiful but violent episodes of star formation. Still, it is understood that interactions with cluster gas and the other cluster galaxies during its headlong flight will ultimately disrupt galaxy NGC 1427A. Many unrelated background galaxies are visible in the sharp Hubble image, including a striking face-on spiral galaxy at the upper left.

APOD: 2005 February 13 - In the Center 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. It contains over 100 galaxies of many types - including spiral, elliptical, and irregular galaxies. The Virgo Cluster is so massive that it is noticeably pulling our Galaxy toward it. 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 center of the Virgo cluster might appear to some as a human face, and includes bright Messier galaxies M86 at the top, M84 on the far right, NGC 4388 at the bottom, and NGC 4387 in the middle.

APOD: 2005 January 18 - NGC 346 in the Small Magellanic Cloud
Explanation: A satellite galaxy of the Milky Way, the Small Magellanic Cloud (SMC) is a wonder of the southern sky, a mere 210,000 light-years distant in the constellation Tucana. Found among the SMC's clusters and nebulae NGC 346 is a star forming region about 200 light-years across, pictured above by the Hubble Space Telescope. Exploring NGC 346, astronomers have identified a population of embryonic stars strung along the dark, intersecting dust lanes visible here on the right. Still collapsing within their natal clouds, the stellar infants' light is reddened by the intervening dust. A small, irregular galaxy, the SMC itself represents a type of galaxy more common in the early Universe. But these small galaxies are thought to be a building blocks for the larger galaxies present today. Within the SMC, stellar nurseries like NGC 346 are also thought to be similar to those found in the early Universe.

APOD: 2004 December 30 - M81 and M82: GALEX Full Field
Explanation: Intriguing galaxy pair M81 and M82 shine in this full-field view from the orbiting GALEX observatory. GALEX - the Galaxy Evolution Explorer - scans the cosmos in ultraviolet light, a view that follows star formation and galaxy evolution through the Universe. Near the bottom, magnificent spiral galaxy M81, similar in size to our own Milky Way, shows off young stars in winding spiral arms. Less than 100 million years old, the young stars are blue in the false-color GALEX image and seen to be well separated from the older yellowish stars of the galactic core. But near the top, turbulent, irregular galaxy M82 shows the results of extreme rates of star birth and death. Supernovae, the death explosions of massive stars, contribute to a violent wind of material expelled from M82's central regions. The striking irregular and spiral galaxy pair are located only about 10 million light-years away in the northern constellation Ursa Major.

APOD: 2004 December 11 - M87's Energetic Jet
Explanation: An energetic jet from the core of giant elliptical galaxy M87 stretches outward for 5,000 light-years. This monstrous jet appears in the panels above to be a knotted and irregular structure, detected across the spectrum, from x-ray to optical to radio wavelengths. In all these bands, the observed emission is likely created as high energy electrons spiral along magnetic field lines, so called synchrotron radiation. But what powers this cosmic blowtorch? Ultimately, the jet is thought to be produced as matter near the center of M87 swirls toward a spinning, supermassive black hole. Strong electromagnetic forces are generated and eject material away from the black hole along the axis of rotation in a narrow jet. Galaxy M87 is about 50 million light-years away and reigns as the large central elliptical galaxy in the Virgo cluster.

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.

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.

APOD: 2004 September 2 - The Large Cloud of Magellan
Explanation: Portuguese navigator Fernando de Magellan and his crew had plenty of time to study the southern sky during the first circumnavigation of planet Earth. As a result, two fuzzy cloud-like objects easily visible for southern hemisphere skygazers are known as the Clouds of Magellan. Of course, these star clouds are now understood to be dwarf irregular galaxies, satellites of our larger spiral Milky Way galaxy. The Large Magellanic Cloud (LMC) pictured above is only about 180,000 light-years distant in the constellation Dorado. Spanning about 15,000 light-years or so, it is the most massive of the Milky Way's satellite galaxies and is the site of the closest supernova in modern times. The prominent red knot on the right is 30 Doradus, or the Tarantula Nebula, a giant star-forming region in the Large Magellanic Cloud.

APOD: 2004 February 5 - NGC 1569: Starburst in a Small 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, irregular galaxies form stars too. In fact, as pictured here, dwarf galaxy NGC 1569 is apparently undergoing a burst of star forming activity, thought to have begun over 25 million years ago. The resulting turbulent environment is fed by supernova explosions as the cosmic detonations spew out material and trigger further star formation. Two massive star clusters - youthful counterparts to globular star clusters in our own spiral Milky Way galaxy - are seen left of center in the gorgeous Hubble Space Telescope image. The picture spans about 1,500 light-years across NGC 1569. A mere 7 million light-years distant, this relatively close starburst galaxy offers astronomers an excellent opportunity to study stellar populations in rapidly evolving galaxies. NGC 1569 lies in the long-necked constellation Camelopardalis.

APOD: 2003 November 23 - A Superwind from the Cigar Galaxy
Explanation: What's lighting up the Cigar Galaxy? M82, as this irregular galaxy is also known, was stirred up by a recent pass near large spiral galaxy M81. This doesn't fully explain the source of the red-glowing outwardly expanding gas, however. Recent evidence indicates that this gas is being driven out by the combined emerging particle winds of many stars, together creating a galactic "superwind." The above recently released photograph from the new Subaru Telescope highlights the specific color of red light strongly emitted by ionized hydrogen gas, showing detailed filaments of this gas. The filaments extend for over 10,000 light years. The 12-million light-year distant Cigar Galaxy is the brightest galaxy in the sky in infrared light, and can be seen in visible light with a small telescope towards the constellation of Ursa Major.

APOD: 2003 August 4 - In the Center 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. It contains over 100 galaxies of many types - including spiral, elliptical, and irregular galaxies. The Virgo Cluster is so massive that it is noticeably pulling our Galaxy toward it. 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 center of the Virgo cluster might appear to some as a human face, and includes bright Messier galaxies M86 at the top, M84 on the far right, NGC 4388 at the bottom, and NGC 4387 in the middle.

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.

APOD: 2003 April 23 - The Stars of NGC 1705
Explanation: Some 2,000 light-years across, NGC 1705 is small as galaxies go, similar to our Milky Way's own satellite galaxies, the Magellanic Clouds. At a much larger distance of 17 million light-years, the stars of NGC 1705 are still easily resolved in this beautiful image constructed from data taken in 1999 and 2000 with the Hubble Space Telescope. Most of the younger, hot, blue stars in the galaxy are seen to be concentrated in a large central star cluster with the older, cooler, red stars more evenly distributed. Possibly 13 billion years old, NGC 1705 could well have been forming stars through out its lifetime while light from its most recent burst of star formation reached Earth only 30 million years ago. This gradually evolving dwarf irregular galaxy lacks organized structures like spiral arms and is thought to be a nearby analog to the first galaxies to form in the early Universe.

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.

APOD: 2001 December 25 - Star Forming Region Hubble V
Explanation: How did stars form in the early universe? Astronomers are gaining insight by studying NGC 6822, a nearby galaxy classified as irregular by modern standards but appearing more typical of galaxies billions of years ago. Inspection of NGC 6822 shows several bright star groups, including two dubbed Hubble-X and Hubble-V. Pictured above, the Hubble Space Telescope has resolved Hubble V into the energetic stars that are lighting up the surrounding gas. Each star in the central dense knot of Hubble V shines brighter than 100,000 Suns. The Hubble V gas cloud spans about 200 light years and lies about 1.5 million light-years away toward the constellation Sagittarius.

APOD: 2001 November 3 - Bright Stars, Dim Galaxy
Explanation: These two clusters of bright, newly formed stars surrounded by a glowing nebula lie 10 million light-years away in the dim, irregular galaxy cataloged as NGC 2366. The Hubble Space Telescope image shows that the youngest cluster, the bottom one at about 2 million years old, is still surrounded by the gas and dust cloud it condensed from, while powerful stellar winds from the stars in the older cluster at the top (4-5 million years old), have begun to clear away its central areas giving the entire nebula an apparent inverted hook shape. Compared to the sun, the stars in these clusters are massive and short lived. The brightest one, near the tip of the hook, is a rare Luminous Blue Variable with 30 to 60 times the mass of the sun - similar to the erruptive Eta Carinae in our own Milky Way. Stars this massive are extremely variable. A comparison with ground based images indicates that in three years this star's brightness increased by about 40 times making it currently the brightest star in this dim galaxy.

APOD: 2001 November 1 - M87's Energetic Jet
Explanation: An energetic jet from the core of giant elliptical galaxy M87 stretches outward for 5,000 light-years. This monstrous jet appears in the panels above to be a knotted and irregular structure, dectected across the spectrum, from x-ray to optical to radio wavelengths. In all these bands, the observed emission is likely created as high energy electrons spiral along magnetic field lines, so called synchrotron radiation. But what powers this cosmic blowtorch? Ultimately, the jet is thought to be produced as matter near the center of M87 swirls toward a spinning, supermassive black hole. Strong electromagnetic forces are generated and eject material away from the black hole along the axis of rotation in a narrow jet. Galaxy M87 is about 50 million light-years away and reigns as the large central elliptical galaxy in the Virgo cluster.

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.

APOD: 2001 April 27 - Visitors' Galaxy Gallery
Explanation: A tantalizing assortment of island universes is assembled here. From top left to bottom right are the lovely but distant galaxies M61, NGC 4449, NGC 4725, NGC 5068, NGC 5247, and NGC 5775/5774. Most are spiral galaxies more or less like our own Milky Way. The color images reveal distinct pink patches marking the glowing hydrogen gas clouds in star forming regions along the graceful spiral arms. While Virgo cluster galaxy M61 is perhaps the most striking of these spirals, the interesting galaxy pair NGC 5775/5774 neatly contrasts the characteristic spiral edge-on and face-on appearance. The one exception to this parade of photogenic spiral galaxies is the small and relatively close irregular galaxy NGC 4449 (top middle). Similar to the Large Magellanic Cloud, companion galaxy to the Milky Way, NGC 4449 also sports young blue star clusters and pink star forming regions. All the galaxies in this gallery were imaged with a small (16 inch diameter) reflecting telescope and digital camera by public participants in the Kitt Peak National Observatory Visitor Center's Advanced Observing Program.

APOD: 2001 March 24 - The UV SMC from UIT
Explanation: Translated from the "acronese" the title reads - The UltraViolet Small Magellanic Cloud from the Ultraviolet Imaging Telescope. FYI, the four ultraviolet images used in this mosaic of the nearby irregular galaxy known as the Small Magellanic Cloud were taken by the UIT instrument during the Astro-1 and Astro-2 shuttle missions in 1990 and 1995. Each separate image field is slightly wider than the apparent size of the full moon. These ultraviolet pictures, shown in false color, must be taken above the Earth's absorbing atmosphere. They highlight concentrations of hot, newly formed stars only a few millions of years old, and reveal the progress of recent star formation in the SMC.

APOD: 2000 November 3 - New Moons For Saturn
Explanation: Which planet has the most moons? For now, it's Saturn. Four newly discovered satellites bring the ringed planet's total to twenty-two, just edging out Uranus' twenty-one for the most known moons in the solar system. Of course, the newfound Saturnian satellites are not large and photogenic. The faint S/2000 S 1, the first discovered in the year 2000, is the tiny dot indicated at the lower right of this August 7th image made with the ESO 2.2 meter telescope at La Silla, Chile. (An eye-catching spiral galaxy at the upper left is in the very distant background!) Unlike Saturn's larger moons whose almost circular orbits lie near the planet's equatorial plane, all four newly discovered moons have irregular, skewed orbits drifting far from the planet. With sizes in the 10 to 50 kilometer range, they are are likely captured asteroids. The international team of astronomers involved in the discoveries hopes to get many observations of the tiny satellites allowing accurate orbital computations before Saturn is lost in the solar glare around March 2001. The team has also found several other irregular satellite candidates which are now being followed. Saturn's only previously known irregular satellite is Phoebe, discovered over 100 years ago by W. H. Pickering,

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.

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.

APOD: 2000 April 4 - A Superwind from the Cigar Galaxy
Explanation: What's lighting up the Cigar Galaxy? M82, as this irregular galaxy is also known, was stirred up by a recent pass near large spiral galaxy M81. This doesn't fully explain the source of the red-glowing outwardly expanding gas, however. Recent evidence indicates that this gas is being driven out by the combined emerging particle winds of many stars, together creating a galactic "superwind." The above recently released photograph from the new Subaru Telescope highlights the specific color of red light strongly emitted by ionized hydrogen gas, showing detailed filaments of this gas. The filaments extend for over 10,000 light years. The 12-million light-year distant Cigar Galaxy is the brightest galaxy in the sky in infrared light, and can be seen in visible light with a small telescope towards the constellation of Ursa Major.

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.

APOD: 2000 February 20 - The Virgo Cluster of Galaxies
Explanation: Pictured are several galaxies of the Virgo Cluster, the closest cluster of galaxies to our Milky Way Galaxy. The Virgo Cluster spans more than 5 degrees on the sky - about 10 times the angle made by a full Moon. It contains over 100 galaxies of many types - including spirals, ellipticals, and irregular galaxies. The Virgo Cluster is so massive that it is noticeably pulling our Galaxy toward it. 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. Notable bright galaxies in the Virgo Cluster include bright Messier objects such as M61, M87, M90, and M100.

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.

APOD: July 29, 1999 - Hydrogen Blob N88A in the Small Magellanic Cloud
Explanation: The bright blob of hydrogen gas cataloged as N88A is seen at the right. It measures a mere 3 light years across. Emerging from the cool, dusty interstellar medium in a nearby irregular galaxy known as the Small Magellanic Cloud, N88A hides hot young stars at its core. The false-color Hubble Space Telescope image was recorded in the characteristic "H-alpha" light emitted by hydrogen atoms as they are ionized by the young star's energetic ultraviolet light and then recombine. Other regions of ionized hydrogen (H II regions) which surround new born stars can be over a thousand light-years across but astronomers now recognize that these small ionized hydrogen blobs contain some of the most massive stars known.

APOD: February 10, 1999 - GRB 990123 Host Galaxy Imaged
Explanation: Do the powerful explosions known as gamma-ray bursts (GRBs) originate in galaxies? This subject took on new light yesterday with the release of a Hubble Space Telescope image of the sky surrounding GRB 990123. This burst was first detected only two weeks ago and cataloged as one of the most powerful GRBs ever. The optical transient (OT) counterpart to the GRB can be seen as the bright spot just below center. Once so bright it was briefly visible with just binoculars, this OT has since become four million times dimmer and continues to fade. Now, it can be seen easily with only a large telescope. The diffuse object above is of particular interest because it appears to be the host galaxy of GRB 990123. This distant galaxy seems to have a normal brightness but an irregular shape. This discovery increases the evidence that most OTs do occur in galaxies. Are all host galaxies this strange?

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.

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.

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.

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.

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.

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.

APOD: July 25, 1997 - Stellar Laboratories in the LMC
Explanation: Stars are evolving in the Large Magellanic Clouds (LMC). This is particularly evident in the above close-up of the LMC's edge, which appears about as large as the full moon. Visible only in the southern hemisphere, the LMC is one of the closest galaxies to our own Milky Way Galaxy at only 175,000 light years away. The LMC is one of only five galaxies visible to the unaided eye. The red bubbles are gas clouds dominated by dust and young stars which are continually forming. The green bubbles are clouds left over from relatively recent supernova explosions of massive stars. Astronomers are still unsure of many details in the tortured history of this irregular galaxy.