Astronomy Picture of the Day Search Results for ""open cluster""

APOD: 2023 May 15 – M16: Eagle Nebula Deep Field
Explanation: From afar, the whole thing looks like an eagle. A closer look at the Eagle Nebula, however, shows the bright region is actually a window into the center of a larger dark shell of dust. Through this window, a brightly-lit workshop appears where a whole open cluster of stars is being formed. In this cavity, tall pillars and round globules of dark dust and cold molecular gas remain where stars are still forming. Already visible are several young bright blue stars whose light and winds are burning away and pushing back the remaining filaments and walls of gas and dust. The Eagle emission nebula, tagged M16, lies about 6500 light years away, spans about 20 light-years, and is visible with binoculars toward the constellation of the Serpent (Serpens). This picture involved long and deep exposures and combined three specific emitted colors emitted by sulfur (colored as yellow), hydrogen (red), and oxygen (blue).

APOD: 2023 May 1 – Carina Nebula North
Explanation: The Great Carina Nebula is home to strange stars and iconic nebulas. Named for its home constellation, the huge star-forming region is larger and brighter than the Great Orion Nebula but less well known because it is so far south -- and because so much of humanity lives so far north. The featured image shows in great detail the northernmost part of the Carina Nebula. On the bottom left is the Gabriela Mistral Nebula consisting of an emission nebula of glowing gas (IC 2599) surrounding the small open cluster of stars (NGC 3324). Above the image center is the larger star cluster NGC 3293, while to its right is the emission nebula Loden 153. The most famous occupant of the Carina Nebula, however, is not shown. Off the image to the lower right is the bright, erratic, and doomed star known as Eta Carinae -- a star once one of the brightest stars in the sky and now predicted to explode in a supernova sometime in the next few million years.

APOD: 2023 January 3 – Kembles Cascade of Stars
Explanation: This line of stars is real. A little too faint to see with the unaided eye, Kemble’s Cascade of stars inspires awe when seen with binoculars. Like the Big Dipper though, Kemble’s Cascade is an asterism, not a constellation. The asterism is visible in the northern sky toward the long-necked constellation of the Giraffe (Camelopardalis). This string of about 20 unrelated stars, each of similar brightness, spans over five times the angular width of the full moon. Stretching diagonally from the upper left to the lower right, Kemble's Cascade was popularized last century by astronomy enthusiast Lucian Kemble. The bright object near the top left of the image is the relatively compact Jolly Roger open cluster of stars, officially designated as NGC 1502.

APOD: 2022 August 8 - The Lagoon Nebula without Stars
Explanation: Ridges of glowing interstellar gas and dark dust clouds inhabit the turbulent, cosmic depths of the Lagoon Nebula. Also known as M8, the bright star forming region is about 5,000 light-years distant. But it still makes for a popular stop on telescopic tours of the constellation Sagittarius, toward the center of our Milky Way Galaxy. Dominated by the telltale red emission of ionized hydrogen atoms recombining with stripped electrons, this stunning, deep view of the Lagoon is nearly 100 light-years across. Right of center, the bright, compact, hourglass shape is gas ionized and sculpted by energetic radiation and extreme stellar winds from a massive young star. In fact, although digitally removed from the featured image, the many bright stars of open cluster NGC 6530 drift within the nebula, just formed in the Lagoon several million years ago.

APOD: 2022 April 30 - M44: The Beehive Cluster
Explanation: A mere 600 light-years away, M44 is one of the closest star clusters to our solar system. Also known as the Praesepe or the Beehive cluster its stars are young though, about 600 million years old compared to our Sun's 4.5 billion years. Based on similar ages and motion through space, M44 and the even closer Hyades star cluster in Taurus are thought to have been born together in the same large molecular cloud. An open cluster spanning some 15 light-years, M44 holds 1,000 stars or so and covers about 3 full moons (1.5 degrees) on the sky in the constellation Cancer. Visible to the unaided eye, M44 has been recognized since antiquity. Described as a faint cloud or celestial mist long before being included as the 44th entry in Charles Messier's 18th century catalog, the cluster was not resolved into its individual stars until telescopes were available. A popular target for modern, binocular-equipped sky gazers, the cluster's few yellowish tinted, cool, red giants are scattered through the field of its brighter hot blue main sequence stars in this telescopic group snapshot. Dramatic diffraction spikes highlighting the brighter cluster members were created with string crossed in front of the telescope's objective lens.

APOD: 2022 February 18 - Three Clusters in Puppis
Explanation: Galactic or open star clusters are young. The swarms of stars are born together near the plane of the Milky Way, but their numbers steadily dwindle as cluster members are ejected by galactic tides and gravitational interactions. Caught in this telescopic frame over three degrees across are three good examples of galactic star clusters, seen toward the southern sky's nautical constellation Puppis. Below and left, M46 is some 5,500 light-years in the distance. Right of center M47 is only 1,600 light-years away and NGC 2423 (top) is about 2500 light-years distant. Around 300 million years young M46 contains a few hundred stars in a region about 30 light-years across. Sharp eyes can spot a planetary nebula, NGC 2438, at about 11 o'clock against the M46 cluster stars. But that nebula's central star is billions of years old, and NGC 2438 is likely a foreground object only by chance along the line of sight to youthful M46. Even younger, aged around 80 million years, M47 is a smaller and looser star cluster spanning about 10 light-years. Star cluster NGC 2423 is pushing about 750 million years in age though. NGC 2423 is known to harbor an extrasolar planet, detected orbiting one of its red giant stars.

APOD: 2022 February 14 - In the Heart of the Heart Nebula
Explanation: What excites the Heart Nebula? First, the large emission nebula dubbed IC 1805 looks, in whole, like a human heart. Its shape perhaps fitting of the Valentine's Day, this heart glows brightly in red light emitted by its most prominent element: excited hydrogen. The red glow and the larger shape are all created by a small group of stars near the nebula's center. In the heart of the Heart Nebula are young stars from the open star cluster Melotte 15 that are eroding away several picturesque dust pillars with their energetic light and winds. The open cluster of stars contains a few bright stars nearly 50 times the mass of our Sun, many dim stars only a fraction of the mass of our Sun, and an absent microquasar that was expelled millions of years ago. The Heart Nebula is located about 7,500 light years away toward the constellation of the mythological Queen of Aethiopia (Cassiopeia).

APOD: 2022 January 31 - Carina Nebula North
Explanation: The Great Carina Nebula is home to strange stars and iconic nebulas. Named for its home constellation, the huge star-forming region is larger and brighter than the Great Orion Nebula but less well known because it is so far south -- and because so much of humanity lives so far north. The featured image shows in great detail the northern-most part of the Carina Nebula. Visible nebulas include the semi-circular filaments surrounding the active star Wolf-Rayet 23 (WR23) on the far left. Just left of center is the Gabriela Mistral Nebula consisting of an emission nebula of glowing gas (IC 2599) surrounding the small open cluster of stars (NGC 3324). Above the image center is the larger star cluster NGC 3293, while to its right is the relatively faint emission nebula designated Loden 153. The most famous occupant of the Carina Nebula, however, is not shown. Off the image to the lower right is the bright, erratic, and doomed star star known as Eta Carinae -- a star once one of the brightest stars in the sky and now predicted to explode in a supernova sometime in the next few million years.

APOD: 2021 October 8 - The Double Cluster in Perseus
Explanation: This pretty starfield spans about three full moons (1.5 degrees) across the heroic northern constellation of Perseus. It holds the famous pair of open star clusters, h and Chi Persei. Also cataloged as NGC 869 (top) and NGC 884, both clusters are about 7,000 light-years away and contain stars much younger and hotter than the Sun. Separated by only a few hundred light-years, the clusters are both 13 million years young based on the ages of their individual stars, evidence that they were likely a product of the same star-forming region. Always a rewarding sight in binoculars, the Double Cluster is even visible to the unaided eye from dark locations. But a shroud of guitar strings was used to produce diffraction spikes on the colorful stars imaged in this vibrant telescopic view.

APOD: 2021 September 25 - The Bubble and the Star Cluster
Explanation: To the eye, this cosmic composition nicely balances the Bubble Nebula at the right with open star cluster M52. The pair would be lopsided on other scales, though. Embedded in a complex of interstellar dust and gas and blown by the winds from a single, massive O-type star, the Bubble Nebula, also known as NGC 7635, is a mere 10 light-years wide. On the other hand, M52 is a rich open cluster of around a thousand stars. The cluster is about 25 light-years across. Seen toward the northern boundary of Cassiopeia, distance estimates for the Bubble Nebula and associated cloud complex are around 11,000 light-years, while star cluster M52 lies nearly 5,000 light-years away. The wide telescopic field of view spans about 1.5 degrees on the sky or three times the apparent size of a full Moon.

APOD: 2021 February 21 - NGC 2244: A Star Cluster in the Rosette Nebula
Explanation: In the heart of the Rosette Nebula lies a bright open cluster of stars that lights up the nebula. The stars of NGC 2244 formed from the surrounding gas only a few million years ago. The featured image taken in January using multiple exposures and very specific colors of Sulfur (shaded red), Hydrogen (green), and Oxygen (blue), captures the central region in tremendous detail. A hot wind of particles streams away from the cluster stars and contributes to an already complex menagerie of gas and dust filaments while slowly evacuating the cluster center. The Rosette Nebula's center measures about 50 light-years across, lies about 5,200 light-years away, and is visible with binoculars towards the constellation of the Unicorn (Monoceros).

APOD: 2021 February 14 - Long Stem Rosette Nebula
Explanation: Would the Rosette Nebula by any other name look as sweet? The bland New General Catalog designation of NGC 2237 doesn't appear to diminish the appearance of this flowery emission nebula, at the top of the image, atop a long stem of glowing hydrogen gas. Inside the nebula lies an open cluster of bright young stars designated NGC 2244. These stars formed about four million years ago from the nebular material and their stellar winds are clearing a hole in the nebula's center, insulated by a layer of dust and hot gas. Ultraviolet light from the hot cluster stars causes the surrounding nebula to glow. The Rosette Nebula spans about 100 light-years across, lies about 5000 light-years away, and can be seen with a small telescope towards the constellation of the Unicorn (Monoceros).

APOD: 2020 November 18 - A Double Star Cluster in Perseus
Explanation: Most star clusters are singularly impressive. Open clusters NGC 869 and NGC 884, however, could be considered doubly impressive. Also known as "h and chi Persei", this unusual double cluster, shown above, is bright enough to be seen from a dark location without even binoculars. Although their discovery surely predates recorded history, the Greek astronomer Hipparchus notably cataloged the double cluster. The clusters are over 7,000 light years distant toward the constellation of Perseus, but are separated by only hundreds of light years. In addition to being physically close together, the clusters' ages based on their individual stars are similar - evidence that both clusters were likely a product of the same star-forming region.

APOD: 2020 September 9 - Pleiades: The Seven Sisters Star Cluster
Explanation: Have you ever seen the Pleiades star cluster? Even if you have, you probably have never seen it as large and clear as this. Perhaps the most famous star cluster on the sky, the bright stars of the Pleiades can be seen without binoculars from even the depths of a light-polluted city. With a long exposure from a dark location, though, the dust cloud surrounding the Pleiades star cluster becomes very evident. The featured exposure covers a sky area several times the size of the full moon. Also known as the Seven Sisters and M45, the Pleiades lies about 400 light years away toward the constellation of the Bull (Taurus). A common legend with a modern twist is that one of the brighter stars faded since the cluster was named, leaving only six of the sister stars visible to the unaided eye. The actual number of Pleiades stars visible, however, may be more or less than seven, depending on the darkness of the surrounding sky and the clarity of the observer's eyesight.

APOD: 2020 September 4 - The Wizard Nebula
Explanation: Open star cluster NGC 7380 is still embedded in its natal cloud of interstellar gas and dust popularly known as the Wizard Nebula. Seen on the left, with foreground and background stars along the plane of our Milky Way galaxy it lies some 8,000 light-years distant, toward the constellation Cepheus. In apparent size on the sky, a full moon would cover the 4 million year young cluster and associated nebula, normally much too faint to be seen by eye. Made with telescope and camera firmly planted on Earth, the image reveals multi light-year sized shapes and structures of cosmic gas and dust within the Wizard though, in a color palette made popular in Hubble Space Telescope images. Recorded with narrowband filters, the visible wavelength light from the nebula's hydrogen, oxygen, and sulfur atoms is transformed into green, blue, and red colors in the final digital composite.

APOD: 2020 August 30 - NGC 6357: Cathedral to Massive Stars
Explanation: How massive can a normal star be? Estimates made from distance, brightness and standard solar models had given one star in the open cluster Pismis 24 over 200 times the mass of our Sun, making it one of the most massive stars known. This star is the brightest object located just above the gas front in the featured image. Close inspection of images taken with the Hubble Space Telescope, however, have shown that Pismis 24-1 derives its brilliant luminosity not from a single star but from three at least. Component stars would still remain near 100 solar masses, making them among the more massive stars currently on record. Toward the bottom of the image, stars are still forming in the associated emission nebula NGC 6357. Appearing perhaps like a Gothic cathedral, energetic stars near the center appear to be breaking out and illuminating a spectacular cocoon.

APOD: 2020 August 6 - Messier 20 and 21
Explanation: The beautiful Trifid Nebula, also known as Messier 20, is easy to find with a small telescope in the nebula rich constellation Sagittarius. About 5,000 light-years away, the colorful study in cosmic contrasts shares this well-composed, nearly 1 degree wide field with open star cluster Messier 21 (right). Trisected by dust lanes the Trifid itself is about 40 light-years across and a mere 300,000 years old. That makes it one of the youngest star forming regions in our sky, with newborn and embryonic stars embedded in its natal dust and gas clouds. Estimates of the distance to open star cluster M21 are similar to M20's, but though they share this gorgeous telescopic skyscape there is no apparent connection between the two. In fact, M21's stars are much older, about 8 million years old.

APOD: 2020 January 30 - Two Clusters and a Comet
Explanation: This lovely starfield spans some four full moons (about 2 degrees) across the heroic northern constellation of Perseus. In telescopic exposures made during the nights of January 24, 26, and 28 it holds the famous pair of open or galactic star clusters h and Chi Persei with comet PanSTARRS (C/2017 T2) captured each night as it swept left to right across the field of view. Also cataloged as NGC 869 (right) and NGC 884, both star clusters are about 7,000 light-years away and contain stars much younger and hotter than the Sun. Separated by only a few hundred light-years, the clusters are both 13 million years young based on the ages of their individual stars, evidence that they were likely a product of the same star-forming region. Discovered in 2017 while still beyond the orbit of Saturn, Comet PanSTARRs is a new visitor to the inner solar system and just over 13 light-minutes from planet Earth. Always a rewarding sight in binoculars, the Double Cluster is even visible to the unaided eye from dark locations. C/2017 T2 could remain a telescopic comet though. One of the brightest comets anticipated in 2020 it makes its closest approach to the Sun in early May.

APOD: 2020 January 22 - The Hyades Star Cluster
Explanation: It is the closest cluster of stars to the Sun. The Hyades open cluster is bright enough to have been remarked on even thousands of years ago, yet is not as bright or compact as the nearby Pleiades (M45) star cluster. Pictured here is a particularly deep image of the Hyades which has brings out vivid star colors and faint coincidental nebulas. The brightest star in the field is yellow Aldebaran, the eye of the bull toward the constellation of Taurus. Aldebaran, at 65 light-years away, is now known to be unrelated to the Hyades cluster, which lies about 150 light-years away. The central Hyades stars are spread out over about 15 light-years. Formed about 625 million years ago, the Hyades likely shares a common origin with the Beehive cluster (M44), a naked-eye open star cluster toward the constellation of Cancer, based on M44's motion through space and remarkably similar age.

APOD: 2019 December 30 - Messier 20 and 21
Explanation: The beautiful Trifid Nebula, also known as Messier 20, is easy to find with a small telescope in the nebula rich constellation Sagittarius. About 5,000 light-years away, the colorful study in cosmic contrasts shares this well-composed, nearly 1 degree wide field with open star cluster Messier 21 (top left). Trisected by dust lanes the Trifid itself is about 40 light-years across and a mere 300,000 years old. That makes it one of the youngest star forming regions in our sky, with newborn and embryonic stars embedded in its natal dust and gas clouds. Estimates of the distance to open star cluster M21 are similar to M20's, but though they share this gorgeous telescopic skyscape there is no apparent connection between the two. In fact, M21's stars are much older, about 8 million years old.

APOD: 2019 October 13 - A Stellar Jewel Box: Open Cluster NGC 290
Explanation: Jewels don't shine this bright -- only stars do. Like gems in a jewel box, though, the stars of open cluster NGC 290 glitter in a beautiful display of brightness and color. The photogenic cluster, pictured here, was captured in 2006 by the orbiting Hubble Space Telescope. Open clusters of stars are younger, contain few stars, and contain a much higher fraction of blue stars than do globular clusters of stars. NGC 290 lies about 200,000 light-years distant in a neighboring galaxy called the Small Cloud of Magellan (SMC). The open cluster contains hundreds of stars and spans about 65 light years across. NGC 290 and other open clusters are good laboratories for studying how stars of different masses evolve, since all the open cluster's stars were born at about the same time.

APOD: 2019 September 11 - IC 1805: The Heart Nebula
Explanation: What energizes the Heart Nebula? First, the large emission nebula dubbed IC 1805 looks, in whole, like a human heart. The nebula glows brightly in red light emitted by its most prominent element: hydrogen. The red glow and the larger shape are all powered by a small group of stars near the nebula's center. In the center of the Heart Nebula are young stars from the open star cluster Melotte 15 that are eroding away several picturesque dust pillars with their energetic light and winds. The open cluster of stars contains a few bright stars nearly 50 times the mass of our Sun, many dim stars only a fraction of the mass of our Sun, and an absent microquasar that was expelled millions of years ago. The Heart Nebula is located about 7,500 light years away toward the constellation of Cassiopeia. Coincidentally, a small meteor was captured in the foreground during imaging and is visible above the dust pillars. At the top right is the companion Fishhead Nebula.

APOD: 2019 September 1 - M45: The Pleiades Star Cluster
Explanation: Have you ever seen the Pleiades star cluster? Even if you have, you probably have never seen it as dusty as this. Perhaps the most famous star cluster on the sky, the bright stars of the Pleiades can be seen without binoculars from even the depths of a light-polluted city. With a long exposure from a dark location, though, the dust cloud surrounding the Pleiades star cluster becomes very evident. The featured exposure took over 12 hours and covers a sky area several times the size of the full moon. Also known as the Seven Sisters and M45, the Pleiades lies about 400 light years away toward the constellation of the Bull (Taurus). A common legend with a modern twist is that one of the brighter stars faded since the cluster was named, leaving only six stars visible to the unaided eye. The actual number of Pleiades stars visible, however, may be more or less than seven, depending on the darkness of the surrounding sky and the clarity of the observer's eyesight.

APOD: 2019 August 30 - NGC 7129 and NGC 7142
Explanation: This wide-field telescopic image looks toward the constellation Cepheus and an intriguing visual pairing of dusty reflection nebula NGC 7129 (right) and open star cluster NGC 7142. The two appear separated by only half a degree on the sky, but they actually lie at quite different distances. In the foreground, dusty nebula NGC 7129 is about 3,000 light-years distant, while open cluster NGC 7142 is likely over 6,000 light-years away. In fact, pervasive and clumpy foreground dust clouds in this region redden the light from NGC 7142, complicating astronomical explorations of the cluster. Still, NGC 7142 is thought to be an older open star cluster, while the bright stars embedded in NGC 7129 are perhaps a few million years young. The telltale reddish crescent shapes around NGC 7129 are associated with energetic jets streaming away from newborn stars.

APOD: 2019 May 28 - Stars, Dust, and Gas near NGC 3572
Explanation: Star formation can be colorful. This chromatic cosmic portrait features glowing gas and dark dust near some recently formed stars of NGC 3572, a little-studied star cluster near the Carina Nebula. Stars from NGC 3572 are visible near the bottom of the image, while the expansive gas cloud above is likely what remains of their formation nebula. The image's striking hues were created by featuring specific colors emitted by hydrogen, oxygen, and sulfur, and blending them with images recorded through broadband filters in red, green, and blue. This nebula near NGC 3572 spans about 100 light years and lies about 9,000 light years away toward the southern constellation of the Ship's Keel (Carina). Within a few million years the pictured gas will likely disperse, while gravitational encounters will likely disperse the cluster stars over about a billion years.

APOD: 2019 May 14 - Young Star Cluster Trumpler 14 from Hubble
Explanation: Why does star cluster Trumpler 14 have so many bright stars? Because it is so young. Many cluster stars have formed only in the past 5 million years and are so hot they emit detectable X-rays. In older star clusters, most stars this young have already died -- typically exploding in a supernova -- leaving behind stars that are fainter and redder. Trumpler 14 spans about 40 light years and lies about 9,000 light years away on the edge of the famous Carina Nebula. A discerning eye can spot two unusual objects in this detailed 2006 image of Trumpler 14 by the Hubble Space Telescope. First, a dark cloud just left of center may be a planetary system trying to form before being destroyed by the energetic winds of Trumpler 14's massive stars. Second is the arc at the bottom left, which one hypothesis holds is the supersonic shock wave of a fast star ejected 100,000 years ago from a completely different star cluster.

APOD: 2019 March 16 - NGC 3324 in Carina
Explanation: This bright cosmic cloud was sculpted by stellar winds and radiation from the hot young stars of open cluster NGC 3324. With dust clouds in silhouette against its glowing atomic gas, the pocket-shaped star-forming region actually spans about 35 light-years. It lies some 7,500 light-years away toward the nebula rich southern constellation Carina. A composite of narrowband image data, the telescopic view captures the characteristic emission from ionized sulfur, hydrogen, and oxygen atoms mapped to red, green, and blue hues in the popular Hubble Palette. For some, the celestial landscape of bright ridges of emission bordered by cool, obscuring dust along the right side create a recognizable face in profile. The region's popular name is the Gabriela Mistral Nebula for the Nobel Prize winning Chilean poet.

APOD: 2018 November 22 - Portrait of NGC 281
Explanation: Look through the cosmic cloud cataloged as NGC 281 and you might miss the stars of open cluster IC 1590. Still, formed within the nebula that cluster's young, massive stars ultimately power the pervasive nebular glow. The eye-catching shapes looming in this portrait of NGC 281 are sculpted dusty columns and dense Bok globules seen in silhouette, eroded by intense, energetic winds and radiation from the hot cluster stars. If they survive long enough, the dusty structures could also be sites of future star formation. Playfully called the Pacman Nebula because of its overall shape, NGC 281 is about 10,000 light-years away in the constellation Cassiopeia. This sharp composite image was made through narrow-band filters. It combines emission from the nebula's hydrogen and oxygen atoms to synthesize red, green, and blue colors. The scene spans well over 80 light-years at the estimated distance of NGC 281.

APOD: 2018 November 12 - The Lagoon Nebula is Stars, Gas, and Dust
Explanation: The majestic Lagoon Nebula is filled with hot gas and the home for many young stars. Spanning 100 light years across while lying only about 5000 light years distant, the Lagoon Nebula is so big and bright that it can be seen without a telescope toward the constellation of the Archer (Sagittarius). Many bright stars are visible from NGC 6530, an open cluster that formed in the nebula only several million years ago. The greater nebula, also known as M8 and NGC 6523, is named "Lagoon" for the band of dust seen to the left of the open cluster's center. The featured image was taken in three colors with details are brought out by light emitted by Hydrogen. Star formation continues in the Lagoon Nebula as witnessed by the many dark dust-laden globules that exist there.

APOD: 2018 October 15 - M16: In and Around the Eagle Nebula
Explanation: From afar, the whole thing looks like an Eagle. A closer look at the Eagle Nebula, however, shows the bright region is actually a window into the center of a larger dark shell of dust. Through this window, a brightly-lit workshop appears where a whole open cluster of stars is being formed. In this cavity tall pillars and round globules of dark dust and cold molecular gas remain where stars are still forming. Already visible are several young bright blue stars whose light and winds are burning away and pushing back the remaining filaments and walls of gas and dust. The Eagle emission nebula, tagged M16, lies about 6500 light years away, spans about 20 light-years, and is visible with binoculars toward the constellation of the Serpent (Serpens). This picture involved over 25 hours of imaging and combines three specific emitted colors emitted by sulfur (colored as red), hydrogen (yellow), and oxygen (blue).

APOD: 2018 August 24 - Messier 20 and 21
Explanation: The beautiful Trifid Nebula, also known as Messier 20, is easy to find with a small telescope in the nebula rich constellation Sagittarius. About 5,000 light-years away, the colorful study in cosmic contrasts shares this well-composed, nearly 1 degree wide field with open star cluster Messier 21 (bottom right). Trisected by dust lanes the Trifid itself is about 40 light-years across and a mere 300,000 years old. That makes it one of the youngest star forming regions in our sky, with newborn and embryonic stars embedded in its natal dust and gas clouds. Estimates of the distance to open star cluster M21 are similar to M20's, but though they share this gorgeous telescopic skyscape there is no apparent connection between the two. In fact, M21's stars are much older, about 8 million years old.

APOD: 2018 April 6 - NGC 3324 in Carina
Explanation: This bright cosmic cloud was sculpted by stellar winds and radiation from the hot young stars of open cluster NGC 3324. With dust clouds in silhouette against its glowing atomic gas, the pocket-shaped star-forming region actually spans about 35 light-years. It lies some 7,500 light-years away toward the nebula rich southern constellation Carina. A composite of narrowband image data, the telescopic view captures the characteristic emission from ionized sulfur, hydrogen, and oxygen atoms mapped to red, green, and blue hues in the popular Hubble Palette. For some, the celestial landscape of bright ridges of emission bordered by cool, obscuring dust along the right side create a recognizable face in profile. The region's popular name is the Gabriela Mistral Nebula for the Nobel Prize winning Chilean poet.

APOD: 2018 February 14 - In the Heart of the Heart Nebula
Explanation: What's that inside the Heart Nebula? First, the large emission nebula dubbed IC 1805 looks, in whole, like a human heart. It's shape perhaps fitting of the Valentine's Day, this heart glows brightly in red light emitted by its most prominent element: hydrogen. The red glow and the larger shape are all created by a small group of stars near the nebula's center. In the heart of the Heart Nebula are young stars from the open star cluster Melotte 15 that are eroding away several picturesque dust pillars with their energetic light and winds. The open cluster of stars contains a few bright stars nearly 50 times the mass of our Sun, many dim stars only a fraction of the mass of our Sun, and an absent microquasar that was expelled millions of years ago. The Heart Nebula is located about 7,500 light years away toward the constellation of the mythological Queen of Aethiopia (Cassiopeia).

APOD: 2017 November 15 - NGC 7789: Caroline's Rose
Explanation: Found among the rich starfields of the Milky Way, star cluster NGC 7789 lies about 8,000 light-years away toward the constellation Cassiopeia. A late 18th century deep sky discovery of astronomer Caroline Lucretia Herschel, the cluster is also known as Caroline's Rose. Its flowery visual appearance in small telescopes is created by the cluster's nestled complex of stars and voids. Now estimated to be 1.6 billion years young, the galactic or open cluster of stars also shows its age. All the stars in the cluster were likely born at the same time, but the brighter and more massive ones have more rapidly exhausted the hydrogen fuel in their cores. These have evolved from main sequence stars like the Sun into the many red giant stars shown with a yellowish cast in this lovely color composite. Using measured color and brightness, astronomers can model the mass and hence the age of the cluster stars just starting to "turn off" the main sequence and become red giants. Over 50 light-years across, Caroline's Rose spans about half a degree (the angular size of the Moon) near the center of the wide-field telescopic image.

APOD: 2017 November 14 - The Pleiades Deep and Dusty
Explanation: The well-known Pleiades star cluster is slowly destroying part of a passing cloud of gas and dust. The Pleiades is the brightest open cluster of stars on Earth's sky and can be seen from almost any northerly location with the unaided eye. The passing young dust cloud is thought to be part of Gould's Belt, an unusual ring of young star formation surrounding the Sun in the local Milky Way Galaxy. Over the past 100,000 years, part of Gould's Belt is by chance moving right through the older Pleiades and is causing a strong reaction between stars and dust. Pressure from the stars' light significantly repels the dust in the surrounding blue reflection nebula, with smaller dust particles being repelled more strongly. A short-term result is that parts of the dust cloud have become filamentary and stratified. The featured deep image also captured Comet C/2015 ER61 (PanSTARRS) on the lower left.

APOD: 2017 October 2 - Two Comets and a Star Cluster
Explanation: Two unusual spots are on the move near the famous Pleiades star cluster. Shifting only a small amount per night, these spots are actually comets in our nearby Solar System that by chance wandered into the field of the light-years distant stars. On the far left is comet C/2017 O1 ASAS-SN, a multi-kilometer block of evaporating ice sporting a bright coma of surrounding gas dominated by green-glowing carbon. Comet ASAS-SN1 shows a slight tail to its lower right. Near the frame center is comet C/2015 ER61 PanSTARRS, also a giant block of evaporating ice, but sporting a rather long tail to its right. On the upper right is the Pleiades, an open cluster dominated by bright blue stars illuminating nearby reflecting dust. This exposure, taken about two weeks ago, is so deep that the filamentary interstellar dust can be traced across the entire field. The Pleiades is visible to the unaided eye, but it should require binoculars to see the comets.

APOD: 2017 September 30 - Portrait of NGC 281
Explanation: Look through the cosmic cloud cataloged as NGC 281 and you might miss the stars of open cluster IC 1590. Still, formed within the nebula that cluster's young, massive stars ultimately power the pervasive nebular glow. The eye-catching shapes looming in this portrait of NGC 281 are sculpted columns and dense dust globules seen in silhouette, eroded by intense, energetic winds and radiation from the hot cluster stars. If they survive long enough, the dusty structures could also be sites of future star formation. Playfully called the Pacman Nebula because of its overall shape, NGC 281 is about 10,000 light-years away in the constellation Cassiopeia. This sharp composite image was made through narrow-band filters, combining emission from the nebula's hydrogen, sulfur, and oxygen atoms in green, red, and blue hues. It spans over 80 light-years at the estimated distance of NGC 281.

APOD: 2017 July 4 - Celestial Fireworks: Into Star Cluster Westerlund 2
Explanation: What if you could go right into a cluster where stars are forming? A one-minute, time-lapse, video visualization of just this has been made with 3D computer modeling of the region surrounding the star cluster Westerlund 2, based on images from the Hubble Space Telescope in visible and infrared light. Westerlund 2 spans about 10 light years across and lies about 20,000 light years distant toward the constellation of the Ship's Keel (Carina). As the illustrative animation begins, the greater Gum 29 nebula fills the screen, with the young cluster of bright stars visible in the center. Stars zip past you as you approach the cluster. Soon your imaginary ship pivots and you pass over light-year long pillars of interstellar gas and dust. Strong winds and radiation from massive young stars destroy all but the densest nearby dust clumps, leaving these pillars in their shadows -- many pointing back toward the cluster center. Last, you pass into the top of the star cluster and survey hundreds of the most massive stars known.

APOD: 2017 June 28 - Composite Messier 20 and 21
Explanation: The beautiful Trifid Nebula, also known as Messier 20, lies about 5,000 light-years away, a colorful study in cosmic contrasts. It shares this nearly 1 degree wide field with open star cluster Messier 21 (top left). Trisected by dust lanes the Trifid itself is about 40 light-years across and a mere 300,000 years old. That makes it one of the youngest star forming regions in our sky, with newborn and embryonic stars embedded in its natal dust and gas clouds. Estimates of the distance to open star cluster M21 are similar to M20's, but though they share this gorgeous telescopic skyscape there is no apparent connection between the two. M21's stars are much older, about 8 million years old. M20 and M21 are easy to find with even a small telescope in the nebula rich constellation Sagittarius. In fact, this well-composed scene is a composite from two different telescopes. Using narrowband data it blends a high resolution image of M20 with a wider field image extending to M21.

APOD: 2017 June 20 - The Massive Stars in Westerlund 1
Explanation: Star cluster Westerlund 1 is home to some of the largest and most massive stars known. It is headlined by the star Westerlund 1-26, a red supergiant star so big that if placed in the center of our Solar System, it would extend out past the orbit of Jupiter. Additionally, the young star cluster is home to 3 other red supergiants, 6 yellow hypergiant stars, 24 Wolf-Rayet stars, and several even-more unusual stars that continue to be studied. Westerlund 1 is relatively close-by for a star cluster at a distance of 15,000 light years, giving astronomers a good laboratory to study the development of massive stars. The featured image of Westerlund 1 was taken by the Hubble Space Telescope toward the southern constellation of the Altar (Ara). Although presently classified as a "super" open cluster, Westerlund 1 may evolve into a low mass globular cluster over the next billion years.

APOD: 2017 May 25 - Star Cluster, Spiral Galaxy, Supernova
Explanation: A cosmic snapshot from May 19, this colorful telescopic field of view spans about 1 degree or 2 full moons on the sky. Spiky in appearance, foreground Milky Way stars are scattered toward the royal constellation Cepheus while stars of open cluster NGC 6939 gather about 5 thousand light-years in the distance near the top of the frame. Face-on spiral galaxy NGC 6946 is toward the lower left nearly 22 million light-years away. The helpful red lines identify recently discovered supernova SN 2017eaw, the death explosion of a massive star nestled in the galaxy's bluish spiral arms. In fact in the last 100 years, 10 supernovae have been discovered in NGC 6946. By comparison, the average rate of supernovae in our Milky Way is about 1 every 100 years or so. Of course, NGC 6946 is also known as The Fireworks Galaxy.

APOD: 2017 February 14 - The Rosette Nebula
Explanation: Would the Rosette Nebula by any other name look as sweet? The bland New General Catalog designation of NGC 2237 doesn't appear to diminish the appearance of this flowery emission nebula. Inside the nebula lies an open cluster of bright young stars designated NGC 2244. These stars formed about four million years ago from the nebular material and their stellar winds are clearing a hole in the nebula's center, insulated by a layer of dust and hot gas. Ultraviolet light from the hot cluster stars causes the surrounding nebula to glow. The Rosette Nebula spans about 100 light-years across, lies about 5000 light-years away, and can be seen with a small telescope towards the constellation of the Unicorn (Monoceros).

APOD: 2016 November 6 - Starburst Cluster in NGC 3603
Explanation: A mere 20,000 light-years from the Sun lies NGC 3603, a resident of the nearby Carina spiral arm of our Milky Way Galaxy. NGC 3603 is well known to astronomers as one of the Milky Way's largest star-forming regions. The central open star cluster contains thousands of stars more massive than our Sun, stars that likely formed only one or two million years ago in a single burst of star formation. In fact, nearby NGC 3603 is thought to contain a convenient example of the massive star clusters that populate much more distant starburst galaxies. Surrounding the cluster are natal clouds of glowing interstellar gas and obscuring dust, sculpted by energetic stellar radiation and winds. Recorded by the Hubble Space Telescope, the image spans about 17 light-years.

APOD: 2016 November 4 - Portrait of NGC 281
Explanation: Look through the cosmic cloud cataloged as NGC 281 and you might miss the stars of open cluster IC 1590. Still, formed within the nebula that cluster's young, massive stars ultimately power the pervasive nebular glow. The eye-catching shapes looming in this portrait of NGC 281 are sculpted columns and dense dust globules seen in silhouette, eroded by intense, energetic winds and radiation from the hot cluster stars. If they survive long enough, the dusty structures could also be sites of future star formation. Playfully called the Pacman Nebula because of its overall shape, NGC 281 is about 10,000 light-years away in the constellation Cassiopeia. This sharp composite image was made through narrow-band filters, combining emission from the nebula's hydrogen, sulfur, and oxygen atoms in green, red, and blue hues. It spans over 80 light-years at the estimated distance of NGC 281.

APOD: 2016 October 19 - M45: The Pleiades Star Cluster
Explanation: Have you ever seen the Pleiades star cluster? Even if you have, you probably have never seen it as dusty as this. Perhaps the most famous star cluster on the sky, the bright stars of the Pleiades can be seen without binoculars even from the heart of a light-polluted city. With a long exposure from a dark location, though, the dust cloud surrounding the Pleiades star cluster becomes very evident. The featured image was a long duration exposure taken last month from Namibia and covers a sky area many times the size of the full moon. Also known as the Seven Sisters and M45, the Pleiades lies about 400 light years away toward the constellation of the Bull (Taurus). A common legend with a modern twist is that one of the brighter stars faded since the cluster was named, leaving only six stars visible to the unaided eye. The actual number of visible Pleiades stars, however, may be more or less than seven, depending on the darkness of the surrounding sky and the clarity of the observer's eyesight.

APOD: 2016 March 27 - NGC 6357: Cathedral to Massive Stars
Explanation: How massive can a normal star be? Estimates made from distance, brightness and standard solar models had given one star in the open cluster Pismis 24 over 200 times the mass of our Sun, making it one of the most massive stars known. This star is the brightest object located just above the gas front in the featured image. Close inspection of images taken with the Hubble Space Telescope, however, have shown that Pismis 24-1 derives its brilliant luminosity not from a single star but from three at least. Component stars would still remain near 100 solar masses, making them among the more massive stars currently on record. Toward the bottom of the image, stars are still forming in the associated emission nebula NGC 6357. Appearing perhaps like a Gothic cathedral, energetic stars near the center appear to be breaking out and illuminating a spectacular cocoon.

APOD: 2016 January 5 - The Lagoon Nebula in Hydrogen Sulfur and Oxygen
Explanation: The majestic Lagoon Nebula is filled with hot gas and the home for many young stars. Spanning 100 light years across while lying only about 5000 light years distant, the Lagoon Nebula is so big and bright that it can be seen without a telescope toward the constellation of the Archer (Sagittarius). Many bright stars are visible from NGC 6530, an open cluster that formed in the nebula only several million years ago. The greater nebula, also known as M8 and NGC 6523, is named "Lagoon" for the band of dust seen to the right of the open cluster's center. The featured image was taken in the light emitted by Hydrogen (shown in brown), Sulfur (red), and Oxygen (blue) and displayed in enhanced color. The featured picture is a newly processed panorama of M8, capturing twice the diameter of the Full Moon. Star formation continues in the Lagoon Nebula as witnessed by the many globules that exist there.

APOD: 2015 December 30 - The Fox Fur Nebula
Explanation: This interstellar canine is formed of cosmic dust and gas interacting with the energetic light and winds from hot young stars. The shape, visual texture, and color, combine to give the region the popular name Fox Fur Nebula. The characteristic blue glow on the left is dust reflecting light from the bright star S Mon, the bright star just below the top edge of the featured image. Textured red and black areas are a combination of the cosmic dust and reddish emission from ionized hydrogen gas. S Mon is part of a young open cluster of stars, NGC 2264, located about 2,500 light years away toward the constellation of the Unicorn (Monoceros).

APOD: 2015 October 27 - Bright from the Heart Nebula
Explanation: What's that inside the Heart Nebula? First, the large emission nebula dubbed IC 1805 looks, in whole, like a human heart. The nebula glows brightly in red light emitted by its most prominent element: hydrogen. The red glow and the larger shape are all created by a small group of stars near the nebula's center. In the center of the Heart Nebula are young stars from the open star cluster Melotte 15 that are eroding away several picturesque dust pillars with their energetic light and winds. The open cluster of stars contains a few bright stars nearly 50 times the mass of our Sun, many dim stars only a fraction of the mass of our Sun, and an absent microquasar that was expelled millions of years ago. The Heart Nebula is located about 7,500 light years away toward the constellation of Cassiopeia. At the top right is the companion Fishhead Nebula.

APOD: 2015 August 26 - Collinder 399: The Coat Hanger
Explanation: Is this coat hanger a star cluster or an asterism? This cosmic hang-up has been debated over much of last century, as astronomers wondered whether this binocular-visible object is really a physically associated open cluster or a chance projection. Chance star projections are known as asterisms, an example of which is the popular Big Dipper. Recent precise measurements from different vantage points in the Earth's orbit around the Sun have uncovered discrepant angular shifts indicating that the Coat Hanger is better described as an asterism. Known more formally as Collinder 399, this bright stellar grouping is wider than the full moon and lies in the constellation of the Fox (Vulpecula).

APOD: 2015 June 17 - M45: The Pleiades Star Cluster
Explanation: Have you ever seen the Pleiades star cluster? Even if you have, you probably have never seen it as dusty as this. Perhaps the most famous star cluster on the sky, the bright stars of the Pleiades can be seen without binoculars from even the depths of a light-polluted city. With a long exposure from a dark location, though, the dust cloud surrounding the Pleiades star cluster becomes very evident. The featured exposure took over 12 hours and covers a sky area several times the size of the full moon. Also known as the Seven Sisters and M45, the Pleiades lies about 400 light years away toward the constellation of the Bull (Taurus). A common legend with a modern twist is that one of the brighter stars faded since the cluster was named, leaving only six stars visible to the unaided eye. The actual number of Pleiades stars visible, however, may be more or less than seven, depending on the darkness of the surrounding sky and the clarity of the observer's eyesight.

APOD: 2015 June 8 - The Milky Way over the Temple of Poseidon
Explanation: What's that glowing in the distance? Although it may look like a lighthouse, the rays of light near the horizon actually emanate from the Temple of Poseidon at Cape Sounion, Greece. Some temple lights are even reflected in the Aegean Sea in the foreground. Although meant to be a monument to the sea, in this image, the temple's lights seem to be pointing out locations on the sky. For example, the wide ray toward the right fortuitously points toward the Lagoon Nebula in the central band of our Milky Way, which runs diagonally down the image from the upper left. Also, the nearly vertical beam seems to point toward the star clouds near the direction of the Wild Duck open cluster of stars. The featured image was taken less than three weeks ago.

APOD: 2015 April 17 - M46 Plus Two
Explanation: Galactic or open star clusters are young. These swarms of stars are born together near the plane of the Milky Way, but their numbers steadily dwindle as cluster members are ejected by galactic tides and gravitational interactions. In fact, this bright open cluster, known as M46, is around 300 million years young. It still contains a few hundred stars within a span of 30 light-years or so. Located about 5,000 light-years away toward the constellation Puppis, M46 also seems to contain contradictions to its youthful status. In this pretty starscape, the colorful, circular patch above and right of the center of M46 is the planetary nebula NGC 2438. Fainter still, a second planetary nebula, PK231+4.1, is identified by the box at the right and enlarged in the inset. Planetary nebulae are a brief, final phase in the life of a sun-like star a billion years old or more, whose central reservoir of hydrogen fuel has been exhausted. NGC 2438 is estimated to be only 3,000 light-years distant, though, and moves at a different speed than M46 cluster members. Along with its fainter cohort, planetary nebula NGC 2438 is likely only by chance appearing near our line-of-sight to the young stars of M46.

APOD: 2015 April 6 - NGC 3293: A Bright Young Star Cluster
Explanation: Hot blue stars shine brightly in this beautiful, recently formed galactic or "open" star cluster. Open cluster NGC 3293 is located in the constellation Carina, lies at a distance of about 8000 light years, and has a particularly high abundance of these young bright stars. A study of NGC 3293 implies that the blue stars are only about 6 million years old, whereas the cluster's dimmer, redder stars appear to be about 20 million years old. If true, star formation in this open cluster took at least 15 million years. Even this amount of time is short, however, when compared with the billions of years stars like our Sun live, and the over-ten billion year lifetimes of many galaxies and our universe. Pictured, NGC 3293 appears just in front of a dense dust lane and red glowing hydrogen gas emanating from the Carina Nebula.

APOD: 2015 March 7 - NGC 602 in the Flying Lizard Nebula
Explanation: Near the outskirts of the Small Magellanic Cloud, a satellite galaxy some 200 thousand light-years distant, lies 5 million year young star cluster NGC 602. Surrounded by natal gas and dust, NGC 602 is just below center in this telescopic field of view with the angular size of the Full Moon on the sky. The cluster itself is about 200 light-years in diameter. Glowing interior ridges and swept back shapes strongly suggest that energetic radiation and shock waves from NGC 602's massive young stars have eroded the dusty material and triggered a progression of star formation moving away from the cluster's center. Of course, the more extended wings of emission in the region suggest a popular name for the complex cosmic environment, The Flying Lizard Nebula.

APOD: 2015 January 5 - A Fox Fur, a Unicorn, and a Christmas Tree
Explanation: What do the following things have in common: a cone, the fur of a fox, and a Christmas tree? Answer: they all occur in the constellation of the unicorn (Monoceros). Pictured as a star forming region and cataloged as NGC 2264, the complex jumble of cosmic gas and dust is about 2,700 light-years distant and mixes reddish emission nebulae excited by energetic light from newborn stars with dark interstellar dust clouds. Where the otherwise obscuring dust clouds lie close to the hot, young stars they also reflect starlight, forming blue reflection nebulae. The image spans about the diameter of a full moon, covering about 30 light-years at the distance of NGC 2264. Its cast of cosmic characters includes the Fox Fur Nebula, whose convoluted pelt lies on the lower right, bright variable star S Mon visible just above the Fox Fur, and the Cone Nebula on the image left. Given their distribution, the stars of NGC 2264 are also known as the Christmas Tree star cluster. The triangular tree shape traced by the stars appears here with its apex at the Cone Nebula on the left with its broader base near S Mon on the right.

APOD: 2014 November 28 - Portrait of NGC 281
Explanation: Look through the cosmic cloud cataloged as NGC 281 and you might miss the stars of open cluster IC 1590. But, formed within the nebula, that cluster's young, massive stars ultimately power the pervasive nebular glow. The eye-catching shapes looming in this portrait of NGC 281 are sculpted columns and dense dust globules seen in silhouette, eroded by intense, energetic winds and radiation from the hot cluster stars. If they survive long enough, the dusty structures could also be sites of future star formation. Playfully called the Pacman Nebula because of its overall shape, NGC 281 is about 10,000 light-years away in the constellation Cassiopeia. This sharp composite image was made through narrow-band filters, combining emission from the nebula's hydrogen, sulfur, and oxygen atoms in green, red, and blue hues. It spans over 80 light-years at the estimated distance of NGC 281.

APOD: 2014 October 8 - NGC 6823: Cloud Sculpting Star Cluster
Explanation: Star cluster NGC 6823 is slowly turning gas clouds into stars. The center of the open cluster, visible on the upper right, formed only about two million years ago and is dominated in brightness by a host of bright young blue stars. Some outer parts of the cluster, visible in the featured image's center as the stars and pillars of emission nebula NGC 6820, contain even younger stars. The huge pillars of gas and dust likely get their elongated shape by erosion from hot radiation emitted from the brightest cluster stars. Striking dark globules of gas and dust are also visible across the upper left of the featured image. Open star cluster NGC 6823 spans about 50 light years and lies about 6000 light years away toward the constellation of the Fox (Vulpecula).

APOD: 2014 September 24 - The Lagoon Nebula in Stars Dust and Gas
Explanation: The large majestic Lagoon Nebula is home for many young stars and hot gas. Spanning 100 light years across while lying only about 5000 light years distant, the Lagoon Nebula is so big and bright that it can be seen without a telescope toward the constellation of Sagittarius. Many bright stars are visible from NGC 6530, an open cluster that formed in the nebula only several million years ago. The greater nebula, also known as M8 and NGC 6523, is named "Lagoon" for the band of dust seen to the left of the open cluster's center. A bright knot of gas and dust in the nebula's center is known as the Hourglass Nebula. The featured picture is a newly processed panorama of M8, capturing five times the diameter of the Moon. Star formation continues in the Lagoon Nebula as witnessed by the many globules that exist there.

APOD: 2014 September 3 - M6: The Butterfly Cluster
Explanation: To some, the outline of the open cluster of stars M6 resembles a butterfly. M6, also known as NGC 6405, spans about 20 light-years and lies about 2,000 light years distant. M6, pictured above, can best be seen in a dark sky with binoculars towards the constellation of the Scorpion (Scorpius), coving about as much of the sky as the full moon. Like other open clusters, M6 is composed predominantly of young blue stars, although the brightest star is nearly orange. M6 is estimated to be about 100 million years old. Determining the distance to clusters like M6 helps astronomers calibrate the distance scale of the universe.

APOD: 2014 August 29 - The Wizard Nebula
Explanation: Open star cluster NGC 7380 is still embedded in its natal cloud of interstellar gas and dust popularly known as the Wizard Nebula. Seen with foreground and background stars along the plane of our Milky Way galaxy it lies some 8,000 light-years distant, toward the constellation Cepheus. A full moon would easily fit inside this telescopic view of the 4 million year young cluster and associated nebula, normally much too faint to be seen by eye. Made with telescope and camera firmly planted on Earth, the image reveals multi light-year sized shapes and structures within the Wizard in a color palette made popular in Hubble Space Telescope images. Recorded with narrowband filters, the visible wavelength light from the nebula's hydrogen, oxygen, and sulfur atoms is transformed into green, blue, and red colors in the final digital composite. But there is still a trick up the Wizard's sleeve. Sliding your cursor over the image (or following this link) will make the stars disappear, leaving only the cosmic gas and dust of the Wizard Nebula.

APOD: 2014 August 28 - Messier 20 and 21
Explanation: The beautiful Trifid Nebula, also known as Messier 20, is easy to find with a small telescope in the nebula rich constellation Sagittarius. About 5,000 light-years away, the colorful study in cosmic contrasts shares this well-composed, nearly 1 degree wide field with open star cluster Messier 21 (top right). Trisected by dust lanes the Trifid itself is about 40 light-years across and a mere 300,000 years old. That makes it one of the youngest star forming regions in our sky, with newborn and embryonic stars embedded in its natal dust and gas clouds. Estimates of the distance to open star cluster M21 are similar to M20's, but though they share this gorgeous telescopic skyscape there is no apparent connection between the two. In fact, M21's stars are much older, about 8 million years old.

APOD: 2014 June 8 - Open Cluster NGC 290: A Stellar Jewel Box
Explanation: Jewels don't shine this bright -- only stars do. Like gems in a jewel box, though, the stars of open cluster NGC 290 glitter in a beautiful display of brightness and color. The photogenic cluster, pictured above, was captured recently by the orbiting Hubble Space Telescope. Open clusters of stars are younger, contain few stars, and contain a much higher fraction of blue stars than do globular clusters of stars. NGC 290 lies about 200,000 light-years distant in a neighboring galaxy called the Small Cloud of Magellan (SMC). The open cluster contains hundreds of stars and spans about 65 light years across. NGC 290 and other open clusters are good laboratories for studying how stars of different masses evolve, since all the open cluster's stars were born at about the same time.

APOD: 2014 April 8 - M42: Inside the Orion Nebula
Explanation: The Great Nebula in Orion, an immense, nearby starbirth region, is probably the most famous of all astronomical nebulas. Here, glowing gas surrounds hot young stars at the edge of an immense interstellar molecular cloud only 1500 light-years away. In the above deep image composite in assigned colors taken by the Hubble Space Telescope wisps and sheets of dust and gas are particularly evident. The Great Nebula in Orion can be found with the unaided eye near the easily identifiable belt of three stars in the popular constellation Orion. In addition to housing a bright open cluster of stars known as the Trapezium, the Orion Nebula contains many stellar nurseries. These nurseries contain much hydrogen gas, hot young stars, proplyds, and stellar jets spewing material at high speeds. Also known as M42, the Orion Nebula spans about 40 light years and is located in the same spiral arm of our Galaxy as the Sun.

APOD: 2014 March 11 - In the Heart of the Rosette Nebula
Explanation: In the heart of the Rosette Nebula lies a bright open cluster of stars that lights up the nebula. The stars of NGC 2244 formed from the surrounding gas only a few million years ago. The above image taken in January using multiple exposures and very specific colors of Sulfur (shaded red), Hydrogen (green), and Oxygen (blue), captures the central region in tremendous detail. A hot wind of particles streams away from the cluster stars and contributes to an already complex menagerie of gas and dust filaments while slowly evacuating the cluster center. The Rosette Nebula's center measures about 50 light-years across, lies about 4,500 light-years away, and is visible with binoculars towards the constellation of the Unicorn (Monoceros).

APOD: 2014 February 25 - The Pleiades Deep and Dusty
Explanation: The well known Pleiades star cluster is slowly destroying part of a passing cloud of gas and dust. The Pleiades is the brightest open cluster of stars on Earth's sky and can be seen from almost any northerly location with the unaided eye. The passing young dust cloud is thought to be part of Gould's belt, an unusual ring of young star formation surrounding the Sun in the local Milky Way Galaxy. Over the past 100,000 years, part of Gould's belt is by chance moving right through the older Pleiades and is causing a strong reaction between stars and dust. Pressure from the stars' light significantly repels the dust in the surrounding blue reflection nebula, with smaller dust particles being repelled more strongly. A short-term result is that parts of the dust cloud have become filamentary and stratified, as seen in the above deep-exposure image.

APOD: 2014 February 22 - M44: The Beehive Cluster
Explanation: A mere 600 light-years away, M44 is one of the closest star clusters to our solar system. Also known as the Praesepe or the Beehive cluster its stars are young though, about 600 million years old compared to our Sun's 4.5 billion years. Based on similar ages and motion through space, M44 and the even closer Hyades star cluster in Taurus are thought to have been born together in the same large molecular cloud. An open cluster spanning some 15 light-years, M44 holds 1,000 stars or so and covers about 3 full moons (1.5 degrees) on the sky in the constellation Cancer. Visible to the unaided eye, M44 has been recognized since antiquity. Described as a faint cloud or celestial mist long before being included as the 44th entry in Charles Messier's 18th century catalog, the cluster was not resolved into its individual stars until telescopes were available. A popular target for modern, binocular-equiped sky gazers, the cluster's few yellowish tinted, cool, red giants are scattered through the field of its brighter hot blue main sequence stars in this colorful stellar group snapshot.

APOD: 2014 January 23 - Double Cluster in Perseus
Explanation: This lovely starfield spans some seven full moons (about 3.5 degrees) across the heroic northern constellation of Perseus. Just right of center it holds the famous pair of open or galactic star clusters, h and Chi Persei. Also cataloged as NGC 869 (right) and NGC 884, both clusters are about 7,000 light-years away and contain stars much younger and hotter than the Sun. Separated by only a few hundred light-years, the clusters are both 13 million years young based on the ages of their individual stars, evidence that they were likely a product of the same star-forming region. Always a rewarding sight in binoculars, the Double Cluster is even visible to the unaided eye from dark locations. Not seen in binoculars though, and not often depicted in telescopic images of the region are faint clouds of reddish ionized hydrogen gas found throughout this remarkable cosmic skyscape. A color composite, the image includes narrowband data to enhance emission from the hydrogen clouds. Visible toward the upper left of the wide field of view is another, smaller open star cluster, NGC 957, also of similar age, distance, and possibly related to the more famous Double Cluster in Perseus.

APOD: 2014 January 7 - M7: Open Star Cluster in Scorpius
Explanation: M7 is one of the most prominent open clusters of stars on the sky. The cluster, dominated by bright blue stars, can be seen with the naked eye in a dark sky in the tail of the constellation of the Scorpion (Scorpius). M7 contains about 100 stars in total, is about 200 million years old, spans 25 light-years across, and lies about 1000 light-years away. The above deep image, taken last June from Hungary through a small telescope, combines over 60 two-minute exposures. The M7 star cluster has been known since ancient times, being noted by Ptolemy in the year 130 AD. Also visible are a dark dust cloud and literally millions of unrelated stars towards the Galactic center.

APOD: 2013 October 26 - NGC 7789: Caroline's Rose
Explanation: Found among the rich starfields of the Milky Way toward the constellation Cassiopeia, star cluster NGC 7789 lies about 8,000 light-years away. A late 18th century deep sky discovery of astronomer Caroline Lucretia Herschel, the cluster is also known as Caroline's Rose. Its suggestive appearance is created by the cluster's nestled complex of stars and voids. Now estimated to be 1.6 billion years young, the galactic or open cluster of stars also shows its age. All the stars in the cluster were likely born at the same time, but the brighter and more massive ones have more rapidly exhausted the hydrogen fuel in their cores. These have evolved from main sequence stars like the Sun into the many red giant stars shown with a yellowish cast in this lovely color composite. Using measured color and brightness, astronomers can model the mass and hence the age of the cluster stars just starting to "turn off" the main sequence and become red giants. Over 50 light-years across, Caroline's Rose spans about half a degree (the angular size of the moon) near the center of the wide-field telescopic image.

APOD: 2013 October 8 - The Bubble and M52
Explanation: To the eye, this cosmic composition nicely balances the Bubble Nebula at the lower left with open star cluster M52 above it and to the right. The pair would be lopsided on other scales, though. Embedded in a complex of interstellar dust and gas and blown by the winds from a single, massive O-type star, the Bubble Nebula, also known as NGC 7635, is a mere 10 light-years wide. On the other hand, M52 is a rich open cluster of around a thousand stars. The cluster is about 25 light-years across. Seen toward the northern boundary of Cassiopeia, distance estimates for the Bubble Nebula and associated cloud complex are around 11,000 light-years, while star cluster M52 lies nearly 5,000 light-years away. The wide telescopic field of view spans about two degrees on the sky or four times the apparent size of the Full Moon.

APOD: 2013 September 18 - M45: The Pleiades Star Cluster
Explanation: Have you ever seen the Pleiades star cluster? Even if you have, you probably have never seen it as dusty as this. Perhaps the most famous star cluster on the sky, the bright stars of the Pleiades can be seen without binoculars from even the depths of a light-polluted city. With a long exposure from a dark location, though, the dust cloud surrounding the Pleiades star cluster becomes very evident. The above exposure took about 20 minutes and covers a sky area several times the size of the full moon. Also known as the Seven Sisters and M45, the Pleiades lies about 400 light years away toward the constellation of the Bull (Taurus). A common legend with a modern twist is that one of the brighter stars faded since the cluster was named, leaving only six stars visible to the unaided eye. The actual number of Pleiades stars visible, however, may be more or less than seven, depending on the darkness of the surrounding sky and the clarity of the observer's eyesight.

APOD: 2013 August 6 - In the Vicinity of the Cone Nebula
Explanation: Strange shapes and textures can be found in neighborhood of the Cone Nebula. The unusual shapes originate from fine interstellar dust reacting in complex ways with the energetic light and hot gas being expelled by the young stars. The brightest star on the right of the above picture is S Mon, while the region just below it has been nicknamed the Fox Fur Nebula for its color and structure. The blue glow directly surrounding S Mon results from reflection, where neighboring dust reflects light from the bright star. The red glow that encompasses the whole region results not only from dust reflection but also emission from hydrogen gas ionized by starlight. S Mon is part of a young open cluster of stars named NGC 2264, located about 2500 light years away toward the constellation of the Unicorn (Monoceros). Even though it points right at S Mon, details of the origin of the mysterious geometric Cone Nebula, visible on the far left, remain a mystery.

APOD: 2013 January 13 - NGC 602 and Beyond
Explanation: Near the outskirts of the Small Magellanic Cloud, a satellite galaxy some 200 thousand light-years distant, lies 5 million year young star cluster NGC 602. Surrounded by natal gas and dust, NGC 602 is featured in this stunning Hubble image of the region. Fantastic ridges and swept back shapes strongly suggest that energetic radiation and shock waves from NGC 602's massive young stars have eroded the dusty material and triggered a progression of star formation moving away from the cluster's center. At the estimated distance of the Small Magellanic Cloud, the picture spans about 200 light-years, but a tantalizing assortment of background galaxies are also visible in the sharp Hubble view. The background galaxies are hundreds of millions of light-years or more beyond NGC 602.

APOD: 2013 January 3 - Open Star Clusters M35 and NGC 2158
Explanation: Open clusters of stars can be near or far, young or old, and diffuse or compact. Found near the plane of our Milky Way galaxy, they contain from 100 to 10,000 stars, all of which formed at nearly the same time. Bright blue stars frequently distinguish younger open clusters. M35, on the upper left, is relatively nearby at 2800 light years distant, relatively young at 150 million years old, and relatively diffuse, with about 2500 stars spread out over a volume 30 light years across. An older and more compact open cluster, NGC 2158, is at the lower right. NGC 2158 is four times more distant than M35, over 10 times older, and much more compact with many more stars in roughly the same volume of space. NGC 2158's bright blue stars have self-destructed, leaving cluster light to be dominated by older and yellower stars. Both clusters are seen toward the constellation of Gemini.

APOD: 2013 January 1 - A Double Star Cluster
Explanation: Few star clusters are seen to be so close to each other. Some 7,000 light-years away, though, this pair of open or galactic star clusters is an easy binocular target, a lovely starfield in the northern constellation Perseus. Also visible to the unaided eye from dark sky areas, it was cataloged in 130 BC by Greek astronomer Hipparchus. Now known as h and chi Persei, or NGC 869 (above right) and NGC 884, the clusters themselves are separated by only a few hundred light-years and contain stars much younger and hotter than the Sun. In addition to being physically close together, the clusters' ages based on their individual stars are similar - evidence that both clusters were likely a product of the same star-forming region.

APOD: 2012 December 24 - Hyades for the Holidays
Explanation: Recognized since antiquity and depicted on the shield of Achilles according to Homer, stars of the Hyades cluster form the head of the constellation Taurus the Bull. Their general V-shape is anchored by Aldebaran, the eye of the Bull and by far the constellation's brightest star. Yellowish in appearance, red giant Aldebaran is not a Hyades cluster member, though. Modern astronomy puts the Hyades cluster 151 light-years away making it the nearest established open star cluster, while Aldebaran lies at less than half that distance, along the same line-of-sight. Along with colorful Hyades stars, this stellar holiday portrait locates Aldebaran just below center, as well as another open star cluster in Taurus, NGC 1647 at the left, some 2,000 light-years or more in the background. Just slide your cursor over the image to identify the stars. The central Hyades stars are spread out over about 15 light-years. Formed some 800 million years ago, the Hyades star cluster may share a common origin with M44 (Praesepe), a naked-eye open star cluster in Cancer, based on M44's motion through space and remarkably similar age.

APOD: 2012 September 12 - M7: Open Star Cluster in Scorpius
Explanation: M7 is one of the most prominent open clusters of stars on the sky. The cluster, dominated by bright blue stars, can be seen with the naked eye in a dark sky in the tail of the constellation of the Scorpion (Scorpius). M7 contains about 100 stars in total, is about 200 million years old, spans 25 light-years across, and lies about 1000 light-years away. The above deep exposure was taken from Hakos Farm in Namibia. The M7 star cluster has been known since ancient times, being noted by Ptolemy in the year 130 AD. Also visible are a dark dust cloud and literally millions of unrelated stars towards the Galactic center.

APOD: 2012 February 15 - Meropes Reflection Nebula
Explanation: Reflection nebulas reflect light from a nearby star. Many small carbon grains in the nebula reflect the light. The blue color typical of reflection nebula is caused by blue light being more efficiently scattered by the carbon dust than red light. The brightness of the nebula is determined by the size and density of the reflecting grains, and by the color and brightness of the neighboring star(s). NGC 1435, pictured above, surrounds Merope (23 Tau), one of the brightest stars in the Pleiades (M45). The Pleiades nebulosity is caused by a chance encounter between an open cluster of stars and a dusty molecular cloud.

APOD: 2011 December 13 - In the Vicinity of the Cone Nebula
Explanation: Strange shapes and textures can be found in neighborhood of the Cone Nebula. The unusual shapes originate from fine interstellar dust reacting in complex ways with the energetic light and hot gas being expelled by the young stars. The brightest star on the right of the above picture is S Mon, while the region just below it has been nicknamed the Fox Fur Nebula for its color and structure. The blue glow directly surrounding S Mon results from reflection, where neighboring dust reflects light from the bright star. The red glow that encompasses the whole region results not only from dust reflection but also emission from hydrogen gas ionized by starlight. S Mon is part of a young open cluster of stars named NGC 2264, located about 2500 light years away toward the constellation of the Unicorn (Monoceros). The origin of the mysterious geometric Cone Nebula, visible on the far left, remains a mystery.

APOD: 2011 November 2 - NGC 7380: The Wizard Nebula
Explanation: What powers are being wielded in the Wizard Nebula? Gravitation strong enough to form stars, and stellar winds and radiations powerful enough to create and dissolve towers of gas. Located only 8,000 light years away, the Wizard nebula, pictured above, surrounds developing open star cluster NGC 7380. Visually, the interplay of stars, gas, and dust has created a shape that appears to some like a fictional medieval sorcerer. The active star forming region spans 100 about light years, making it appear larger than the angular extent of the Moon. The Wizard Nebula can be located with a small telescope toward the constellation of the King of Aethiopia (Cepheus). Although the nebula may last only a few million years, some of the stars being formed may outlive our Sun.

APOD: 2011 September 21 - Pleiades Deep Field
Explanation: Have you ever seen the Pleiades star cluster? Even if you have, you probably have never seen it like this: all dusty. Perhaps the most famous star cluster on the sky, the bright stars of the Pleiades can be seen without binoculars from even the depths of a light-polluted city. With a long exposure from a dark location, though, the dust cloud surrounding the Pleiades star cluster becomes very evident. The above exposure took about 30 hours and covers a sky area several times the size of the full moon. Also known as the Seven Sisters and M45, the Pleiades lies about 400 light years away toward the constellation of the Bull (Taurus). A common legend with a modern twist is that one of the brighter stars faded since the cluster was named, leaving only six stars visible to the unaided eye. The actual number of Pleiades stars visible, however, may be more or less than seven, depending on the darkness of the surrounding sky and the clarity of the observer's eyesight.

APOD: 2011 September 14 - The Bubble and M52
Explanation: To the eye, this cosmic composition nicely balances the Bubble Nebula at the lower right with open star cluster M52. The pair would be lopsided on other scales, though. Embedded in a complex of interstellar dust and gas and blown by the winds from a single, massive O-type star, the Bubble Nebula, also known as NGC 7635, is a mere 10 light-years wide. On the other hand, M52 is a rich open cluster of around a thousand stars. The cluster is about 25 light-years across. Seen toward the northern boundary of Cassiopeia, distance estimates for the Bubble Nebula and associated cloud complex are around 11,000 light-years, while star cluster M52 lies nearly 5,000 light-years away. The wide telescopic field of view spans about 1.5 degrees on the sky or three times the apparent size of the Full Moon.

APOD: 2011 September 6 - M6: The Butterfly Cluster
Explanation: To some, the outline of the open cluster of stars M6 resembles a butterfly. M6, also known as NGC 6405, spans about 20 light-years and lies about 2,000 light years distant. M6 can best be seen in a dark sky with binoculars towards the constellation of Scorpius, coving about as much of the sky as the full moon. Like other open clusters, M6 is composed predominantly of young blue stars, although the brightest star is nearly orange. M6 is estimated to be about 100 million years old. Determining the distance to clusters like M6 helps astronomers calibrate the distance scale of the universe.

APOD: 2011 September 3 - Comet Garradd Passes Ten Thousand Stars
Explanation: Comet Garradd continues to brighten as it drifts across the northern sky. Last week the comet, visible with binoculars and discernible by its green coma, passed nearly in front of globular cluster M71. M71 was once thought to be an open cluster, but is now known to be an older globular cluster containing over 10,000 stars. The photogenic duo was captured with a standard digital camera in a 10-minute, wide-angle exposure toward the northern constellation of the Arrow (Sagitta). The stars Sham (alpha Sagittae), beta Sagittae, gamma Sagittae, and the double star delta Sagittae are all visible in a diagonal band running down from the upper left. Comet C/2009 P1 (Garradd), will remain visible in northern skies for months and will reach its closest approach to the Sun in December.

APOD: 2011 August 25 - Portrait of NGC 281
Explanation: Look through the cosmic cloud cataloged as NGC 281 and it's almost easy to miss stars of open cluster IC 1590. But, formed within the nebula, that cluster's young, massive stars ultimately power the pervasive nebular glow. The eye-catching shapes looming in this portrait of NGC 281 are sculpted columns and dense dust globules seen in silhouette, eroded by intense, energetic winds and radiation from the hot cluster stars. If they survive long enough, the dusty structures could also be sites of future star formation. Playfully called the Pacman Nebula because of its overall shape, NGC 281 is about 10,000 light-years away in the constellation Cassiopeia. This composite image was made through narrow-band filters, but combines emission from the nebula's hydrogen, sulfur, and oxygen atoms in a visible spectrum palette. It spans over 80 light-years at the estimated distance of NGC 281.

APOD: 2011 February 14 - The Rosette Nebula
Explanation: Would the Rosette Nebula by any other name look as sweet? The bland New General Catalog designation of NGC 2237 doesn't appear to diminish the appearance of this flowery emission nebula. Inside the nebula lies an open cluster of bright young stars designated NGC 2244. These stars formed about four million years ago from the nebular material and their stellar winds are clearing a hole in the nebula's center, insulated by a layer of dust and hot gas. Ultraviolet light from the hot cluster stars causes the surrounding nebula to glow. The Rosette Nebula spans about 100 light-years across, lies about 5000 light-years away, and can be seen with a small telescope towards the constellation of the Unicorn (Monoceros).

APOD: 2010 December 2 - Hartley 2 Star Cluster Tour
Explanation: Early in November, small but active Comet Hartley 2 (103/P Hartley) became the fifth comet imaged close-up by a spacecraft from planet Earth. Continuing its own tour of the solar system with a 6 year orbital period, Hartley 2 is now appearing in the nautical constellation Puppis. Still a target for binoculars or small telescopes from dark sky locations, the comet is captured in this composite image from November 27, sharing the rich 2.5 degree wide field of view with some star clusters well known to earthbound skygazers. Below and right of the comet's alluring green coma lies bright M47, a young open star cluster some 80 milion years old, about 1,600 light-years away. Below and left open cluster M46 is older, around 300 million years of age, and 5,400 light-years distant. Hartley 2's short, faint tail even extends up and right toward another fainter star cluster in the scene, NGC 2423. On November 27, Comet Hartley 2 was about 2.25 light-minutes from Earth. Sweeping toward the bottom of this field, by November 28 the comet's path had carried it between M46 and M47.

APOD: 2010 October 26 - Comet Hartley Passes a Double Star Cluster
Explanation: Most star clusters are singularly impressive. Open clusters NGC 869 and NGC 884, however, are doubly impressive. Also known as "h and chi Persei", this unusual double cluster, shown above, is bright enough to be seen from a dark location without even binoculars. Although their discovery surely predates written history, the Greek astronomer Hipparchus notably cataloged the double cluster. The clusters are over 7,000 light years distant toward the constellation of Perseus, but are separated by only hundreds of light years. Captured earlier this month, the bright comet 103P/Hartley, informally called Comet Hartley 2, passed well in front but only a few degrees away from the famous double cluster. Comet Hartley 2, visible on the right, is now fading but still discernable to northern observers with binoculars. No binoculars are needed, of course, if you go right up to the comet's nucleus, as is the plan for NASA's EPOXI spacecraft on November 4.

APOD: 2010 August 17 - NGC 4755: A Jewel Box of Stars
Explanation: The great variety of star colors in this open cluster underlies its name: The Jewel Box. One of the bright central stars is a red supergiant, in contrast to the many blue stars that surround it. The cluster, also known as Kappa Crucis contains just over 100 stars, and is about 10 million years old. Open clusters are younger, contain few stars, and contain a much higher fraction of blue stars than do globular clusters. This Jewel Box lies about 6,400 light-years away, so the light that we see today was emitted from the cluster before even the Great Pyramids in Egypt were built. The Jewel Box, pictured above, spans about 20 light-years, and can be seen with binoculars towards the southern constellation of the cross (Crux).

APOD: 2010 February 14 - Field of Rosette
Explanation: What surrounds the florid Rosette nebula? To better picture this area of the sky, the famous flowery emission nebula on the far right has been captured recently in a deep and dramatic wide field image that features several other sky highlights. Designated NGC 2237, the center of the Rosette nebula is populated by the bright blue stars of open cluster NGC 2244, whose winds and energetic light are evacuating the nebula's center. Below the famous flower, a symbol of Valentine's Day, is a column of dust and gas that appears like a rose's stem but extends hundreds of light years. Across the above image, the bright blue star just left and below the center is called S Monocerotis. The star is part of the open cluster of stars labelled NGC 2264 and known as the Snowflake cluster. To the right of S Mon is a dark pointy featured called the Cone nebula, a nebula likely shaped by winds flowing out a massive star obscured by dust. To the left of S Mon is the Fox Fur nebula, a tumultuous region created by the rapidly evolving Snowflake cluster. The Rosette region, at about 5,000 light years distant, is about twice as far away as the region surrounding S Mon. The entire field can be seen with a small telescope toward the constellation of the Unicorn (Monoceros).

APOD: 2010 February 11 - Star Cluster M34
Explanation: This pretty, open cluster of stars, M34, is about the size of the Full Moon on the sky. Easy to appreciate in small telescopes, it lies some 1,800 light-years away in the constellation Perseus. At that distance, M34 physically spans about 15 light-years. Formed at the same time from the same cloud of dust and gas, all the stars of M34 are about 200 million years young. But like any open star cluster orbiting in the plane of our galaxy, M34 will eventually disperse as it experiences gravitational tides and encounters with the Milky Way's interstellar clouds and other stars. Over four billion years ago, our own Sun was likely formed in a similar open star cluster.

APOD: 2010 January 13 - The Spider and the Fly
Explanation: Bright clusters and nebulae abound in the ancient northern constellation of Auriga. The region includes the open star cluster M38, emission nebula IC 410 with Tadpoles, Auriga's own Flaming Star Nebula IC 405, and this interesting pair IC 417 (lower left) and NGC 1931. An imaginative eye toward the expansive IC 417 and diminutive NGC 1931 suggests a cosmic spider and fly. About 10,000 light-years distant, both represent young, open star clusters formed in interstellar clouds and still embedded in glowing hydrogen gas. For scale, the more compact NGC 1931 is about 10 light-years across.

APOD: 2009 December 4 - The Double Cluster
Explanation: A lovely starfield in the heroic northern constellation Perseus holds this famous pair of open or galactic star clusters, h and Chi Persei. Also cataloged as NGC 869 (right) and NGC 884, both clusters are about 7,000 light-years away and contain stars much younger and hotter than the Sun. Separated by only a few hundred light-years, the clusters' ages based on their individual stars are similar - evidence that they were likely a product of the same star-forming region. Always a rewarding sight in binoculars, the Double Cluster is even visible to the unaided eye from dark locations. Star colors (and spikes) are enhanced in this beautiful, wide field, telescopic image.

APOD: 2009 November 8 - M7: Open Star Cluster in Scorpius
Explanation: M7 is one of the most prominent open clusters of stars on the sky. The cluster, dominated by bright blue stars, can be seen with the naked eye in a dark sky in the tail of the constellation of the Scorpion (Scorpius). M7 contains about 100 stars in total, is about 200 million years old, spans 25 light-years across, and lies about 1000 light-years away. The above deep exposure was taken last month over several nights from Yalbraith, NSW, Australia. The M7 star cluster has been known since ancient times, being noted by Ptolemy in the year 130 AD. Also visible are a dark dust cloud and literally millions of unrelated stars towards the Galactic center.

APOD: 2009 October 30 - The Bubble and M52
Explanation: To the eye, this cosmic composition nicely balances the Bubble Nebula at the upper right with open star cluster M52. The pair would be lopsided on other scales, though. Embedded in a complex of interstellar dust and gas and blown by the winds from a single, massive O-type star, the Bubble Nebula (aka NGC 7635) is a mere 10 light-years wide. On the other hand, M52 is a rich open cluster of around a thousand stars. The cluster is about 25 light-years across. Seen toward the northern boundary of Cassiopeia, distance estimates for the Bubble Nebula and associated cloud complex are around 11,000 light-years, while star cluster M52 lies nearly 5,000 light-years away.

APOD: 2009 October 14 - Pleiades and Stardust
Explanation: Have you ever seen the Pleiades star cluster? Perhaps the most famous star cluster on the sky, the Pleiades can be seen without binoculars from even the depths of a light-polluted city. Also known as the Seven Sisters and M45, the Pleiades is one of the brightest and closest open clusters. Hurtling through a cosmic dust cloud a mere 400 light-years away, the Pleiades or Seven Sisters star cluster is well-known for its striking blue reflection nebulae. This remarkable wide-field (3 degree) image of the region shows the famous star cluster near the center, while highlighting lesser known dusty reflection nebulas nearby, across an area that would span over 20 light-years. In this case, the sister stars and cosmic dust clouds are not related, they just happen to be passing through the same region of space.

APOD: 2009 October 6 - The Lagoon Nebula from GigaGalaxy Zoom
Explanation: The large majestic Lagoon Nebula is home for many young stars and hot gas. Spanning 100 light years across while lying only about 5000 light years distant, the Lagoon Nebulae is so big and bright that it can be seen without a telescope toward the constellation of Sagittarius. Many bright stars are visible from NGC 6530, an open cluster that formed in the nebula only several million years ago. The greater nebula, also known as M8 and NGC 6523, is named "Lagoon" for the band of dust seen to the left of the open cluster's center. A bright knot of gas and dust in the nebula's center is known as the Hourglass Nebula. The above picture is a newly released, digitally stitched panorama of M8 taken as part of the GigaGalaxy Zoom project by the Wide Field Imager attached to the MPG/ESO 2.2-meter Telescope at the La Silla Observatory in Chile. The vista spans three times the diameter of the Moon, while the highest resolution image version occupies over 350 million pixels. Star formation continues in the Lagoon Nebula as witnessed by the many globules that exist there.

APOD: 2009 August 31 - Open Cluster M25
Explanation: Many stars like our Sun were formed in open clusters. The above pictured open cluster, M25, contains thousands of stars and is about two thousand light years distant. The stars in this cluster all formed together about 90 million years ago. The bright young stars in M25 appear blue. Open clusters, also called galactic clusters, contain fewer and younger stars than globular clusters. Also unlike globular clusters, open clusters are generally confined to the plane of our Galaxy. M25 is visible with binoculars towards the constellation of the Archer ( Sagittarius).

APOD: 2009 April 12 - M39: Open Cluster in Cygnus
Explanation: Lying just at the limit of human perception is a picturesque starfield containing one of the larger open clusters on the northern sky. Spanning an angle larger than the Moon, M39's relatively few stars lie only about 800 light years distant toward the constellation of Cygnus. The above picture of M39 is a mosaic of 33 images taken by the WIYN telescope on Kitt Peak in Arizona, USA. The stars in M39 are all about 300 million years old, much younger than the 5,000 million years of our Sun. Open clusters, also called galactic clusters, contain fewer and younger stars than globular clusters. Also unlike globular clusters, open clusters are generally confined to the plane of our Galaxy.

APOD: 2009 March 26 - Stars Young and Old
Explanation: Galactic or open star clusters are relatively young. These swarms of stars are born near the plane of the Milky Way, but their numbers steadily dwindle as cluster members are strewn through the Galaxy by gravitational interactions. This bright open cluster, known as M46, is around 300 million years young and still contains a few hundred stars within its span of 30 light-years or so. Located about 5,000 light-years away toward the constellation Puppis, M46 also seems to contain a contradiction to its youthful status. In the lovely starscape, the colorful, circular patch just below the center of M46 (also inset at upper left) is the planetary nebula NGC 2438. Planetary nebulae are a brief, final phase in the life of a solar-type star a few billion years old whose central reservoir of hydrogen fuel has been exhausted. In fact, old NGC 2438 is estimated to be only 3,000 light-years distant and moves at a different speed than M46 cluster members. It likely represents a foreground object, only by chance appearing along our line-of-sight to young M46.

APOD: 2008 December 23 - Collinder 399: The Coat Hanger
Explanation: Is this coat hanger a star cluster or an asterism? This cosmic hang-up has been debated over much of last century, as astronomers wondered whether this binocular-visible object is really a physically associated open cluster or a chance projection. Chance star projections are known as asterisms, an example of which is the popular Big Dipper. Recent precise measurements from different vantage points in the Earth's orbit around the Sun have uncovered discrepant angular shifts indicating that the Coat Hanger is better described as an asterism. Known more formally as Collinder 399, this bright stellar grouping is wider than the full moon and lies in the constellation of the Fox (Vulpecula). On the far right of the image is the open cluster of stars NGC 6802.

APOD: 2008 December 10 - Portrait of NGC 281
Explanation: Look through the cosmic cloud cataloged as NGC 281 and it's almost easy to miss stars of open cluster IC 1590. But, formed within the nebula, that cluster's young, massive stars ultimately power the pervasive nebular glow. The eye-catching shapes looming in this colorful portrait of NGC 281 are sculpted columns and dense dust globules seen in silhouette, eroded by intense, energetic winds and radiation from the hot cluster stars. If they survive long enough, the dusty structures could also be sites of future star formation. Sometimes called the Pacman Nebula because of its overall shape in wider-field views, NGC 281 is about 10,000 light-years away in the constellation Cassiopeia. This composite image was made through narrow-band filters and shows emission from the nebula's hydrogen, sulfur, and oxygen atoms in green, red, and blue hues. It spans over 80 light-years at the estimated distance of NGC 281.

APOD: 2008 October 26 - Massive Stars in Open Cluster Pismis 24
Explanation: How massive can a normal star be? Estimates made from distance, brightness and standard solar models had given one star in the open cluster Pismis 24 over 200 times the mass of our Sun, making it a record holder. This star is the brightest object located just above the gas front in the above image. Close inspection of images taken recently with the Hubble Space Telescope, however, have shown that Pismis 24-1 derives its brilliant luminosity not from a single star but from three at least. Component stars would still remain near 100 solar masses, making them among the more massive stars currently on record. Toward the bottom of the image, stars are still forming in the associated emission nebula NGC 6357, including several that appear to be breaking out and illuminating a spectacular cocoon.

APOD: 2008 August 3 - Open Cluster NGC 290: A Stellar Jewel Box
Explanation: Jewels don't shine this bright -- only stars do. Like gems in a jewel box, though, the stars of open cluster NGC 290 glitter in a beautiful display of brightness and color. The photogenic cluster, pictured above, was captured recently by the orbiting Hubble Space Telescope. Open clusters of stars are younger, contain few stars, and contain a much higher fraction of blue stars than do globular clusters of stars. NGC 290 lies about 200,000 light-years distant in a neighboring galaxy called the Small Cloud of Magellan (SMC). The open cluster contains hundreds of stars and spans about 65 light years across. NGC 290 and other open clusters are good laboratories for studying how stars of different masses evolve, since all the open cluster's stars were born at about the same time.

APOD: 2007 December 9 - The Fairy of Eagle Nebula
Explanation: The dust sculptures of the Eagle Nebula are evaporating. As powerful starlight whittles away these cool cosmic mountains, the statuesque pillars that remain might be imagined as mythical beasts. Pictured above is one of several striking dust pillars of the Eagle Nebula that might be described as a gigantic alien fairy. This fairy, however, is ten light years tall and spews radiation much hotter than common fire. The greater Eagle Nebula, M16, is actually a giant evaporating shell of gas and dust inside of which is a growing cavity filled with a spectacular stellar nursery currently forming an open cluster of stars. The above image in scientifically re-assigned colors was released as part of the fifteenth anniversary celebration of the launch of the Hubble Space Telescope.

APOD: 2007 November 22 - Pleiades and Stardust
Explanation: Hurtling through a cosmic dust cloud a mere 400 light-years away, the lovely Pleiades or Seven Sisters star cluster is well-known for its striking blue reflection nebulae. This remarkable wide-field (3 degree) image of the region shows the famous star cluster at the right, while highlighting lesser known dusty reflection nebulae nearby, across an area that would span over 20 light-years. In this case, the sister stars and cosmic dust clouds are not related, they just happen to be passing through the same region of space. But astronomers using infrared detectors have recently found a dusty disk that really does belong to one young Pleiades star -- HD 23514. Surrounding HD 23514, the disk is estimated to be comparable in size to the terrestrial planet zone in our own solar system and likely represents the debris from the process of rocky planet formation.

APOD: 2007 November 18 - M45: The Pleiades Star Cluster
Explanation: Perhaps the most famous star cluster on the sky, the Pleiades can be seen without binoculars from even the depths of a light-polluted city. Also known as the Seven Sisters and M45, the Pleiades is one of the brightest and closest open clusters. The Pleiades contains over 3000 stars, is about 400 light years away, and only 13 light years across. Quite evident in the above photograph are the blue reflection nebulae that surround the brighter cluster stars. Low mass, faint, brown dwarfs have also been found in the Pleiades.

APOD: 2007 September 13 - NGC 7129 and NGC 7142
Explanation: This alluring telescopic image looks toward the constellation Cepheus and an intriguing visual pairing of dusty reflection nebula NGC 7129 (left) and open star cluster NGC 7142. The two appear separated by only half a degree on the sky, but they actually lie at quite different distances. In the foreground, dusty nebula NGC 7129 is about 3,000 light-years distant, while open cluster NGC 7142 is likely over 6,000 light-years away. In fact, the pervasive and clumpy foreground dust clouds in this region redden the light from NGC 7142, complicating astronomical studies of the cluster. Still, NGC 7142 is thought to be an older open star cluster, while the bright stars embedded in NGC 7129 are perhaps a million years young. The telltale reddish crescent shapes around NGC 7129 are associated with energetic jets streaming away from newborn stars. Surprisingly, despite the dust, far off background galaxies can be seen in the colorful cosmic vista.

APOD: 2007 August 9 - Star Cluster Messier 67
Explanation: Gathered at the center of this sharp skyview are the stars of Messier 67, one of the oldest known open star clusters. In fact, though open star clusters are usually much younger, the stars of M67 are likely around 4 billion years old, about the same age and with about the same elemental abundances as the Sun. Open clusters are almost always younger because they are dispersed over time as they encounter other stars, interstellar clouds, and experience gravitational tides while orbiting the center of our galaxy. Still, M67 contains over 500 stars or so and lies some 2,800 light-years away in the constellation Cancer. At that estimated distance, M67 would be about 12 light-years across.

APOD: 2007 May 9 - The Snowflake Cluster versus the Cone Nebula
Explanation: Strange shapes and textures can be found in the neighborhood of the Cone Nebula. These patterns result from the tumultuous unrest that accompanies the formation of the open cluster of stars known as NGC 2264, the Snowflake cluster. To better understand this process, a detailed image of this region was taken in two colors of infrared light by the orbiting Spitzer Space Telescope. Bright stars from the Snowflake cluster dot the field. These stars soon heat up and destroy the gas and dust mountains in which they formed. One such dust mountain is the famous Cone Nebula, visible in the above image on the left, pointing toward a bright star near the center of the field. The entire NGC 2264 region is located about 2,500 light years away toward the constellation of the Unicorn (Monoceros).

APOD: 2007 February 14 - The Rosette Nebula
Explanation: Would the Rosette Nebula by any other name look as sweet? The bland New General Catalog designation of NGC 2237 doesn't appear to diminish the appearance of the this flowery emission nebula. Inside the nebula lies an open cluster of bright young stars designated NGC 2244. These stars formed about four million years ago from the nebular material and their stellar winds are clearing a hole in the nebula's center, insulated by a layer of dust and hot gas. Ultraviolet light from the hot cluster stars causes the surrounding nebula to glow. The Rosette Nebula spans about 100 light-years across, lies about 5000 light-years away, and can be seen with a small telescope towards the constellation of Monoceros.

APOD: 2007 January 12 - Collinder 399: The Coat Hanger
Explanation: Is this coat hanger a star cluster or an asterism? This cosmic hang-up has been debated over much of last century, as astronomers wondered whether this binocular-visible object is really a physically associated open cluster or a chance projection. Chance star projections are known as asterisms, an example of which is the popular Big Dipper. Recent precise measurements from different vantage points in the Earth's orbit around the Sun have recently uncovered discrepant angular shifts indicating that the Coat Hanger is better described as an asterism. Known more formally as Collinder 399, this bright stellar grouping can be seen spanning more than a full moon toward the direction of the constellation of the Fox (Vulpecula). On the far right of the above image is the open cluster of stars NGC 6802.

APOD: 2006 December 20 - Star Forming Region NGC 6357
Explanation: For reasons unknown, NGC 6357 is forming some of the most massive stars ever discovered. Near the more obvious Cat's Paw nebula, NGC 6357 houses the open star cluster Pismis 24, home to these tremendously bright and blue stars. The overall red glow near the inner star forming region results from the emission of ionized hydrogen gas. The surrounding nebula, shown above, holds a complex tapestry of gas, dark dust, stars still forming, and newly born stars. The intricate patterns are caused by complex interactions between interstellar winds, radiation pressures, magnetic fields, and gravity. NGC 6357 spans about 400 light years and lies about 8,000 light years away toward the constellation of the Scorpion.

APOD: 2006 December 19 - Massive Stars in Open Cluster Pismis 24
Explanation: How massive can a normal star be? Estimates made from distance, brightness and standard solar models had given one star in the open cluster Pismis 24 over 200 times the mass of our Sun, making it a record holder. This star is the brightest object located just to the right of the gas front in the above image. Close inspection of images taken recently with the Hubble Space Telescope, however, have shown that Pismis 24-1 derives its brilliant luminosity not from a single star but from three at least. Component stars would still remain near 100 solar masses, making them among the more massive stars currently on record. Toward the image left, stars are still forming in the associated emission nebula NGC 6357, including several that appear to be breaking out and illuminating a spectacular cocoon.

APOD: 2006 October 27 - The Spider and The Fly
Explanation: Star clusters and nebulae abound in the ancient northern constellation Auriga - a region that includes the interesting pair NGC 1931 (lower left) and IC 417. In this gorgeous color image, an imaginative eye toward the expansive IC 417 and diminutive NGC 1931 suggests a cosmic spider and fly. About 10,000 light-years distant, both are young open star clusters formed in interstellar clouds and still embedded in glowing hydrogen gas. The more compact NGC 1931 is about 10 light-years across with contrasting blue hues characteristic of dust reflected starlight.

APOD: 2006 October 21 - Tombaugh 4
Explanation: Clyde Tombaugh discovered planet Pluto in 1930 while surveying the skies with the 13-inch Lawrence Lowell Telescope. But the skilled and careful astronomer also went on to discover star clusters, comets, asteroids, and clusters of galaxies. For example, pictured is galactic or open star cluster Tombaugh 4 in the northern constellation Cassiopeia. Published in 1941, Tombaugh's description, based on his photographic images from the Lowell 13-inch, indicates the cluster is small and faint, and comprised of about 30 stars. Using the apparent brightness of the cluster stars he estimated the distance to be 20 to 30 thousand light-years, making Tombaugh 4 over 10 light-years in diameter. This deep color image, made with a modern ccd camera and another 13-inch telescope, includes the region's foreground stars and faint nebulosities.

APOD: 2006 June 1 - Reflections on NGC 6188
Explanation: NGC 6188 is an interstellar carnival of young blue stars, hot red gas, and cool dark dust. Located 4,000 light years away in the disk of our Galaxy, NGC 6188 is home to the Ara OB1 association, a group of bright young stars whose nucleus forms the open cluster NGC 6193. These stars are so bright that some of their blue light reflects off of interstellar dust forming the diffuse blue glow surrounding the stars in the above photograph. Open cluster NGC 6193 formed about three million years ago from the surrounding gas, and appears unusually rich in close binary stars. The red glow visible throughout the photograph arises from hydrogen gas heated by the bright stars in Ara OB1. The dark dust that blocks much of NGC 6188's light was likely formed in the outer atmospheres of cooler stars and in supernovae ejecta.

APOD: 2006 May 1 - Open Cluster NGC 290: A Stellar Jewel Box
Explanation: Jewels don't shine this bright -- only stars do. Like gems in a jewel box, though, the stars of open cluster NGC 290 glitter in a beautiful display of brightness and color. The photogenic cluster, pictured above, was captured recently by the orbiting Hubble Space Telescope. Open clusters of stars are younger, contain few stars, and contain a much higher fraction of blue stars than do globular clusters of stars. NGC 290 lies about 200,000 light-years distant in a neighboring galaxy called the Small Cloud of Magellan (SMC). The open cluster contains hundreds of stars and spans about 65 light years across. NGC 290 and other open clusters are good laboratories for studying how stars of different masses evolve, since all the open cluster's stars were born at about the same time.

APOD: 2006 February 26 - Inside the Eagle Nebula
Explanation: From afar, the whole thing looks like an Eagle. A closer look at the Eagle Nebula, however, shows the bright region is actually a window into the center of a larger dark shell of dust. Through this window, a brightly-lit workshop appears where a whole open cluster of stars is being formed. In this cavity tall pillars and round globules of dark dust and cold molecular gas remain where stars are still forming. Already visible are several young bright blue stars whose light and winds are burning away and pushing back the remaining filaments and walls of gas and dust. The Eagle emission nebula, tagged M16, lies about 6500 light years away, spans about 20 light-years, and is visible with binoculars toward the constellation of Serpens. The above picture combines three specific emitted colors and was taken with the 0.9-meter telescope on Kitt Peak, Arizona, USA.

APOD: 2006 January 28 - Saturn in the Hive
Explanation: If you can find Saturn in tonight's sky, then you can also find M44, popularly known as the Beehive star cluster. In fact, with a pair of binoculars most casual skygazers should find it fairly easy to zero in on this celestial scene. Saturn is at opposition - opposite the Sun in Earth's sky - so, the bright planet rises in the east at sunset and is visible throughout the night. Near the stationary part of its wandering path through the heavens, Saturn will obligingly linger for a while in the vicinity of M44 in the relatively faint constellation Cancer. Seen here in a photograph from January 25, Saturn (lower right) is strongly overexposed with the stars of M44 swarming above and to the left. The picture approximately corresponds to the view when looking through a typical pair of binoculars. Saturn is about 64 light-minutes from our fair planet while M44, one of the closest star clusters, is around 600 light-years away.

APOD: 2006 January 9 - M45: The Pleiades Star Cluster
Explanation: Perhaps the most famous star cluster on the sky, the Pleiades can be seen without binoculars from even the depths of a light-polluted city. Also known as the Seven Sisters and M45, the Pleiades is one of the brightest and closest open clusters. The Pleiades contains over 3000 stars, is about 400 light years away, and only 13 light years across. Quite evident in the above photograph are the blue reflection nebulae that surround the brighter cluster stars. Low mass, faint, brown dwarfs have also been found in the Pleiades. (Editors' note: The prominent diffraction spikes are caused by the telescope itself and may be either distracting or provide aesthetic enhancement, depending on your point of view.)

APOD: 2005 November 18 - The 37 Cluster
Explanation: For the mostly harmless denizens of planet Earth, the brighter stars of open cluster NGC 2169 seem to form a cosmic 37. (Did you expect 42?.) Of course, the improbable numerical asterism appears solely by chance and lies at an estimated distance of 3,600 light-years toward the constellation Orion. As far as galactic or open star clusters go, NGC 2169 is a small one, spanning about 7 light-years. Formed at the same time from the same cloud of dust and gas, the stars of NGC 2169 are only about 8 million years old. Such clusters are expected to disperse over time as they encounter other stars, interstellar clouds, and experience gravitational tides while traveling through the galaxy. Over four billion years ago, our own Sun was likely formed in a similar open cluster of stars.

APOD: 2005 October 11 - NGC 869 and NGC 884: A Double Open Cluster
Explanation: Most star clusters are singularly impressive. Open clusters NGC 869 and NGC 884, however, are doubly impressive. Also known as "h and chi Persei", this unusual double cluster, shown above, is bright enough to be seen from a dark location without even binoculars. Although their discovery surely predates written history, the Greek astronomer Hipparchus notably cataloged the "double cluster". The clusters are over 7,000 light years distant toward the constellation of Perseus, but are separated by only hundreds of light years.

APOD: 2005 September 27 - The Star Pillars of Sharpless 171
Explanation: Towering pillars of cold gas and dark dust adorn the center star forming region of Sharpless 171. An open cluster of stars is forming there from the gas in cold molecular clouds. As energetic light emitted by young massive stars boils away the opaque dust, the region fragments and picturesque pillars of the remnant gas and dust form and slowly evaporate. The energetic light also illuminates the surrounding hydrogen gas, energize it to glow as a red emission nebula. Pictured above is the active central region of the Sharpless 171 greater emission nebula. Sharpless 171 incorporates NGC 7822 and the active region Cederblad 214, much of which is imaged above. The area above spans about 20 light years, lies about 3,000 light years away, and can be seen with a telescope toward the northern constellation of the King of Ethiopia (Cepheus).

APOD: 2005 August 23 - NGC 281: The Pacman Nebula
Explanation: NGC 281 is a busy workshop of star formation. Prominent features include a small open cluster of stars, a diffuse red-glowing emission nebula, large lanes of obscuring gas and dust, and dense knots of dust and gas in which stars may still be forming. The open cluster of stars IC 1590 visible around the center has formed only in the last few million years. The brightest member of this cluster is actually a multiple-star system shining light that helps ionize the nebula's gas, causing the red glow visible throughout. The lanes of dust visible left of center are likely homes of future star formation. Particularly striking in the above photograph are the dark Bok globules visible against the bright nebula. The NGC 281 system, dubbed the Pacman nebula for its overall shape, lies about 10 thousand light years distant.

APOD: 2005 August 4 - Stars Young and Old
Explanation: Galactic or open star clusters are relatively young swarms of bright stars born together near the plane of our Milky Way Galaxy. Separated by about a degree on the sky, two nice examples are M46 (upper left) 5,400 light-years in the distance and M47 (lower right) only 1,600 light-years away toward the nautical constellation Puppis. Around 300 million years young M46 contains a few hundred stars in a region about 30 light-years across. Aged 80 million years, M47 is a smaller but looser cluster of about 50 stars spanning 10 light-years. But this portrait of stellar youth also contains an ancient interloper. The small, colorful patch of glowing gas in M46 is actually the planetary nebula NGC 2438 - the final phase in the life of a sun-like star billions of years old. NGC 2438 is estimated to be only 3,000 light-years distant and likely represents a foreground object, only by chance appearing along our line of sight to youthful M46.

APOD: 2005 July 10 - In the Center of the Trapezium
Explanation: Start with the constellation of Orion. Near Orion's belt is a fuzzy area known as the Great Nebula of Orion or M42. In this nebula is a bright star cluster known as the Trapezium, shown above. New stellar systems are forming there in gigantic globs of gas and dust known as Proplyds. Looking closely at the above picture also reveals that gas and dust surrounding some of the dimmer stars appears to form structures that point away from the brighter stars. The above false color image was made by combining several exposures from the orbiting Hubble Space Telescope.

APOD: 2005 May 9 - Stars, Dust and Nebula in NGC 6559
Explanation: When stars form, pandemonium reigns. A textbook case is the star forming region NGC 6559. Visible above are red glowing emission nebulas of hydrogen, blue reflection nebulas of dust, dark absorption nebulas of dust, and the stars that formed from them. The first massive stars formed from the dense gas will emit energetic light and winds that erode, fragment, and sculpt their birthplace. And then they explode. The resulting morass can be as beautiful as it is complex. After tens of millions of years, the dust boils away, the gas gets swept away, and all that is left is a naked open cluster of stars.

APOD: 2005 April 6 - The M7 Open Star Cluster in Scorpius
Explanation: M7 is one of the most prominent open clusters of stars on the sky. The cluster, dominated by bright blue stars, can be seen with the naked eye in a dark sky in the tail of the constellation of Scorpius. M7 contains about 100 stars in total, is about 200 million years old, spans 25 light-years across, and lies about 1000 light-years away. This color picture was taken recently at the Kitt Peak National Observatory in Arizona, USA as part of the Advanced Observers Program. The M7 star cluster has been known since ancient times, being noted by Ptolemy in the year 130 AD. Also visible is a dark dust cloud near the bottom of the frame, and literally millions of unrelated stars towards the Galactic center.

APOD: 2005 February 14 - The Rosette Nebula
Explanation: Would the Rosette Nebula by any other name look as sweet? The bland New General Catalog designation of NGC 2237 doesn't appear to diminish the appearance of the this flowery emission nebula. Inside the nebula lies an open cluster of bright young stars designated NGC 2244. These stars formed about four million years ago from the nebular material and their stellar winds are clearing a hole in the nebula's center, insulated by a layer of dust and hot gas. Ultraviolet light from the hot cluster stars causes the surrounding nebula to glow. The Rosette Nebula spans about 100 light-years across, lies about 5000 light-years away, and can be seen with a small telescope towards the constellation of Monoceros.

APOD: 2005 January 3 - The Pleiades Star Cluster
Explanation: Perhaps the most famous star cluster on the sky, the Pleiades can be seen without binoculars from even the depths of a light-polluted city. Also known as the Seven Sisters and M45, the Pleiades is one of the brightest and closest open clusters. The Pleiades contains over 3000 stars, is about 400 light years away, and only 13 light years across. Quite evident in the above photograph are the blue reflection nebulae that surround the brighter cluster stars. Low mass, faint, brown dwarfs have also been found in the Pleiades. (Editors' note: The prominent diffraction spikes are caused by the telescope itself and may be either distracting or provide aesthetic enhancement, depending on your point of view.)

APOD: 2004 October 20 - NGC 281: Cluster, Clouds, and Globules
Explanation: NGC 281 is a busy workshop of star formation. Prominent features include a small open cluster of stars, a diffuse red-glowing emission nebula, large lanes of obscuring gas and dust, and dense knots of dust and gas in which stars may still be forming. The open cluster of stars IC 1590 visible around the center has formed only in the last few million years. The brightest member of this cluster is actually a multiple-star system shining light that helps ionize the nebula's gas, causing the red glow visible throughout. The lanes of dust visible left of center are likely homes of future star formation. Particularly striking in the above photograph are the dark Bok globules visible against the bright nebula. The entire NGC 281 system lies about 10 thousand light years distant.

APOD: 2004 October 4 - NGC 6823: Cloud Sculpting Star Cluster
Explanation: Star cluster NGC 6823 is ready for its close-up. The center of the open cluster, visible on the upper right, formed only about two million years ago and is dominated in brightness by a host of bright young blue stars. Outer parts of the cluster, visible above in the image center as the stars and pillars of emission nebula NGC 6820, contain even younger stars. The huge pillars of gas and dust likely get their elongated shape by erosion from hot radiation emitted from the brightest cluster stars. Striking dark globules of gas and dust are also visible across the bottom of this image by the 25 year old Canada France Hawaii Telescope. Open star cluster NGC 6823 spans about 50 light years and lies about 6000 light years away toward the constellation of Vulpecula (The Fox).

APOD: 2004 September 21 - M24: A Sagittarius Starscape
Explanation: Many vast star fields in the plane of our Milky Way Galaxy are rich in clouds of dust, and gas. First and foremost, visible in the above picture are millions of stars, many of which are similar to our Sun. Next huge filaments of dark interstellar dust run across the image and block the light from millions of more stars yet further across our Galaxy. The bright red region on the left is part of the Omega Nebula, an emission nebula of mostly hot hydrogen gas also known as M17. A small bright grouping of stars near the image center is the open cluster M18, while the long bright streak of stars just right of center is M24. On the far right of the image is the picturesque red emission nebula IC 1283 flanked by two blue reflection nebulas NGC 6589 and NGC 6590. These objects are visible with a small telescope toward the constellation of Sagittarius.

APOD: 2004 June 17 - Comet NEAT and the Beehive Cluster
Explanation: To the unaided eye, they appeared as similar fuzzy patches. But when a bright comet passed in front of a bright star cluster last month, binoculars and cameras were able to show off their marked differences in dramatic fashion. Pictured above, the bright comet, C/2001 Q4 (NEAT) shows many details of its coma and tail, while far in the distance the Beehive open cluster, M44, shows many of its stars. Comet Q4 has now faded to the edge of unaided visibility and can best be found with a sky map and binoculars from the Northern Hemisphere well into June. Star cluster M44 will remain an impressive star cluster toward the constellation of Cancer indefinitely.

APOD: 2004 March 31 - M39: Open Cluster in Cygnus
Explanation: Lying just at the limit of human perception is a picturesque starfield containing one of the larger open clusters on the northern sky. Spanning an angle larger than the Moon, M39's relatively few stars lie only about 800 light years distant toward the constellation of Cygnus. The above picture of M39 is a mosaic of 33 images taken by the WIYN telescope on Kitt Peak in Arizona, USA. The stars in M39 are all about 300 million years old, much younger than the 5000 million years of our Sun. Open clusters, also called galactic clusters, contain fewer and younger stars than globular clusters. Also unlike globular clusters, open clusters are generally confined to the plane of our Galaxy.

APOD: 2004 February 22 - The M7 Open Star Cluster in Scorpius
Explanation: M7 is one of the most prominent open clusters of stars on the sky. The cluster, dominated by bright blue stars, can be seen with the naked eye in a dark sky in the tail of the constellation of Scorpius. M7 contains about 100 stars in total, is about 200 million years old, spans 25 light-years across, and lies about 1000 light-years away. This color picture was taken in 1995 at the Burrell-Schmidt Telescope at Kitt Peak National Observatory in Arizona. The M7 star cluster has been known since ancient times, being noted by Ptolemy in the year 130 AD. Also visible is a dark dust cloud near the bottom of the frame, and literally millions of unrelated stars towards the Galactic center.

APOD: 2003 December 27 - The Pleiades Star Cluster
Explanation: Perhaps the most famous star cluster on the sky, the Pleiades can be seen without binoculars from even the depths of a light-polluted city. Also known as the Seven Sisters and M45, the Pleiades is one of the brightest and closest open clusters. The Pleiades contains over 3000 stars, is about 400 light years away, and only 13 light years across. Quite evident in the above photograph are the blue reflection nebulae that surround the brighter cluster stars. Low mass, faint, brown dwarfs have also been found in the Pleiades. (Editors' note: The prominent diffraction spikes were added to the image for aesthetic reasons, produced by kite string donated by Rob Gendler's kids and placed over the telescope dew shield.)

APOD: 2003 December 15 - Open Star Clusters M35 and NGC 2158
Explanation: Open clusters of stars can be near or far, young or old, and diffuse or compact. Open clusters may contain from 100 to 10,000 stars, all of which formed at nearly the same time. Bright blue stars frequently distinguish younger open clusters. M35, pictured above on the upper left, is relatively nearby at 2800 light years distant, relatively young at 150 million years old, and relatively diffuse, with about 2500 stars spread out over a volume 30 light years across. An older and more compact open cluster, NGC 2158, is visible above on the lower right. NGC 2158 is four times more distant that M35, over 10 times older, and much more compact as it contains many more stars in roughly the same volume of space. NGC 2158's bright blue stars have self-destructed, leaving cluster light to be dominated by older and yellower stars. Both clusters are visible toward the constellation of Gemini -- M35 with binoculars and NGC 2158 with a small telescope.

APOD: 2003 December 2 - NGC 869 and NGC 884: A Double Open Cluster
Explanation: Most star clusters are singularly impressive. Open clusters NGC 869 and NGC 884, however, are doubly impressive. Also known as "h and chi Persei", this unusual double cluster, shown above, is bright enough to be seen from a dark location without even binoculars. Although their discovery surely predates written history, the Greek astronomer Hipparchus notably cataloged the "double cluster". The clusters are over 7000 light years distant toward the constellation of Perseus, but are separated by only hundreds of light years.

APOD: 2003 September 8 - Stars and Dust of the Lagoon Nebula
Explanation: The large majestic Lagoon Nebula is home for many young stars and hot gas. Spanning 100 light years across while lying only about 5000 light years distant, the Lagoon Nebulae is so big and bright that it can be seen without a telescope toward the constellation of Sagittarius. Many bright stars are visible from NGC 6530, an open cluster that formed in the nebula only several million years ago. The greater nebula, also known as M8 and NGC 6523, is named "Lagoon" for the band of dust seen to the left of the open cluster's center. A bright knot of gas and dust in the nebula's center is known as the Hourglass Nebula. The above picture is a digitally sharpened composite of exposures taken in specific colors of light emitted by sulfur (red), hydrogen (green), and oxygen (blue). Star formation continues in the Lagoon Nebula as witnessed by the many globules that exist there.

APOD: 2003 April 29 - In the Center of the Rosette Nebula
Explanation: In the heart of the Rosette Nebula lies a bright open cluster of stars that lights up the nebula. The stars of NGC 2244 formed from the surrounding gas only a few million years ago. This just-released image taken by the CFHT's new MegaPrime camera shows the region in unprecedented detail. Although the emission nebula is dominated by red hydrogen light, the above image has exaggerated the effect of green light emitted primarily by small amounts of oxygen. A hot wind of particles streams away from the cluster stars and contributes to an already complex menagerie of gas and dust filaments while slowly evacuating the cluster center. The Rosette Nebula's center measures about 50 light-years across, lies about 4500 light-years away, and is visible with binoculars towards the constellation of Monoceros.

APOD: 2003 April 7 - NGC 281: Cluster, Clouds, and Globules
Explanation: NGC 281 is a busy workshop of star formation. Prominent features include a small open cluster of stars, a diffuse red-glowing emission nebula, large lanes of obscuring gas and dust, and dense knots of dust and gas in which stars may still be forming. The open cluster of stars IC 1590 visible around the center has formed only in the last few million years. The brightest member of this cluster is actually a multiple-star system shining light that helps ionize the nebula's gas, causing the red glow visible throughout. The lanes of dust visible below the center are likely homes of future star formation. Particularly striking in the above photograph are the dark Bok globules visible against the bright nebula. Stars are surely forming there right now. The entire NGC 281 system lies about 10 thousand light years distant.

APOD: 2003 April 3 - Jupiter in the Hive
Explanation: If you can find planet Jupiter in tonight's sky, then you can also find M44, popularly known as the Beehive star cluster. In fact, with a pair of binoculars most casual skygazers should find it easy to zero in on this celestial scene. It should be easy because after sunset Jupiter presently rules the night as the brightest "star" overhead. Now near the stationary part of its wandering path through the heavens, Jupiter will obligingly linger for a while at a spot only a degree or so southeast of M44 in the relatively faint constellation Cancer. Seen here in a photograph from March 28, Jupiter (lower left) is strongly overexposed with the stars of M44 swarming above and to the right. The picture approximately corresponds to the view when looking through a typical pair of binoculars. Jupiter is about 30 light-minutes from our fair planet while M44, one of the closest star clusters, is around 600 light-years away.

APOD: 2003 February 13 - The Eagle Nebula from CFHT
Explanation: Bright blue stars are still forming in the dark pillars of the Eagle Nebula. Made famous by a picture from the Hubble Space Telescope in 1995, the Eagle Nebula shows the dramatic process of star formation. To the upper right of the nebula in the above picture lies the heart of the open cluster M16. The bright blue stars of M16 have been continually forming over the past 5 million years, most recently in the famous central gas and dust pillars known as elephant trunks. Light takes about 7000 years to reach us from M16, which spans about 20 light years and can be seen with binoculars toward the constellation of Serpens.

APOD: 2003 January 22 - M11: The Wild Duck Cluster
Explanation: Many stars like our Sun were formed in open clusters. The above pictured open cluster, M11, contains thousands of stars and is just over five thousand light years distant. The stars in this cluster all formed together about 250 million years ago. The bright young stars in M11 appear blue. Open clusters, also called galactic clusters, contain fewer and younger stars than globular clusters. Also unlike globular clusters, open clusters are generally confined to the plane of our Galaxy. M11 is visible with binoculars towards the constellation of Scutum.

APOD: 2003 January 7 - Open Star Cluster M38
Explanation: Open cluster M38 can be seen with binoculars toward the constellation of Auriga. M38 is considered an intermediately rich open cluster of stars, each of which is about 200 million years old. Located in the disk of our Milky Way galaxy, M38 is still young enough to house many bright blue stars, although it's brightest star is a yellow giant shining 900 times brighter than our Sun. The cluster spans roughly 25 light-years and lies about 4000 light-years away. M38, pictured above, is found only about 2.5 degrees northwest of open cluster M36. Loosely bound by gravity, open clusters spread out over time as they orbit the galactic center and their member stars slowly escape.

APOD: 2002 December 1 - The Pleiades Star Cluster
Explanation: It is the most famous star cluster on the sky. The Pleiades can be seen without binoculars from even the depths of a light-polluted city. Also known as the Seven Sisters and M45, the Pleiades is one of the brightest and closest open clusters. The Pleiades contains over 3000 stars, is about 400 light years away, and only 13 light years across. Quite evident in the above photograph are the blue reflection nebulae that surround the bright cluster stars. Low mass, faint, brown dwarfs have recently been found in the Pleiades.

APOD: 2002 November 29 - Open Star Clusters M35 and NGC 2158
Explanation: Open clusters of stars can be near or far, young or old, and diffuse or compact. Open clusters may contain from 100 to 10,000 stars, all of which formed at nearly the same time. Bright blue stars frequently distinguish younger open clusters. M35, pictured above on the upper left, is a relatively nearby at 2800 light years distant, relatively young at 150 million years old, and relatively diffuse, with about 2500 stars spread out over a volume 30 light years across. An older and more compact open cluster, NGC 2158, is visible above on the lower right. NGC 2158 is four times more distant that M35, over 10 times older, and much more compact as it contains many more stars in roughly the same volume of space. NGC 2158's bright blue stars have self-destructed, leaving cluster light to be dominated by older and yellower stars. Both clusters are visible toward the constellation of Gemini -- M35 with binoculars and NGC 2158 with a small telescope.

APOD: 2002 October 6 - The Lagoon Nebula in Three Colors
Explanation: The bright Lagoon Nebula is home to a diverse array of astronomical objects. Particularly interesting sources include a bright open cluster of stars and several energetic star-forming regions. When viewed by eye, cluster light is dominated by an overall red glow that is caused by luminous hydrogen gas, while the dark filaments are caused by absorption by dense lanes of dust. The above picture, from the Curtis-Schmidt Telescope, however, shows the nebula's emission in three exact colors specifically emitted by hydrogen, oxygen, and sulfur. The Lagoon Nebula, also known as M8 and NGC 6523, lies about 5000 light-years away. The Lagoon Nebula can be located with binoculars in the constellation of Sagittarius spanning a region over three times the diameter of a full Moon.

APOD: 2002 July 22 - Open Cluster NGC 6520 from CFHT
Explanation: Did you ever have a day when it felt like a dark cloud was following you around? For the open cluster of stars NGC 6520, every day is like this. On the left of the above picture are many of NGC 6520's bright blue stars. They formed only millions of years ago - much more recently than our ancient Sun which formed billions of years ago. On the right is an absorption nebula, molecular cloud Barnard 86, from which the stars of NGC 6520 surely formed. This nebula contains much opaque dust that blocks light from the many stars that would have been visible in the background. Surrounding NGC 6520 is part of the tremendously dense starscape in the bulge of our Milky Way Galaxy, the extended halo of stars that surrounds the center of our Galaxy. NGC 6520 spans about 10 light years and lies about 5500 light years away toward the direction of Sagittarius.

APOD: 2002 July 17 - Star-Forming Region RCW38 from 2MASS
Explanation: The star cluster in RCW38 was hiding. Looking at the star forming region RCW38 will not normally reveal most of the stars in this cluster. The reason is that the open cluster is so young that it is still shrouded in thick dust that absorbs visible light. This dust typically accompanies the gas that condenses to form young stars. When viewed in infrared light, however, many stars in RCW38 are revealed, because dust is less effective at absorbing infrared light. The above representative-color image mosaic of RCW38 taken by the 2MASS sky survey in infrared light shows not only many bright blue stars from the star cluster but clouds of brightly emitting gas and dramatic lanes of dark dust. RCW38 spans about 10 light-years and is located about 5500 light years away towards the constellation of Vela.

APOD: 2002 May 5 - The M7 Open Star Cluster in Scorpius
Explanation: M7 is one of the most prominent open clusters of stars on the sky. The cluster, dominated by bright blue stars, can be seen with the naked eye in a dark sky in the tail of the constellation of Scorpius. M7 contains about 100 stars in total, is about 200 million years old, spans 25 light-years across, and lies about 1000 light-years away. This color picture was taken in 1995 at the Burrell-Schmidt Telescope at Kitt Peak National Observatory in Arizona. The M7 star cluster has been known since ancient times, being noted by Ptolemy in the year 130 AD. Also visible is a dark dust cloud near the bottom of the frame, and literally millions of unrelated stars towards the Galactic center.

APOD: 2002 March 17 - NGC 2244: A Star Cluster in the Rosette Nebula
Explanation: In the heart of the Rosette Nebula lies a bright open cluster of stars that lights up the nebula. The stars of NGC 2244 formed from the surrounding gas only four million years ago and emit light and wind that define the nebula's appearance today. High energy light from the bright young stars of NGC 2244 ionizes the surrounding hydrogen gas clouds to create the red emission nebula appearance. The hot wind of particles that streams away from the cluster stars contributes to an already complex menagerie of gas and dust filaments while slowly evacuating the cluster center. NGC 2244 measures about 50 light-years across, lies about 4500 light-years away, and is visible with binoculars towards the constellation of Monoceros.

APOD: 2001 December 18 - Sharpless 212 in Hydrogen and Sulfur
Explanation: Where do the most massive stars form? Observational evidence indicates that the outskirts of developing open clusters of stars are primary locations. Pictured above is one such open cluster: Sharpless 212. Visible in the image center are massive stars in the open cluster. The energetic light from these stars ionizes surrounding hydrogen atoms creating an HII region. As the hydrogen atoms re-acquire electrons, they emit the red light highlighted. Sharpless 212 also contains small amounts of dust and heavy atoms such as Sulfur. The dust efficiently absorbs light, while emission from Sulfur is highlighted in blue. Particularly striking and well-defined boundaries that separate the ionized material from surrounding neutral material are visible at the edge of the HII region. Sharpless 212 spans about 20 light years and lies about 25,000 light years away.

APOD: 2001 September 9 - NGC 3293: A Bright Young Open Cluster
Explanation: Hot blue stars shine brightly in this beautiful, recently formed galactic or "open" star cluster. Open cluster NGC 3293 is located in the constellation Carina, lies at a distance of about 8000 light years, and has a particularly high abundance of these young bright stars. A study of NGC 3293 implies that the blue stars are only about 6 million years old, whereas the cluster's dimmer, redder stars appear to be about 20 million years old. If true, star formation in this open cluster took at least 15 million years. Even this amount of time is short, however, when compared with the billions of years stars like our Sun live, and the over-ten billion year lifetimes of many galaxies and our universe. NGC 3293 appears just in front of a dense dust lane emanating from the Carina Nebula.

APOD: 2001 August 20 - The Lagoon Nebula in Three Colors
Explanation: The bright Lagoon Nebula is home to a diverse array of astronomical objects. Particularly interesting sources include a bright open cluster of stars and several energetic star-forming regions. When viewed by eye, cluster light is dominated by an overall red glow that is caused by luminous hydrogen gas, while the dark filaments are caused by absorption by dense lanes of dust. The above picture, from the Curtis-Schmidt Telescope, however, shows the nebula's emission in three exact colors specifically emitted by hydrogen, oxygen, and sulfur. The Lagoon Nebula, also known as M8 and NGC 6523, lies about 5000 light-years away. The Lagoon Nebula can be located with binoculars in the constellation of Sagittarius spanning a region over three times the diameter of a full Moon.

APOD: 2001 July 30 - Star Cluster R136 Bursts Out
Explanation: In the center of star-forming region 30 Doradus lies a huge cluster of the largest, hottest, most massive stars known. Known as R136, the cluster's energetic stars are breaking out of the cocoon of gas and dust from which they formed. This disintegrating cocoon, which fills the rest of the recently released above picture by the Hubble Space Telescope, is predominantly ionized hydrogen from 30 Doradus. R136 is composed of thousands of hot blue stars, some about 50 times more massive than our Sun. R136, also known as NGC 2070, lies in the LMC - a satellite galaxy to our own Milky Way Galaxy. Although the young ages of stars in R136 make it similar to a Milky Way open cluster, its high density of stars will likely turn it into a low mass globular cluster in a few billion years.

APOD: 2001 June 18 - NGC 4755: A Jewel Box of Stars
Explanation: The great variety of star colors in this open cluster underlies its name: The Jewel Box. One of the bright central stars is a red supergiant, in contrast to the many blue stars that surround it. The cluster, also known as Kappa Crucis contains just over 100 stars, and is about 10 million years old. Open clusters are younger, contain few stars, and contain a much higher fraction of blue stars than do globular clusters. This Jewel Box lies about 7500 light-years away, so the light that we see today was emitted from the cluster before even the Great Pyramids in Egypt were built. The Jewel Box, pictured above, spans about 20 light-years, and can be seen with binoculars towards the southern constellation of Crux.

APOD: 2001 May 6 - The Pleiades Star Cluster
Explanation: It is the most famous star cluster on the sky. The Pleiades can be seen without binoculars from even the depths of a light-polluted city. Also known as the Seven Sisters and M45, the Pleiades is one of the brightest and closest open clusters. The Pleiades contains over 3000 stars, is about 400 light years away, and only 13 light years across. Quite evident in the above photograph are the blue reflection nebulae that surround the bright cluster stars. Low mass, faint, brown dwarfs have recently been found in the Pleiades.

APOD: 2001 February 14 - The Rosette Nebula
Explanation: Would the Rosette Nebula by any other name look as sweet? The bland New General Catalog designation of NGC 2237 doesn't appear to diminish the appearance of the this flowery emission nebula. Inside the nebula lies an open cluster of bright young stars designated NGC 2244. These stars formed about four million years ago from the nebular material and their stellar winds are clearing a hole in the nebula's center, insulated by a layer of dust and hot gas. Ultraviolet light from the hot cluster stars causes the surrounding nebula to glow. The Rosette Nebula spans about 100 light-years across, lies about 5000 light-years away, and can be seen with a small telescope towards the constellation of Monoceros.

APOD: 2001 February 7 - Distant Open Cluster M103
Explanation: Bright blue stars highlight the open cluster known as M103. The gas clouds from which these stars condensed has long dispersed. Of the stars that were formed, the brightest, bluest, and most massive have already used up their nuclear fuel and self-destructed in supernova explosions. A 20 million-year age for M103 was estimated by finding the brightest main-sequence stars that still survive and theoretically computing their lifetimes. In fact, a formerly blue star has recently evolved off the main sequence and is visible above as the red giant star near the cluster center. In general, yellow stars like our Sun are usually less bright and hence less prominent in open clusters than their massive blue cousins. Light takes about 14 years to cross M103. Although visible with binoculars toward the constellation of Cassiopeia, M103's great distance of 8000 light years makes it appear four times smaller than a full moon.

APOD: 2000 October 22 - Wild Duck Open Cluster M11
Explanation: Many stars like our Sun were formed in open clusters. The above open cluster, M11, contains thousands of stars and is just over three thousand light years distant. The stars in this cluster all formed together about 150 million years ago. The bright young stars in M11 appear blue. Open clusters, also called galactic clusters, contain fewer and younger stars than globular clusters. Also unlike globular clusters, open clusters are generally confined to the plane of our Galaxy. M11 is visible with binoculars towards the constellation of Scutum.

APOD: 2000 August 22 - NGC 2244: A Star Cluster in the Rosette Nebula
Explanation: In the heart of the Rosette Nebula lies a bright open cluster of stars that lights up the nebula. The stars of NGC 2244 formed from the surrounding gas only four million years ago and emit light and wind that define the nebula's appearance today. High energy light from the bright young stars of NGC 2244 ionizes the surrounding hydrogen gas clouds to create the red emission nebula appearance. The hot wind of particles that streams away from the cluster stars contributes to an already complex menagerie of gas and dust filaments while slowly evacuating the cluster center. NGC 2244 measures about 50 light-years across, lies about 4500 light-years away, and is visible with binoculars towards the constellation of Monoceros.

APOD: 2000 April 5 - The M7 Open Star Cluster in Scorpius
Explanation: M7 is one of the most prominent open clusters of stars on the sky. The cluster, dominated by bright blue stars, can be seen with the naked eye in a dark sky in the tail of the constellation of Scorpius. M7 contains about 100 stars in total, is about 200 million years old, spans 25 light-years across, and lies about 1000 light-years away. This color picture was taken in 1995 at the Burrell-Schmidt Telescope at Kitt Peak National Observatory in Arizona. The M7 star cluster has been known since ancient times, being noted by Ptolemy in the year 130 AD. Also visible is a dark dust cloud near the bottom of the frame, and literally millions of unrelated stars towards the Galactic center.

APOD: 2000 February 27 - The Pleiades Star Cluster
Explanation: It is the most famous star cluster on the sky. The Pleiades can be seen without binoculars from even the depths of a light-polluted city. Also known as the Seven Sisters and M45, the Pleiades is one of the brightest and closest open clusters. The Pleiades contains over 3000 stars, is about 400 light years away, and only 13 light years across. Quite evident in the above photograph are the blue reflection nebulae that surround the bright cluster stars. Low mass, faint, brown dwarfs have recently been found in the Pleiades.

APOD: 2000 January 12 - NGC 6791: An Old, Large Open Cluster
Explanation: NGC 6791 is one of the oldest and largest open clusters of stars known. But how did it get so dirty? Open star clusters usually contain a few hundred stars each less than a billion years old. Open star cluster NGC 6791, however, contains thousands of stars recently measured to be about 8 billion years old. What's really confusing, though, is that the stars of NGC 6791 are relatively dirty - the minuscule amounts of heavy elements (generically called metals) are high relative to most other star clusters. Older stars are supposed to be metal poor, since metals have only been slowly accumulating in our Milky Way Galaxy. This enigma makes NGC 6791, pictured above, one of the most studied open clusters and a possible example of how stars might evolve in the centers of galaxies.

APOD: October 18, 1999 - NGC 3603: An Active Star Cluster
Explanation: NGC 3603 is home to a massive star cluster, thick dust pillars, and a star about to explode. The central open cluster contains about 2000 bright stars, each of which is much brighter and more massive than our Sun. Together, radiations from these stars are energizing and pushing away surrounding material, making NGC 3603 one of the most interesting HII regions known. NGC 3603 is about 20,000 light-years away, and the region shown is about 20 light-years across. Possibly most interesting about this recently released, representative-color picture are the large number of dim stars visible. These stars are less massive than our Sun, demonstrating that great numbers of low-mass stars also form in active starburst regions.

APOD: September 21, 1999 - The Quintuplet Star Cluster
Explanation: Bright clusters of stars form and disperse near the center of our Galaxy. Four million years ago the Quintuplet Cluster, pictured above, formed and is now slowly dispersing. The Quintuplet Cluster is located within 100 light-years of the Galactic center, and is home to the brightest star yet cataloged in our Galaxy: the Pistol Star. Objects near our Galactic center are usually hidden from view by opaque dust. This recently-released picture was able to capture the cluster in infrared light, though, with the NICMOS camera onboard the orbiting Hubble Space Telescope. The young Quintuplet Cluster is one of the most massive open clusters yet discovered, but still much less massive than the ancient globular clusters that orbit in the distant halo. Some of the bright white stars visible above may be on the verge of blowing themselves up in a spectacular supernova.

APOD: August 28, 1999 - X-Ray Pleiades
Explanation: The Pleiades star cluster is one of the jewels of the northern sky. To the unaided eye it appears as an alluring group of stars in the constellation Taurus, while telescopic views reveal cluster stars surrounded by delicate blue wisps of dust-reflected starlight. To the X-ray telescopes on board the orbiting ROSAT observatory, the cluster also presents an impressive, but slightly altered, appearance. This false color image was produced from ROSAT observations by translating different X-ray energy bands to visual colors - the lowest energies are shown in red, medium in green, and highest energies in blue. (The green boxes mark the position of the seven brightest visual stars.) The Pleiades stars seen in X-rays have extremely hot, tenuous outer atmospheres called coronas and the range of colors corresponds to different coronal temperatures.

APOD: August 25, 1999 - Reflections on NGC 6188
Explanation: NGC 6188 is an interstellar carnival of young blue stars, hot red gas, and cool dark dust. Located 4000 light years away in the disk of our Galaxy, NGC 6188 is home to the Ara OB1 association, a group of bright young stars whose nucleus forms the open cluster NGC 6193. These stars are so bright that some of their blue light reflects off of interstellar dust forming the diffuse blue glow in the center of the above photograph. Open cluster NGC 6193 formed about three million years ago from the surrounding gas, and appears unusually rich in close binary stars. The red glow visible throughout the photograph arises from hydrogen gas heated by the bright stars in Ara OB1. The dark dust that blocks much of NGC 6188's light was likely formed in the outer atmospheres of cooler stars and in supernovae ejecta.

APOD: March 5, 1999 - M46 And NGC 2438: Young And Old
Explanation: Galactic or open star clusters are relatively young. These swarms of bright stars are born near the plane of the Milky Way, but their numbers steadily dwindle as cluster members are strewn through the Galaxy by gravitational interactions. This bright open cluster, known as M46, is around 300 million years young and still contains a few hundred stars. Located about 5,000 light-years away toward the constellation Puppis, M46 is a familiar object to telescopic stargazers and also seems to contain a contradiction to its youthful status. The striking red patch of glowing gas above center in this lovely photograph is the planetary nebula NGC 2438 - a brief, final phase in the life of a solar-type star a few billion years old whose central reservoir of hydrogen fuel has been exhausted. In fact, NGC 2438 is estimated to be only 3,000 light-years distant and moves at a different speed than M46 cluster members. It likely represents a foreground object, only by chance appearing along our line of sight to young M46.

APOD: February 21, 1999 - In the Center of 30 Doradus
Explanation: In the center of 30 Doradus lies a huge cluster of the largest, hottest, most massive stars known. The center of this cluster, known as R136, is boxed in the upper right portion of the above picture. The gas and dust filling the rest of the picture is predominantly ionized hydrogen from the emission nebula 30 Doradus. R136 is composed of thousands of hot blue stars, some about 50 times more massive than our Sun. 30 Doradus and R136 lie in the LMC - a satellite galaxy to our own Milky Way Galaxy. Although the ages of stars in R136 cause it to be best described as an open cluster, R136's density will likely make it a low mass globular cluster in a few billion years.

APOD: January 6, 1999 - M6: The Butterfly Cluster
Explanation: To some, the outline of the open cluster of stars M6 resembles a butterfly. M6, also known as NGC 6405, spans about 20 light-years and lies about 2,000 light years distant. M6 can best be seen in a dark sky with binoculars towards the constellation of Scorpius, coving about as much of the sky as the full moon. Like other open clusters, M6 is composed predominantly of young blue stars, although the brightest star is nearly orange. M6 is estimated to be about 100 million years old. Determining the distance to clusters like M6 helps astronomers calibrate the distance scale of the universe.

APOD: December 7, 1998 - Star Forming Region RCW38
Explanation: Star cluster RCW38 was hiding. This open cluster of stars is located about 5000 light years away towards the constellation of Vela. Looking there will not normally reveal most of the stars in this cluster, though. The reason is that the open cluster is so young that it is still shrouded in thick dust that absorbs visible light. This dust typically accompanies the gas that condenses to form young stars. When viewed in infrared light, however, the star cluster in RCW38 is revealed, because dust is less effective at absorbing infrared light. The above photograph was one of the first ever taken with the new Infrared Spectrometer and Array Camera (ISAAC) affixed to the 8.2-meter Very Large Telescope.

APOD: October 25, 1998 - The Pleiades Star Cluster
Explanation: It is the most famous star cluster on the sky. The Pleiades can be seen without binoculars from even the depths of a light-polluted city. Also known as the Seven Sisters and M45, the Pleiades is one of the brightest and closest open clusters. The Pleiades contains over 3000 stars, is about 400 light years away, and only 13 light years across. Quite evident in the above photograph are the blue reflection nebulae that surround the bright cluster stars. Low mass, faint, brown dwarfs have recently been found in the Pleiades.

APOD: September 21, 1998 - NGC 281: Cluster, Clouds, and Globules
Explanation: NGC 281 is a busy workshop of star formation. Prominent features include a small open cluster of stars, a diffuse red-glowing emission nebula, large lanes of obscuring gas and dust, and dense knots of dust and gas in which stars may still be forming. The open cluster of stars IC 1590 visible on the upper right has formed only in the last few million years. The brightest member of this cluster is actually a multiple-star system shining light that helps ionize the nebula's gas, causing the red glow visible throughout. The lanes of dust on the lower right might be the home of future star formation. Particularly striking in the above photograph are the dark Bok globules visible against the bright nebula. Stars are probably forming there right now. The entire NGC 281 system lies about 10 thousand light years distant.

APOD: August 3, 1998 - M44: A Beehive of Stars
Explanation: M44 is a prominent open cluster of stars. Nicknamed Praesepe and "The Beehive", it is one of the few open clusters visible to the unaided eye. M44 was thought to be a nebula until Galileo used an early telescope to resolve the cluster's bright blue stars. These stars are visible in the above image. M44, which is thought to have formed about 400 million years ago, is larger and older than most other open clusters. The Beehive Cluster lies about 580 light-years away, and spans about 10 light-years across. When viewed with a powerful telescope, hundreds of stars become visible.

APOD: April 5, 1998 - X-Ray Pleiades
Explanation: The Pleiades star cluster is one of the jewels of the northern sky. To the unaided eye it appears as a lovely and tantalizing grouping of stars in the constellation of Taurus, while telescopic views reveal cluster stars surrounded by delicate blue wisps of dust-reflected starlight. To the X-ray telescopes on board the orbiting ROSAT observatory, the cluster also presents an impressive, but slightly altered, appearance. This false color image was produced from ROSAT observations by translating different X-ray energy bands to visual colors - the lowest energies are shown in red, medium in green, and highest energies in blue. (The green boxes mark the position of the seven brightest visual stars.) The Pleiades stars seen in X-rays have extremely hot, tenuous outer atmospheres called coronas and the range of colors corresponds to different coronal temperatures.

APOD: March 29, 1998 - NGC 3293: A Bright Young Open Cluster
Explanation: Hot blue stars shine brightly in this beautiful, recently formed galactic or "open" star cluster. Open cluster NGC 3293 is located in the constellation Carina, lies at a distance of about 8000 light years, and has a particularly high abundance of these young bright stars. A study of NGC 3293 implies that the blue stars are only about 6 million years old, whereas the cluster's dimmer, redder stars appear to be about 20 million years old. If true, star formation in this open cluster took at least 15 million years. Even this amount of time is short, however, when compared with the billions of years stars like our Sun live, and the over-ten billion year lifetimes of many galaxies and our universe. NGC 3293 appears just in front dense dust lane emanating from the Carina Nebula.

APOD: February 14, 1998 - The Rosette Nebula
Explanation: Would the Rosette Nebula by any other name look as sweet? The bland New General Catalog designation of NGC 2237 doesn't appear to diminish the appearance of the this flowery emission nebula. Inside the nebula lies an open cluster of bright young stars designated NGC 2244. These stars recently formed from the nebular material and their stellar "wind" has cleared a hole in the nebula's center, insulated by a layer of dust and hot gas. Ultraviolet light from the hot cluster stars causes the surrounding nebula to glow.

APOD: January 27, 1998 - The Great Nebula in Orion
Explanation: The Great Nebula in Orion can be found just below and to the left of the easily identifiable belt of three stars in the popular constellation Orion. This fuzzy patch contains one of the closest stellar nurseries, lying at a distance of about 1500 light years. In the above picture, the red region on the left consists of nebulae designated M42 and M43 and contains the bright Trapezium open cluster. The blue region on the right is a nebula primarily reflecting the light from internal bright stars. Recent observations of the Orion Nebula by the Hubble Space Telescope have located solar-system sized star-forming regions.

APOD: November 18, 1997 - In the Center of the Trapezium
Explanation: Start with the constellation of Orion. Below Orion's belt is a fuzzy area known as the Great Nebula of Orion or M42. In this nebula is a bright star cluster known as the Trapezium, shown above. New stellar systems are forming there in gigantic globs of gas and dust known as Proplyds. Looking closely at the above picture also reveals that gas and dust surrounding some of the dimmer stars appears to form structures that point away from the brighter stars. The above false color image was made by combining several exposures from the Hubble Space Telescope.

APOD: October 18, 1997 - The Pleiades Star Cluster
Explanation: It is the most famous star cluster on the sky. The Pleiades can be seen without binoculars from even the depths of a light-polluted city. Also known as the Seven Sisters and M45, the Pleiades is one of the brightest and closest open clusters. The Pleiades contains over 3000 stars, is about 400 light years away, and only 13 light years across. Quite evident in the above photograph are the blue reflection nebulae that surround the bright cluster stars. Low mass, faint, brown dwarfs have recently been found in the Pleiades.

APOD: October 4, 1997 - In the Center of 30 Doradus
Explanation: In the center of 30 Doradus lies a huge cluster of the largest, hottest, most massive stars known. The center of this cluster, known as R136, is boxed in the upper right portion of the above picture. The gas and dust filling the rest of the picture is predominantly ionized hydrogen from the emission nebula 30 Doradus. R136 is composed of thousands of hot blue stars, some about 50 times more massive than our Sun. 30 Doradus and R136 lie in the LMC - a satellite galaxy to our own Milky Way Galaxy. Although the ages of stars in R136 cause it to be best described as an open cluster, R136's density will likely make it a low mass globular cluster in a few billion years.

APOD: July 30, 1997 - Eagle Castle
Explanation: What lights up this castle of star formation? The familiar Eagle Nebula glows much like a neon sign, but in many colors at once. The above photograph is a composite of three of these glowing gas colors. In particular the glowing red Sulfur gas of the nebula nicely outlines some of the denser star forming knots. Energetic light from young massive stars causes the gas to glow and effectively boils away part of the dust and gas from its birth pillar. Many of these stars will explode after several million years, returning most of their elements back to the nebula which formed them. This process is forming an open cluster of stars known as M16.

APOD: July 18, 1997 - Blue Stars and Red Pillars
Explanation: Bright blue stars are still forming in the red pillars of the Eagle Nebula. Made famous by a picture from the Hubble Space Telescope in 1995, the Eagle Nebula shows the dramatic process of star formation. To the upper right of the nebula in the above picture lies the heart of the open cluster M16. This picture closely depicts the true colors of the stars and nebula. The bright blue stars of M16 are continually forming from the Eagle Nebula gas, most recently in the famous gas and dust pillars seen below the photo's center. Of all the young stars in M16, the most massive shine the brightest and the bluest. A typical age for a star in this cluster is about 5 million years, making them only 1/1000 the age of our Sun. Light takes about 7000 years to reach us from M16.

APOD: April 16, 1997 - A Star Cluster Through Hale-Bopp's Tail
Explanation: Comet Hale-Bopp continues to look impressive. The photograph above captured the comet on April 7^th passing nearly in front of M34, a star cluster in the constellation of Perseus. Many of the stars in this open cluster can be seen through Comet Hale-Bopp's white dust tail. The bright blue ion tail now shows several streams. Now receding from both the Sun and the Earth, Comet Hale-Bopp should still remain an impressive sight for weeks to come as it slowly fades.

APOD: January 29, 1997 - NGC 869 & NGC 884: A Double Open Cluster
Explanation: Most star clusters are singularly impressive. But open clusters NGC 869 and NGC 884 are doubly impressive. Also known as "h and chi Persei", this rare double cluster, shown above, is bright enough to be seen from a dark location without even binoculars. Although their discovery surely predates written history, the "double cluster" was notably cataloged by the Greek astronomer Hipparcos. The clusters are over 7000 light years distant toward the constellation of Perseus, but are separated by only hundreds of light years.

APOD: January 28, 1997 - Open Cluster M50
Explanation: Many stars form in clusters. Two types of star clusters are visible in our Milky Way Galaxy: open clusters and globular clusters. Open clusters like M50, shown above, typically contain hundreds of stars, many of which are bright, young, and blue. In fact, most of the bright blue stars in the above picture belong to M50, but most of the dimmer, red stars do not. M50 lies about 3000 light-years from Earth and is about 20 light years across. Open clusters tend to have irregular shapes and are mostly found in the plane of our Galaxy.

APOD: November 11, 1996 - NGC 4755: A Jewel Box of Stars
Explanation: The great variety of star colors in this open cluster underlie it's name: The Jewel Box. The bright central star Kappa Crucis is red, in contrast to the many blue stars that surround it. The cluster contains just over 100 stars, and might be no older than 10 million years. Open clusters are younger, contain few stars, and contain a much higher fraction of blue stars than do globular clusters. This Jewel Box lies about 7500 light-years away, so the light that we see today was emitted from the clusters before even the Great Pyramids in Egypt were built.

APOD: September 25, 1996 - Bright Stars and Dark Clouds
Explanation: Did you ever feel like a black cloud was following you around? Well don't feel bad - this even happened to the bright young stars of the open cluster NGC 6520. On the left are the cluster's bright blue stars. They formed only millions of years ago - much more recently than our ancient Sun which formed billions of years ago. On the right is an absorption nebula from which the stars might have formed. This nebula contains much opaque dust which blocks visible light from the many stars that would have been seen in the background. The study of open clusters is valuable for many reasons which include the understanding star formation and the calibration of the distance scale of our universe.

APOD: August 23, 1996 - NGC 3293: A Bright Young Open Cluster
Explanation: Hot Blue stars shine brightly in this beautiful, recently formed galactic or "open" star cluster. Open cluster NGC 3293 is located in the constellation Carina, lies at a distance of about 8000 light years, and has a particularly high abundance of these young bright stars. A study of NGC 3293 implies that the blue stars are only about 6 million years old, whereas the cluster's dimmer, redder stars appear to be about 20 million years old. If true, star formation in this open cluster took at least 15 million years. Even this amount of time is short, however, when compared with the billions of years stars like our Sun live, and the over-ten billion year lifetimes of many galaxies and our universe. NGC 3293 appears just in front dense dust lane emanating from the Carina Nebula.

APOD: May 24, 1996 - In the Center of 30 Doradus
Explanation: In the center of 30 Doradus lies a huge cluster of the largest, hottest, most massive stars known. The center of this cluster, known as R136, is boxed in the upper right portion of the above picture. The gas and dust filling the rest of the picture is predominantly ionized hydrogen from the emission nebula 30 Doradus. R136 is composed of thousands of hot blue stars, some about 50 times more massive than our Sun. 30 Doradus and R136 lie in the LMC - a satellite galaxy to our own Milky Way Galaxy. Although the ages of stars in R136 cause it to be best described as an open cluster, R136's density will likely make it a low mass globular cluster in a few billion years.

APOD: May 22, 1996 - Star Cluster in the Rosette Nebula
Explanation: Embedded in the center of the colorful and photogenic Rosette Nebula is a bright, young open cluster. The bright blue stars in this cluster, labelled NGC 2244, emit ultraviolet light that knocks electrons away from hydrogen atoms. When the electrons fall back, they emit the red light which distinctively defines the glow of all emission nebulae. The Rosette Nebula is thousands of light years distant, but light would take only about 100 years to cross it. The Rosette Nebula is not difficult to observer and, although faint, actually appears larger than the full moon.

APOD: February 14, 1996 - NGC 2237: The Rosette Nebula
Explanation: Would the Rosette nebula by any other name look as sweet? The bland New General Catalog designation of NGC 2237 doesn't appear to diminish the appearance of the this flowery emission nebula. Inside the nebula lies an open cluster of bright young stars designated NGC 2244. These stars recently formed from the nebular material and their stellar "wind" has cleared a hole in the nebula's center, insulated by a layer of dust and hot gas. Ultraviolet light from the hot cluster stars causes the surrounding nebula to glow.

APOD: January 27, 1996 - Open Cluster M8 in the Lagoon
Explanation: The large majestic Lagoon Nebula is home for many young stars and hot gas. The Lagoon Nebulae is so large and bright it can be seen without a telescope. Formed only several million years ago in the nebula is the open cluster known as NGC 6530, whose young stars show their high temperature by their blue glow. The nebula, also known as M8 and NGC 6523, is named "Lagoon" for the band of dust seen to the left of the open cluster's center. A bright knot of gas and dust in the nebula's center is known as the Hourglass Nebula. Star formation continues in the the Lagoon Nebula as witnessed by the many globules that exist there.

APOD: January 16, 1996 - Wild Duck Open Cluster M11
Explanation: Many stars like our Sun were formed in open clusters. The above open cluster, M11, contains thousands of stars and is just over three thousand light years distant. The stars in this cluster all formed together about 150 million years ago. The many bright stars in the cluster appear blue. Open clusters, also called galactic clusters, contain fewer and younger stars than globular clusters. Also unlike globular clusters, open clusters are generally confined to the plane of our Galaxy.

APOD: November 1, 1995 - M16: Dust and an Open Cluster
Explanation: The photogenic M16 shown above is composed of a young star cluster and a spectacular emission nebulae lined with distinct regions of interstellar dust. Most of the stars in the cluster can be seen offset just above and to the right of the photograph's center. This type of star cluster is called an "open" or "galactic" cluster and typically has a few hundred young bright members. The redness of the surrounding emission nebula gas is caused by electrons recombining with hydrogen nuclei, while the dark regions are dust lanes that absorb much of the radiation that enters it. The dust absorbs so much light it allows astronomers to determine which stars are inside the nebula and which are in the foreground.