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




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Thumbnail image of picture found for this day. APOD: 2022 March 10 - Globular Star Cluster 47 Tuc
Explanation: Globular star cluster 47 Tucanae is a jewel of the southern sky. Also known as NGC 104, it roams the halo of our Milky Way Galaxy along with some 200 other globular star clusters. The second brightest globular cluster (after Omega Centauri) as seen from planet Earth, 47 Tuc lies about 13,000 light-years away. It can be spotted with the naked-eye close on the sky to the Small Magellanic Cloud in the constellation of the Toucan. The dense cluster is made up of hundreds of thousands of stars in a volume only about 120 light-years across. Red giant stars on the outskirts of the cluster are easy to pick out as yellowish stars in this sharp telescopic portrait. Tightly packed globular cluster 47 Tuc is also home to a star with the closest known orbit around a black hole.

Thumbnail image of picture found for this day. APOD: 2022 January 26 - Stars, Dust, and Gas Near Antares
Explanation: Why is the sky near Antares and Rho Ophiuchi so dusty yet colorful? The colors result from a mixture of objects and processes. Fine dust -- illuminated from the front by starlight -- produces blue reflection nebulae. Gaseous clouds whose atoms are excited by ultraviolet starlight produce reddish emission nebulae. Backlit dust clouds block starlight and so appear dark. Antares, a red supergiant and one of the brighter stars in the night sky, lights up the yellow-red clouds on the lower right of the featured image. The Rho Ophiuchi star system lies at the center of the blue reflection nebula on the top left. The distant globular cluster of stars M4 is visible above and to the right of Antares. These star clouds are even more colorful than humans can see, emitting light across the electromagnetic spectrum.

Thumbnail image of picture found for this day. APOD: 2021 December 12 - Comet Leonard Before Star Cluster M3
Explanation: Comet Leonard is now visible to the unaided eye -- but just barely. Passing nearest to the Earth today, the comet is best seen this week soon after sunset, toward the west, low on the horizon. Currently best visible in the north, by late December the comet will best be seen from south of Earth's equator. The featured image of Comet C/2021 A1 (Leonard) was taken a week ago from California, USA. The deep exposure shows in great detail the comet's green gas coma and developing dust tail. The comet -- across our inner Solar System and only light-minutes away -- was captured passing nearly in front of globular star cluster M3. In contrast, M3 is about 35,000 light-years away. In a week, Comet Leonard will pass unusually close to Venus, but will continue on and be at its closest to the Sun in early January.

Thumbnail image of picture found for this day. APOD: 2021 October 19 - Palomar 6: Globular Star Cluster
Explanation: Where did this big ball of stars come from? Palomar 6 is one of about 200 globular clusters of stars that survive in our Milky Way Galaxy. These spherical star-balls are older than our Sun as well as older than most stars that orbit in our galaxy's disk. Palomar 6 itself is estimated to be about 12.5 billion years old, so old that it is close to -- and so constrains -- the age of the entire universe. Containing about 500,000 stars, Palomar 6 lies about 25,000 light years away, but not very far from our galaxy's center. At that distance, this sharp image from the Hubble Space Telescope spans about 15 light-years. After much study including images from Hubble, a leading origin hypothesis is that Palomar 6 was created -- and survives today -- in the central bulge of stars that surround the Milky Way's center, not in the distant galactic halo where most other globular clusters are now found.

Thumbnail image of picture found for this day. APOD: 2021 August 6 - Stars and Dust Across Corona Australis
Explanation: Cosmic dust clouds cross a rich field of stars in this telescopic vista near the northern boundary of Corona Australis, the Southern Crown. Less than 500 light-years away the dust clouds effectively block light from more distant background stars in the Milky Way. Top to bottom the frame spans about 2 degrees or over 15 light-years at the clouds' estimated distance. At top right is a group of lovely reflection nebulae cataloged as NGC 6726, 6727, 6729, and IC 4812. A characteristic blue color is produced as light from hot stars is reflected by the cosmic dust. The dust also obscures from view stars in the region still in the process of formation. Just above the bluish reflection nebulae a smaller NGC 6729 surrounds young variable star R Coronae Australis. To its right are telltale reddish arcs and loops identified as Herbig Haro objects associated with energetic newborn stars. Magnificent globular star cluster NGC 6723 is at bottom left in the frame. Though NGC 6723 appears to be part of the group, its ancient stars actually lie nearly 30,000 light-years away, far beyond the young stars of the Corona Australis dust clouds.

Thumbnail image of picture found for this day. APOD: 2021 June 3 - Millions of Stars in Omega Centauri
Explanation: Globular star cluster Omega Centauri, also known as NGC 5139, is some 15,000 light-years away. The cluster is packed with about 10 million stars much older than the Sun within a volume about 150 light-years in diameter. It's the largest and brightest of 200 or so known globular clusters that roam the halo of our Milky Way galaxy. Though most star clusters consist of stars with the same age and composition, the enigmatic Omega Cen exhibits the presence of different stellar populations with a spread of ages and chemical abundances. In fact, Omega Cen may be the remnant core of a small galaxy merging with the Milky Way. Omega Centauri's red giant stars (with a yellowish hue) are easy to pick out in this sharp, color telescopic view.

Thumbnail image of picture found for this day. APOD: 2021 May 20 - M13: The Great Globular Cluster in Hercules
Explanation: In 1716, English astronomer Edmond Halley noted, "This is but a little Patch, but it shews itself to the naked Eye, when the Sky is serene and the Moon absent." Of course, M13 is now less modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. Sharp telescopic views like this one reveal the spectacular cluster's hundreds of thousands of stars. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter. Approaching the cluster core upwards of 100 stars could be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. The remarkable range of brightness recorded in this image follows stars into the dense cluster core. Distant background galaxies in the medium-wide field of view include NGC 6207 at the lower right.

Thumbnail image of picture found for this day. APOD: 2021 February 7 - Blue Straggler Stars in Globular Cluster M53
Explanation: If our Sun were part of this star cluster, the night sky would glow like a jewel box of bright stars. This cluster, known as M53 and cataloged as NGC 5024, is one of about 250 globular clusters that survive in our Galaxy. Most of the stars in M53 are older and redder than our Sun, but some enigmatic stars appear to be bluer and younger. These young stars might contradict the hypothesis that all the stars in M53 formed at nearly the same time. These unusual stars are known as blue stragglers and are unusually common in M53. After much debate, blue stragglers are now thought to be stars rejuvenated by fresh matter falling in from a binary star companion. By analyzing pictures of globular clusters like the featured image taken by the Hubble Space Telescope, astronomers use the abundance of stars like blue stragglers to help determine the age of the globular cluster and hence a limit on the age of the universe. M53, visible with a binoculars towards the constellation of Bernice's Hair (Coma Berenices), contains over 250,000 stars and is one of the furthest globulars from the center of our Galaxy.

Thumbnail image of picture found for this day. APOD: 2020 October 24 - Globular Star Cluster 47 Tuc
Explanation: Globular star cluster 47 Tucanae is a jewel of the southern sky. Also known as NGC 104, it roams the halo of our Milky Way Galaxy along with some 200 other globular star clusters. The second brightest globular cluster (after Omega Centauri) as seen from planet Earth, it lies about 13,000 light-years away and can be spotted naked-eye close on the sky to the Small Magellanic Cloud in the constellation of the Toucan. The dense cluster is made up of hundreds of thousands of stars in a volume only about 120 light-years across. Red giant stars on the outskirts of the cluster are easy to pick out as yellowish stars in this sharp telescopic portrait. Tightly packed globular cluster 47 Tuc is also home to a star with the closest known orbit around a black hole.

Thumbnail image of picture found for this day. APOD: 2020 October 14 - The Colorful Clouds of Rho Ophiuchi
Explanation: The many spectacular colors of the Rho Ophiuchi (oh'-fee-yu-kee) clouds highlight the many processes that occur there. The blue regions shine primarily by reflected light. Blue light from the Rho Ophiuchi star system and nearby stars reflects more efficiently off this portion of the nebula than red light. The Earth's daytime sky appears blue for the same reason. The red and yellow regions shine primarily because of emission from the nebula's atomic and molecular gas. Light from nearby blue stars - more energetic than the bright star Antares - knocks electrons away from the gas, which then shines when the electrons recombine with the gas. The dark brown regions are caused by dust grains - born in young stellar atmospheres - which effectively block light emitted behind them. The Rho Ophiuchi star clouds, well in front of the globular cluster M4 visible here on the upper right, are even more colorful than humans can see - the clouds emits light in every wavelength band from the radio to the gamma-ray.

Thumbnail image of picture found for this day. APOD: 2020 March 19 - M13: The Great Globular Cluster in Hercules
Explanation: In 1716, English astronomer Edmond Halley noted, "This is but a little Patch, but it shews itself to the naked Eye, when the Sky is serene and the Moon absent." Of course, M13 is now less modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. Sharp telescopic views like this one reveal the spectacular cluster's hundreds of thousands of stars. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter. Approaching the cluster core upwards of 100 stars could be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. The remarkable range of brightness recorded in this image follows stars into the dense cluster core and reveals three subtle dark lanes forming the apparent shape of a propeller just below and slightly left of center. Distant background galaxies in the medium-wide field of view include NGC 6207 at the upper left.

Thumbnail image of picture found for this day. APOD: 2020 January 23 - Globular Star Cluster NGC 6752
Explanation: Some 13,000 light-years away toward the southern constellation Pavo, the globular star cluster NGC 6752 roams the halo of our Milky Way galaxy. Over 10 billion years old, NGC 6752 follows clusters Omega Centauri and 47 Tucanae as the third brightest globular in planet Earth's night sky. It holds over 100 thousand stars in a sphere about 100 light-years in diameter. Telescopic explorations of the NGC 6752 have found that a remarkable fraction of the stars near the cluster's core, are multiple star systems. They also reveal the presence of blue straggle stars, stars which appear to be too young and massive to exist in a cluster whose stars are all expected to be at least twice as old as the Sun. The blue stragglers are thought to be formed by star mergers and collisions in the dense stellar environment at the cluster's core. This sharp color composite also features the cluster's ancient red giant stars in yellowish hues. (Note: The bright, spiky blue star at 11 o'clock from the cluster center is a foreground star along the line-of-sight to NGC 6752)

Thumbnail image of picture found for this day. APOD: 2019 August 27 - Dark Dust and Colorful Clouds near Antares
Explanation: Why is the sky near Antares and Rho Ophiuchi so dusty yet colorful? The colors result from a mixture of objects and processes. Fine dust illuminated from the front by starlight produces blue reflection nebulae. Gaseous clouds whose atoms are excited by ultraviolet starlight produce reddish emission nebulae. Backlit dust clouds block starlight and so appear dark. Antares, a red supergiant and one of the brighter stars in the night sky, lights up the yellow-red clouds on the lower left of the featured image. Rho Ophiuchi lies at the center of the blue nebula near the top. The distant globular cluster M4 is visible to the right of Antares. These star clouds are even more colorful than humans can see, emitting light across the electromagnetic spectrum.

Thumbnail image of picture found for this day. APOD: 2019 August 24 - Millions of Stars in Omega Centauri
Explanation: Globular star cluster Omega Centauri, also known as NGC 5139, is some 15,000 light-years away. The cluster is packed with about 10 million stars much older than the Sun within a volume about 150 light-years in diameter. It's the largest and brightest of 200 or so known globular clusters that roam the halo of our Milky Way galaxy. Though most star clusters consist of stars with the same age and composition, the enigmatic Omega Cen exhibits the presence of different stellar populations with a spread of ages and chemical abundances. In fact, Omega Cen may be the remnant core of a small galaxy merging with the Milky Way.

Thumbnail image of picture found for this day. APOD: 2019 June 13 - The Colors and Magnitudes of M13
Explanation: M13 is modestly recognized as the Great Globular Star Cluster in Hercules. A ball of stars numbering in the hundreds of thousands crowded into a region 150 light years across, it lies some 25,000 light-years away. The sharp, color picture of M13 at upper left is familiar to many telescopic imagers. Still, M13's Color vs Magnitude Diagram in the panel below and right, made from the same image data, can offer a more telling view. Also known as a Hertzsprung Russell (HR) diagram it plots the apparent brightness of individual cluster stars against color index. The color index is determined for each star by subtracting its brightness (in magnitudes) measured through a red filter from its brightness measured with a blue filter (B-R). Blue stars are hot and red stars are cool so that astronomical color index ranging from bluer to redder follows the relative stellar temperature scale from left (hot) to right (cool). In M13's HR diagram, the stars clearly fall into distinct groups. The broad swath extending diagonally from the bottom right is the cluster's main sequence. A sharp turn toward the upper right hand corner follows the red giant branch while the blue giants are found grouped in the upper left. Formed at the same time, at first M13's stars were all located along the main sequence by mass, lower mass stars at the lower right. Over time higher mass stars have evolved off the main sequence into red, then blue giants and beyond. In fact, the position of the turn-off from the main sequence to the red giant branch indicates the cluster's age at about 12 billion years.

Thumbnail image of picture found for this day. APOD: 2019 May 13 - Rho Ophiuchi Wide Field
Explanation: The colorful clouds surrounding the star system Rho Ophiuchi compose one of the closest star forming regions. Rho Ophiuchi itself is a binary star system visible in the blue reflection nebula just to the left of the image center. The star system, located only 400 light years away, is distinguished by its multi-colored surroundings, which include a red emission nebula and numerous light and dark brown dust lanes. Near the lower left of the Rho Ophiuchi molecular cloud system is the yellow star Antares, while a distant but coincidently-superposed globular cluster of stars, M4, is visible just to the right of Antares. Near the image top lies IC 4592, the Blue Horsehead nebula. The blue glow that surrounds the Blue Horsehead's eye -- and other stars around the image -- is a reflection nebula composed of fine dust. On the featured image right is a geometrically angled reflection nebula cataloged as Sharpless 1. Here, the bright star near the dust vortex creates the light of surrounding reflection nebula. Although most of these features are visible through a small telescope pointed toward the constellations of Ophiuchus, Scorpius, and Sagittarius, the only way to see the intricate details of the dust swirls, as featured above, is to use a long exposure camera.

Thumbnail image of picture found for this day. APOD: 2019 May 9 - Messier 5
Explanation: "Beautiful Nebula discovered between the Balance [Libra] & the Serpent [Serpens] ..." begins the description of the 5th entry in 18th century astronomer Charles Messier's famous catalog of nebulae and star clusters. Though it appeared to Messier to be fuzzy and round and without stars, Messier 5 (M5) is now known to be a globular star cluster, 100,000 stars or more, bound by gravity and packed into a region around 165 light-years in diameter. It lies some 25,000 light-years away. Roaming the halo of our galaxy, globular star clusters are ancient members of the Milky Way. M5 is one of the oldest globulars, its stars estimated to be nearly 13 billion years old. The beautiful star cluster is a popular target for earthbound telescopes. Even close to its dense core, the cluster's red and blue giant stars, and rejuvenated blue stragglers stand out with yellowish and blue hues in this sharp color image.

Thumbnail image of picture found for this day. APOD: 2019 April 4 - Messier 2
Explanation: After the Crab Nebula, M1, this giant star cluster is the second entry in 18th century astronomer Charles Messier's famous list of things that are not comets. M2 is one of the largest globular star clusters now known to roam the halo of our Milky Way galaxy. Though Messier originally described it as a nebula without stars, this stunning Hubble image resolves stars across the central 40 light-years of M2. Its population of stars numbers close to 150,000, concentrated within a total diameter of around 175 light-years. About 55,000 light-years distant toward the constellation Aquarius, this ancient denizen of the Milky Way, also known as NGC 7089, is 13 billion years old.

Thumbnail image of picture found for this day. APOD: 2019 March 28 - The Gaia Stars of M15
Explanation: Messier 15 is a 13 billion year old relic of the early formative years of our galaxy, one of about 170 globular star clusters that still roam the halo of the Milky Way. About 200 light-years in diameter, it lies about 35,000 light years away toward the constellation Pegasus. But this realistic looking view of the ancient globular star cluster is not a photograph. Instead it's an animated gif image constructed from remarkably precise individual measurements of star positions, brightness, and color. The astronomically rich data set used was made by the sky-scanning Gaia satellite which also determined parallax distances for 1.3 billion Milky Way stars. In the animated gif, twinkling stars are M15's identified RR Lyrae stars. Plentiful in M15, RR Lyrae stars are evolved pulsating variable stars whose brightness and pulsation period, typically less than a day, are related.

Thumbnail image of picture found for this day. APOD: 2019 March 24 - Zooming in on Star Cluster Terzan 5
Explanation: Globular clusters once ruled the Milky Way. Back in the old days, back when our Galaxy first formed, perhaps thousands of globular clusters roamed our Galaxy. Today, there are less than 200 left. Over the eons, many globular clusters were destroyed by repeated fateful encounters with each other or the Galactic center. Surviving relics are older than any Earth fossil, older than any other structures in our Galaxy, and limit the universe itself in raw age. There are few, if any, young globular clusters in our Milky Way Galaxy because conditions are not ripe for more to form. The featured video shows what it might look like to go from the Earth to the globular cluster Terzan 5, ending with a picture of the cluster taken with the Hubble Space Telescope. This star cluster has been found to contain not only stars formed in the early days of our Milky Way Galaxy, but also, quite surprisingly, others that formed in a separate burst of star formation about 7 billion years later.

Thumbnail image of picture found for this day. APOD: 2018 November 29 - Across Corona Australis
Explanation: Cosmic dust clouds are draped across a rich field of stars in this broad telescopic panorama near the northern boundary of Corona Australis, the Southern Crown. Less than 500 light-years away the denser clouds effectively block light from more distant background stars in the Milky Way. The entire vista spans about 5 degrees or nearly 45 light-years at the clouds' estimated distance. Toward the right lies a group of bluish reflection nebulae cataloged as NGC 6726, 6727, 6729 and IC 4812. The characteristic blue color is produced as light from hot stars is reflected by the cosmic dust. The dust also obscures from view stars in the region still in the process of formation. Smaller yellowish nebula NGC 6729 surrounds young variable star R Coronae Australis. Below it are arcs and loops identified as Herbig Haro (HH) objects associated with energetic newborn stars. Magnificent globular star cluster NGC 6723 is above and right of the nebulae. Though NGC 6723 appears to be part of the group, its ancient stars actually lie nearly 30,000 light-years away, far beyond the young stars of the Corona Australis dust clouds.

Thumbnail image of picture found for this day. APOD: 2018 October 17 - M15: Dense Globular Star Cluster
Explanation: Messier 15 is an immense swarm of over 100,000 stars. A 13 billion year old relic of the early formative years of our galaxy it's one of about 170 globular star clusters that still roam the halo of the Milky Way. Centered in this sharp telescopic view, M15 lies about 35,000 light years away toward the constellation Pegasus, well beyond the spiky foreground stars. Its diameter is about 200 light-years. But more than half its stars are packed into the central 10 light-years or so, one of the densest concentrations of stars known. Hubble-based measurements of the increasing velocities of M15's central stars are evidence that a massive black hole resides at the center of dense globular cluster M15.

Thumbnail image of picture found for this day. APOD: 2018 October 3 - NGC 1898: Globular Cluster in the LMC
Explanation: Jewels don't shine this bright -- only stars do. And almost every spot in this glittering jewel-box of an image from the Hubble Space Telescope is a star. Now some stars are more red than our Sun, and some more blue -- but all of them are much farther away. Although it takes light about 8 minutes to reach Earth from the Sun, NGC 1898 is so far away that it takes light about 160,000 years to get here. This huge ball of stars, NGC 1898, is called a globular cluster and resides in the central bar of the Large Magellanic Cloud (LMC) -- a satellite galaxy of our large Milky Way Galaxy. The featured multi-colored image includes light from the infrared to the ultraviolet and was taken to help determine if the stars of NGC 1898 all formed at the same time, or at different times. There are increasing indications that most globular clusters formed stars in stages, and that, in particular, stars from NGC 1898 formed shortly after ancient encounters with the Small Magellanic Cloud (SMC) and our Milky Way Galaxy.

Thumbnail image of picture found for this day. APOD: 2017 November 4 - Hubble s Messier 5
Explanation: "Beautiful Nebula discovered between the Balance [Libra] & the Serpent [Serpens] ..." begins the description of the 5th entry in 18th century astronomer Charles Messier's famous catalog of nebulae and star clusters. Though it appeared to Messier to be fuzzy and round and without stars, Messier 5 (M5) is now known to be a globular star cluster, 100,000 stars or more, bound by gravity and packed into a region around 165 light-years in diameter. It lies some 25,000 light-years away. Roaming the halo of our galaxy, globular star clusters are ancient members of the Milky Way. M5 is one of the oldest globulars, its stars estimated to be nearly 13 billion years old. The beautiful star cluster is a popular target for Earthbound telescopes. Of course, deployed in low Earth orbit on April 25, 1990, the Hubble Space Telescope has also captured its own stunning close-up view that spans about 20 light-years across the central region of M5. Even close to its dense core the cluster's aging red and blue giant stars and rejuvenated blue stragglers stand out in yellow and blue hues in the sharp color image.

Thumbnail image of picture found for this day. APOD: 2017 October 11 - Star Cluster NGC 362 from Hubble
Explanation: If our Sun were near the center of NGC 362, the night sky would glow like a jewel box of bright stars. Hundreds of stars would glow brighter than Sirius, and in many different colors. Although these stars could become part of breathtaking constellations and intricate folklore, it would be difficult for planetary inhabitants there to see -- and hence understand -- the greater universe beyond. NGC 362 is one of only about 170 globular clusters of stars that exist in our Milky Way Galaxy. This star cluster is one of the younger globulars, forming likely well after our Galaxy. NGC 362 can be found with the unaided eye nearly in front of the Small Magellanic Cloud, and angularly close to the second brightest globular cluster known, 47 Tucanae. The featured image was taken with the Hubble Space Telescope to help better understand how massive stars end up near the center of some globular clusters.

Thumbnail image of picture found for this day. APOD: 2017 July 11 - Star Cluster Omega Centauri in HDR
Explanation: Behold the largest ball of stars in our galaxy. Omega Centauri is packed with about 10 million stars, many older than our Sun and packed within a volume of only about 150 light-years in diameter. The star cluster is the largest and brightest of 200 or so known globular clusters that roam the halo of our Milky Way galaxy. Though most star clusters consist of stars with the same age and composition, the enigmatic Omega Cen exhibits the presence of different stellar populations with a spread of ages and chemical abundances. In fact, Omega Cen may be the remnant core of a small galaxy merging with the Milky Way. The featured image shows so many stars because it merged different exposures with high dynamic range (HDR) techniques. Omega Centauri, also known as NGC 5139, lies about 15,000 light-years away toward the southern constellation of the Centaurus.

Thumbnail image of picture found for this day. 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.

Thumbnail image of picture found for this day. APOD: 2017 May 12 - M13: The Great Globular Cluster in Hercules
Explanation: In 1716, English astronomer Edmond Halley noted, "This is but a little Patch, but it shews itself to the naked Eye, when the Sky is serene and the Moon absent." Of course, M13 is now less modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. Telescopic views reveal the spectacular cluster's hundreds of thousands of stars. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter. Approaching the cluster core upwards of 100 stars could be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. Along with the cluster's dense core, the outer reaches of M13 are highlighted in this sharp color image. The cluster's evolved red and blue giant stars show up in yellowish and blue tints.

Thumbnail image of picture found for this day. APOD: 2016 September 21 - Zooming in on Star Cluster Terzan 5
Explanation: Globular clusters once ruled the Milky Way. Back in the old days, back when our Galaxy first formed, perhaps thousands of globular clusters roamed our Galaxy. Today, there are less than 200 left. Over the eons, many globular clusters were destroyed by repeated fateful encounters with each other or the Galactic center. Surviving relics are older than any Earth fossil, older than any other structures in our Galaxy, and limit the universe itself in raw age. There are few, if any, young globular clusters in our Milky Way Galaxy because conditions are not ripe for more to form. The featured video shows what it might look like to go from the Earth to the globular cluster Terzan 5, ending with a picture of the cluster taken with the Hubble Space Telescope. This star cluster was recently found to contain not only stars formed in the early days of our Milky Way Galaxy, but also, quite surprisingly, others that formed in a separate burst of star formation about 7 billion years later.

Thumbnail image of picture found for this day. APOD: 2016 July 27 - M13: A Great Globular Cluster of Stars
Explanation: M13 is one of the most prominent and best known globular clusters. Visible with binoculars in the constellation of Hercules, M13 is frequently one of the first objects found by curious sky gazers seeking celestials wonders beyond normal human vision. M13 is a colossal home to over 100,000 stars, spans over 150 light years across, lies over 20,000 light years distant, and is over 12 billion years old. At the 1974 dedication of Arecibo Observatory, a radio message about Earth was sent in the direction of M13. The featured image in HDR, taken through a small telescope, spans an angular size just larger than a full Moon, whereas the inset image, taken by Hubble Space Telescope, zooms in on the central 0.04 degrees.

Thumbnail image of picture found for this day. APOD: 2016 July 5 - The Colorful Clouds of Rho Ophiuchi
Explanation: The many spectacular colors of the Rho Ophiuchi (oh'-fee-yu-kee) clouds highlight the many processes that occur there. The blue regions shine primarily by reflected light. Blue light from the star Rho Ophiuchi and nearby stars reflects more efficiently off this portion of the nebula than red light. The Earth's daytime sky appears blue for the same reason. The red and yellow regions shine primarily because of emission from the nebula's atomic and molecular gas. Light from nearby blue stars - more energetic than the bright star Antares - knocks electrons away from the gas, which then shines when the electrons recombine with the gas. The dark brown regions are caused by dust grains - born in young stellar atmospheres - which effectively block light emitted behind them. The Rho Ophiuchi star clouds, well in front of the globular cluster M4 visible here on the upper right, are even more colorful than humans can see - the clouds emits light in every wavelength band from the radio to the gamma-ray.

Thumbnail image of picture found for this day. APOD: 2016 April 27 - Omega Centauri: The Brightest Globular Star Cluster
Explanation: This huge ball of stars predates our Sun. Long before humankind evolved, before dinosaurs roamed, and even before our Earth existed, ancient globs of stars condensed and orbited a young Milky Way Galaxy. Of the 200 or so globular clusters that survive today, Omega Centauri is the largest, containing over ten million stars. Omega Centauri is also the brightest globular cluster, at apparent visual magnitude 3.9 it is visible to southern observers with the unaided eye. Cataloged as NGC 5139, Omega Centauri is about 18,000 light-years away and 150 light-years in diameter. Unlike many other globular clusters, the stars in Omega Centauri show several different ages and trace chemical abundances, indicating that the globular star cluster has a complex history over its 12 billion year age.

Thumbnail image of picture found for this day. APOD: 2016 April 11 - The Comet and the Star Cluster
Explanation: Comet Linear has become unexpectedly bright. The comet, discovered in 2000, underwent a 100-fold outburst just a week before it passed a mere 14 lunar distances from Earth late last month. The comet was captured here last week at about magnitude 6 -- just bright enough to be seen by the unaided eye -- passing in front of the distant globular star cluster M14. Comet 252/P LINEAR is one of a rare group of comets that vacillate between the Earth and Jupiter every 5 years. How the comet will evolve from here is unknown, but hopes run high that it will remain a good object for binoculars in northern skies for the next week or two.

Thumbnail image of picture found for this day. APOD: 2015 September 10 - NGC 4372 and the Dark Doodad
Explanation: The delightful Dark Doodad Nebula drifts through southern skies, a tantalizing target for binoculars in the constellation Musca, The Fly. The dusty cosmic cloud is seen against rich starfields just south of the prominent Coalsack Nebula and the Southern Cross. Stretching for about 3 degrees across this scene the Dark Doodad is punctuated at its southern tip (lower left) by globular star cluster NGC 4372. Of course NGC 4372 roams the halo of our Milky Way Galaxy, a background object some 20,000 light-years away and only by chance along our line-of-sight to the Dark Doodad. The Dark Doodad's well defined silhouette belongs to the Musca molecular cloud, but its better known alliterative moniker was first coined by astro-imager and writer Dennis di Cicco in 1986 while observing Comet Halley from the Australian outback. The Dark Doodad is around 700 light-years distant and over 30 light-years long.

Thumbnail image of picture found for this day. APOD: 2015 July 6 - Colorful Clouds Near Rho Ophiuchi
Explanation: Why is the sky near Antares and Rho Ophiuchi so colorful? The colors result from a mixture of objects and processes. Fine dust illuminated from the front by starlight produces blue reflection nebulae. Gaseous clouds whose atoms are excited by ultraviolet starlight produce reddish emission nebulae. Backlit dust clouds block starlight and so appear dark. Antares, a red supergiant and one of the brighter stars in the night sky, lights up the yellow-red clouds on the lower center of the featured image. Rho Ophiuchi lies at the center of the blue nebula on the left. The distant globular cluster M4 is visible to the upper right of center. These star clouds are even more colorful than humans can see, emitting light across the electromagnetic spectrum.

Thumbnail image of picture found for this day. APOD: 2015 June 20 - Hubble's Messier 5
Explanation: "Beautiful Nebula discovered between the Balance [Libra] & the Serpent [Serpens] ..." begins the description of the 5th entry in 18th century astronomer Charles Messier's famous catalog of nebulae and star clusters. Though it appeared to Messier to be fuzzy and round and without stars, Messier 5 (M5) is now known to be a globular star cluster, 100,000 stars or more, bound by gravity and packed into a region around 165 light-years in diameter. It lies some 25,000 light-years away. Roaming the halo of our galaxy, globular star clusters are ancient members of the Milky Way. M5 is one of the oldest globulars, its stars estimated to be nearly 13 billion years old. The beautiful star cluster is a popular target for Earthbound telescopes. Of course, deployed in low Earth orbit on April 25, 1990, the Hubble Space Telescope has also captured its own stunning close-up view that spans about 20 light-years near the central region of M5. Even close to its dense core at the left, the cluster's aging red and blue giant stars and rejuvenated blue stragglers stand out in yellow and blue hues in the sharp color image.

Thumbnail image of picture found for this day. APOD: 2015 May 20 - A Cliff Looming on Comet 67P
Explanation: What's that looming behind this gravel-strewn hill on Comet Churyumov–Gerasimenko? A jagged cliff. The unusual double-lobed nucleus of Comet 67P/Churyumov–Gerasimenko lends itself to unusual and dramatic vistas, another of which has been captured by the Rosetta spacecraft that arrived at the comet last September. The featured cometscape, taken last October and digitally enhanced, spans about 850 meters across. Meanwhile, Comet Churyumov–Gerasimenko continues to sprout jets as it nears its closest approach to the Sun in August. Along the way, Rosetta will continue listening for signals from Philae, a probe that landed on the nucleus but rebounded to an unknown surface location last November. If newly exposed to sunlight, Philae might regain enough energy to again signal Rosetta.

Thumbnail image of picture found for this day. APOD: 2015 May 19 - Globular Star Cluster 47 Tuc
Explanation: Globular star cluster 47 Tucanae is a jewel box of the southern sky. Also known as NGC 104, it roams the halo of our Milky Way Galaxy along with over 150 other globular star clusters. The second brightest globular cluster (after Omega Centauri) as seen from planet Earth, 47 Tuc lies about 17,000 light-years away and can be spotted naked-eye near the Small Magellanic Cloud in the constellation of the Toucan. The dense cluster is made up of hundreds of thousands of stars in a volume only about 120 light-years across. Recent observations have shown that 47 Tuc's white dwarf stars are in the process of being gravitationally expelled to the outer parts of the cluster due to their relatively low mass. Other colorful low mass stars including yellowish red giant stars are easy to pick out on the outskirts of the cluster in this recently released sharp telescopic portrait by the Hubble Space Telescope.

Thumbnail image of picture found for this day. APOD: 2015 February 19 - Palomar 12
Explanation: Palomar 12 was not born here. The stars of the globular cluster, first identified in the Palomar Sky Survey, are younger than those in other globular star clusters that roam the halo of our Milky Way Galaxy. Palomar 12's position in our galaxy and measured motion suggest its home was once the Sagittarius Dwarf Elliptical Galaxy, a small satellite of the Milky Way. Disrupted by gravitational tides during close encounters the satellite galaxy has lost its stars to the larger Milky Way. Now part of the Milky Way's halo, the tidal capture of Palomar 12 likely took place some 1.7 billion years ago. Seen behind spiky foreground stars in the sharp Hubble image, Palomar 12 spans nearly 60 light-years. Still much closer than the faint, fuzzy, background galaxies scattered throughout the field of view, it lies about 60,000 light-years away, toward the constellation Capricornus.

Thumbnail image of picture found for this day. APOD: 2014 December 31 - Comet Lovejoy before a Globular Star Cluster
Explanation: Comet Lovejoy has become visible to the unaided eye. To see the comet, just go outside an hour or so after sunset and look for a fuzzy patch to the right of Orion's belt. Binoculars and a star chart may help. Pictured here, Comet C/2014 Q2 (Lovejoy) was captured three days ago passing nearly in front of M79, the globular star cluster visible as the bright spot slightly above and to the left of the comet's green-hued coma. The nucleus of Comet Lovejoy is a giant dirty iceberg that is shedding gas into a long and intricate ion tail, extending across the image, as it nears the Sun. The comet is expected to become even easier to spot for northern observers during January, as it is rises earlier and, hopefully, continues to brighten.

Thumbnail image of picture found for this day. APOD: 2014 December 10 - The Reddening of M71
Explanation: Now known to be a globular star cluster at the tender age of 10 billion years, M71 is a mere 13,000 light-years away within the narrow boundaries of the faint constellation Sagitta. Close to the plane of the Milky Way galaxy in planet Earth's sky, its 10,000 or so member stars are gathered into a region about 27 light-years across near the center of this color composite view. In fact, the line-of-sight to M71 passes along the galactic plane through much intervening diffuse interstellar dust. The dust dims starlight and scatters blue light more efficiently, masking the brightness of M71's stars and shifting true star colors toward the red. How much are the star colors shifted? Slide your cursor over the image (or follow this link) to use an estimate of the dust reddening or galactic extinction to correct the star colors in M71. Corrections to the brightness and colors of M71 member stars are needed to measure the cluster's distance and age using a Color-Magnitude diagram.

Thumbnail image of picture found for this day. APOD: 2014 July 27 - Rho Ophiuchi Wide Field
Explanation: The clouds surrounding the star system Rho Ophiuchi compose one of the closest star forming regions. Rho Ophiuchi itself is a binary star system visible in the light-colored region on the image right. The star system, located only 400 light years away, is distinguished by its colorful surroundings, which include a red emission nebula and numerous light and dark brown dust lanes. Near the upper right of the Rho Ophiuchi molecular cloud system is the yellow star Antares, while a distant but coincidently-superposed globular cluster of stars, M4, is visible between Antares and the red emission nebula. Near the image bottom lies IC 4592, the Blue Horsehead nebula. The blue glow that surrounds the Blue Horsehead's eye -- and other stars around the image -- is a reflection nebula composed of fine dust. On the above image left is a geometrically angled reflection nebula cataloged as Sharpless 1. Here, the bright star near the dust vortex creates the light of surrounding reflection nebula. Although most of these features are visible through a small telescope pointed toward the constellations of Ophiuchus, Scorpius, and Sagittarius, the only way to see the intricate details of the dust swirls, as featured above, is to use a long exposure camera.

Thumbnail image of picture found for this day. APOD: 2014 May 29 - Millions of Stars in Omega Centauri
Explanation: Globular star cluster Omega Centauri, also known as NGC 5139, is some 15,000 light-years away. The cluster is packed with about 10 million stars much older than the Sun within a volume about 150 light-years in diameter, the largest and brightest of 200 or so known globular clusters that roam the halo of our Milky Way galaxy. Though most star clusters consist of stars with the same age and composition, the enigmatic Omega Cen exhibits the presence of different stellar populations with a spread of ages and chemical abundances. In fact, Omega Cen may be the remnant core of a small galaxy merging with the Milky Way. This astronomically sharp color image of the classic globular cluster was recorded in March under Chilean skies from Hacienda Los Andes.

Thumbnail image of picture found for this day. APOD: 2014 April 25 - Hubble's Messier 5
Explanation: "Beautiful Nebula discovered between the Balance [Libra] & the Serpent [Serpens] ..." begins the description of the 5th entry in 18th century astronomer Charles Messier's famous catalog of nebulae and star clusters. Though it appeared to Messier to be fuzzy and round and without stars, Messier 5 (M5) is now known to be a globular star cluster, 100,000 stars or more, bound by gravity and packed into a region around 165 light-years in diameter. It lies some 25,000 light-years away. Roaming the halo of our galaxy, globular star clusters are ancient members of the Milky Way. M5 is one of the oldest globulars, its stars estimated to be nearly 13 billion years old. The beautiful star cluster is a popular target for Earthbound telescopes. Of course, deployed in low Earth orbit on April 25, 1990, the Hubble Space Telescope has also captured its own stunning close-up view that spans about 20 light-years near the central region of M5. Even close to its dense core at the left, the cluster's aging red and blue giant stars and rejuvenated blue stragglers stand out in yellow and blue hues in the sharp color image.

Thumbnail image of picture found for this day. APOD: 2013 December 3 - The Colorful Clouds of Rho Ophiuchi
Explanation: The many spectacular colors of the Rho Ophiuchi (oh'-fee-yu-kee) clouds highlight the many processes that occur there. The blue regions shine primarily by reflected light. Blue light from the star Rho Ophiuchi and nearby stars reflects more efficiently off this portion of the nebula than red light. The Earth's daytime sky appears blue for the same reason. The red and yellow regions shine primarily because of emission from the nebula's atomic and molecular gas. Light from nearby blue stars - more energetic than the bright star Antares - knocks electrons away from the gas, which then shines when the electrons recombine with the gas. The dark brown regions are caused by dust grains - born in young stellar atmospheres - which effectively block light emitted behind them. The Rho Ophiuchi star clouds, well in front of the globular cluster M4 visible above on lower left, are even more colorful than humans can see - the clouds emits light in every wavelength band from the radio to the gamma-ray.

Thumbnail image of picture found for this day. APOD: 2013 November 19 - Globular Cluster M15 from Hubble
Explanation: Stars, like bees, swarm around the center of bright globular cluster M15. This ball of over 100,000 stars is a relic from the early years of our Galaxy, and continues to orbit the Milky Way's center. M15, one of about 170 globular clusters remaining, is noted for being easily visible with only binoculars, having at its center one of the densest concentrations of stars known, and containing a high abundance of variable stars and pulsars. Released only recently, this sharp image taken by the Earth-orbiting Hubble Space Telescope spans about 120 light years. It shows the dramatic increase in density of stars toward the cluster's center. M15 lies about 35,000 light years away toward the constellation of the Winged Horse (Pegasus).

Thumbnail image of picture found for this day. APOD: 2013 September 12 - Stars and Dust Across Corona Australis
Explanation: Cosmic dust clouds sprawl across a rich field of stars in this sweeping telescopic vista near the northern boundary of Corona Australis, the Southern Crown. Less than 500 light-years away the dust clouds effectively block light from more distant background stars in the Milky Way. The entire frame spans about 2 degrees or over 15 light-years at the clouds' estimated distance. Near center is a group of lovely reflection nebulae cataloged as NGC 6726, 6727, 6729, and IC 4812. A characteristic blue color is produced as light from hot stars is reflected by the cosmic dust. The dust also obscures from view stars in the region still in the process of formation. Smaller yellowish nebula NGC 6729 surrounds young variable star R Coronae Australis. Below it are arcs and loops identified as Herbig Haro objects associated with energetic newborn stars. Magnificent globular star cluster NGC 6723 is at the right. Though NGC 6723 appears to be part of the group, its ancient stars actually lie nearly 30,000 light-years away, far beyond the young stars of the Corona Australis dust clouds.

Thumbnail image of picture found for this day. APOD: 2013 July 5 - Globular Star Cluster NGC 6752
Explanation: Some 13,000 light-years away toward the southern constellation Pavo, the globular star cluster NGC 6752 roams the halo of our Milky Way galaxy. Over 10 billion years old, NGC 6752 follows clusters Omega Centauri and 47 Tucanae as the third brightest globular in planet Earth's night sky. It holds over 100 thousand stars in a sphere about 100 light-years in diameter. Telescopic explorations of the NGC 6752 have found that a remarkable fraction of the stars near the cluster's core, are multiple star systems. They also reveal the presence of blue straggle stars, stars which appear to be too young and massive to exist in a cluster whose stars are all expected to be at least twice as old as the Sun. The blue stragglers are thought to be formed by star mergers and collisions in the dense stellar environment at the cluster's core. This sharp color composite also features the cluster's ancient red giant stars in yellowish hues.

Thumbnail image of picture found for this day. APOD: 2013 May 1 - Omega Centauri: The Brightest Globular Cluster
Explanation: This huge ball of stars predates our Sun. Long before humankind evolved, before dinosaurs roamed, and even before our Earth existed, ancient globs of stars condensed and orbited a young Milky Way Galaxy. Of the 200 or so globular clusters that survive today, Omega Centauri is the largest, containing over ten million stars. Omega Centauri is also the brightest globular cluster, at apparent visual magnitude 3.9 it is visible to southern observers with the unaided eye. Cataloged as NGC 5139, Omega Centauri is about 18,000 light-years away and 150 light-years in diameter. Unlike many other globular clusters, the stars in Omega Centauri show several different ages and trace chemical abundances, indicating that the globular star cluster has a complex history over its 12 billion year age.

Thumbnail image of picture found for this day. APOD: 2013 February 16 - Sweeping Through Southern Skies
Explanation: For now, Comet Lemmon (C/2012 F6), and Comet PanSTARRS (C/2011 L4) are sweeping through southern skies. Lemmon's lime green coma and thin tail are near the left edge of this telephoto scene, a single frame from a timelapse video (vimeo here) recorded on February 12, tracking its motion against the background stars. Comet Lemmon's path brought it close to the line-of-sight to prominent southern sky treasures the Small Magellanic Cloud and globular cluster 47 Tucanae (right). Sporting a broader, whitish tail, Comet PanSTARRS appears in later video frames moving through the faint constellation Microscopium. Visible in binoculars and small telescopes, both comets are getting brighter and headed toward northern skies in coming months.

Thumbnail image of picture found for this day. APOD: 2013 January 31 - NGC 4372 and the Dark Doodad
Explanation: The delightful Dark Doodad Nebula drifts through southern skies, a tantalizing target for binoculars in the constellation Musca, The Fly. The dusty cosmic cloud is seen against rich starfields just south of the prominent Coalsack Nebula and the Southern Cross. Stretching for about 3 degrees across this scene the Dark Doodad seems punctuated at its southern tip (lower left) by globular star cluster NGC 4372. Of course NGC 4372 roams the halo of our Milky Way Galaxy, a background object some 20,000 light-years away and only by chance along our line-of-sight to the Dark Doodad. The Dark Doodad's well defined silhouette belongs to the Musca molecular cloud, but its better known alliterative moniker was first coined by astro-imager and writer Dennis di Cicco in 1986 while observing comet Halley from the Australian outback. The Dark Doodad is around 700 light-years distant and over 30 light-years long.

Thumbnail image of picture found for this day. APOD: 2012 December 11 - NGC 604: Giant Stellar Nursery
Explanation: Stars are sometimes born in the midst of chaos. About 3 million years ago in the nearby galaxy M33, a large cloud of gas spawned dense internal knots which gravitationally collapsed to form stars. NGC 604 was so large, however, it could form enough stars to make a globular cluster. Many young stars from this cloud are visible in the above image from the Hubble Space Telescope, along with what is left of the initial gas cloud. Some stars were so massive they have already evolved and exploded in a supernova. The brightest stars that are left emit light so energetic that they create one of the largest clouds of ionized hydrogen gas known, comparable to the Tarantula Nebula in our Milky Way's close neighbor, the Large Magellanic Cloud.

Thumbnail image of picture found for this day. APOD: 2012 December 6 - 47 Tuc Near the Small Magellanic Cloud
Explanation: Globular star cluster 47 Tucanae is a jewel of the southern sky. Also known as NGC 104, it roams the halo of our Milky Way Galaxy along with around 200 other globular star clusters. The second brightest globular cluster (after Omega Centauri) as seen from planet Earth, it lies about 13,000 light-years away and can be spotted naked-eye near the Small Magellanic Cloud (SMC) in the constellation of the Toucan. Of course, the SMC is some 210,000 light-years distant, a satellite galaxy of our Milky Way and not physically close to 47 Tuc. Stars on the outskirts of the SMC are seen at the upper left of this broad southern skyscape. Toward the lower right with about the same apparent diameter as a Full Moon, dense cluster 47 Tuc is made up of several million stars in a volume only about 120 light-years across. Away from the bright cluster core, the red giants of 47 Tuc are easy to pick out as yellowish tinted stars. Globular cluster 47 Tuc is also home to exotic x-ray binary star systems.

Thumbnail image of picture found for this day. APOD: 2012 September 27 - Stars and Dust Across Corona Australis
Explanation: Cosmic dust clouds sprawl across a rich field of stars in this sweeping telescopic vista near the northern boundary of Corona Australis, the Southern Crown. Probably less than 500 light-years away and effectively blocking light from more distant, background stars in the Milky Way, the densest part of the dust cloud is about 8 light-years long. At its tip (upper right) is a group of lovely reflection nebulae cataloged as NGC 6726, 6727, 6729, and IC 4812. A characteristic blue color is produced as light from hot stars is reflected by the cosmic dust. The smaller yellowish nebula (NGC 6729) surrounds young variable star R Coronae Australis. Magnificent globular star cluster NGC 6723 is toward the upper right corner of the view. While NGC 6723 appears to be part of the group, it actually lies nearly 30,000 light-years away, far beyond the Corona Australis dust clouds.

Thumbnail image of picture found for this day. APOD: 2012 August 28 - Colorful Clouds Near Rho Ophiuchi
Explanation: Why is the sky near Antares and Rho Ophiuchi so colorful? The colors result from a mixture of objects and processes. Fine dust illuminated from the front by starlight produces blue reflection nebulae. Gaseous clouds whose atoms are excited by ultraviolet starlight produce reddish emission nebulae. Backlit dust clouds block starlight and so appear dark. Antares, a red supergiant and one of the brighter stars in the night sky, lights up the yellow-red clouds on the lower center. Rho Ophiuchi lies at the center of the blue nebula near the top. The distant globular cluster M4 is visible just to the right of Antares, and to the lower left of the red cloud engulfing Sigma Scorpii. These star clouds are even more colorful than humans can see, emitting light across the electromagnetic spectrum.

Thumbnail image of picture found for this day. APOD: 2012 August 19 - M72: A Globular Cluster of Stars
Explanation: Globular clusters once ruled the Milky Way. Back in the old days, back when our Galaxy first formed, perhaps thousands of globular clusters roamed our Galaxy. Today, there are less than 200 left. Many globular clusters were destroyed over the eons by repeated fateful encounters with each other or the Galactic center. Surviving relics are older than any Earth fossil, older than any other structures in our Galaxy, and limit the universe itself in raw age. There are few, if any, young globular clusters in our Milky Way Galaxy because conditions are not ripe for more to form. Pictured above by the Hubble Space Telescope are about 100,000 of M72's stars. M72, which spans about 50 light years and lies about 50,000 light years away, can be seen with a small telescope toward the constellation of the Water Bearer (Aquarius).

Thumbnail image of picture found for this day. APOD: 2012 August 3 - Messier 5
Explanation: "Beautiful Nebula discovered between the Balance [Libra] & the Serpent [Serpens] ..." begins the description of the 5th entry in 18th century astronomer Charles Messier's famous catalog of nebulae and star clusters. Though it appeared to Messier to be fuzzy and round and without stars, Messier 5 (M5) is now known to be a globular star cluster, 100,000 stars or more, bound by gravity and packed into a region around 165 light-years in diameter. It lies some 25,000 light-years away. Roaming the halo of our galaxy, globular star clusters are ancient members of the Milky Way. M5 is one of the oldest globulars, its stars estimated to be nearly 13 billion years old. The beautiful star cluster is a popular target for earthbound telescopes. Even close to its dense core, the cluster's red and blue giant stars stand out with yellowish and blue hues in this sharp color image.

Thumbnail image of picture found for this day. APOD: 2012 June 14 - M13: The Great Globular Cluster in Hercules
Explanation: In 1716, English astronomer Edmond Halley noted, "This is but a little Patch, but it shews itself to the naked Eye, when the Sky is serene and the Moon absent." Of course, M13 is now modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. Telescopic views reveal the spectacular cluster's hundreds of thousands of stars. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter, but approaching the cluster core upwards of 100 stars could be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. Along with the cluster's dense core, the outer reaches of M13 are highlighted in this sharp color image. The cluster's evolved red and blue giant stars show up in yellowish and blue tints.

Thumbnail image of picture found for this day. APOD: 2012 April 10 - 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 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 3/4 degree or nearly 1.5 full moons, covering 40 light-years at the distance of NGC 2264. Its cast of cosmic characters includes the Fox Fur Nebula, whose convoluted pelt lies below center, bright variable star S Mon immersed in the blue-tinted haze, and the Cone Nebula near the tree's top. Of course, 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 and its broader base centered near S Mon.

Thumbnail image of picture found for this day. APOD: 2012 April 9 - Blue Straggler Stars in Globular Cluster M53
Explanation: If our Sun were part of M53, the night sky would glow like a jewel box of bright stars. M53, also known as NGC 5024, is one of about 250 globular clusters that survive in our Galaxy. Most of the stars in M53 are older and redder than our Sun, but some enigmatic stars appear to be bluer and younger. These young stars might contradict the hypothesis that all the stars in M53 formed at nearly the same time. These unusual stars are known as blue stragglers and are unusually common in M53. After much debate, blue stragglers are now thought to be stars rejuvenated by fresh matter falling in from a binary star companion. By analyzing pictures of globular clusters like the above image taken by the Hubble Space Telescope, astronomers use the abundance of stars like blue stragglers to help determine the age of the globular cluster and hence a limit on the age of the universe. M53, visible with a binoculars towards the constellation of Bernice's Hair (Coma Berenices), contains over 250,000 stars and is one of the furthest globulars from the center of our Galaxy.

Thumbnail image of picture found for this day. APOD: 2012 March 23 - Messier 9 Close Up
Explanation: Renown 18th century astronomer Charles Messier described this 9th entry in his famous astronomical catalog as "Nebula, without star, in the right leg of Ophiuchus ...". But Messier 9 (M9) does have stars, known to modern astronomers as a globular cluster of over 300,000 stars within a diameter of about 90 light-years. It lies some 25,000 light-years distant, near the central bulge of our Milky Way galaxy. This Hubble Space Telescope close-up resolves the dense swarm of stars across the cluster's central 25 light-years. At least twice the age of the Sun and deficient in heavy elements, the cluster stars have colors corresponding to their temperatures, redder stars are cooler, bluer stars are hotter. Many of the cluster's cool red giant stars show a yellowish tint in the sharp Hubble view.

Thumbnail image of picture found for this day. 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.

Thumbnail image of picture found for this day. APOD: 2011 June 15 - Millions of Stars in Omega Centauri
Explanation: Featured in this sharp telescopic image, globular star cluster Omega Centauri (NGC 5139) is some 15,000 light-years away. Some 150 light-years in diameter, the cluster is packed with about 10 million stars much older than the Sun. Omega Cen is the largest of 200 or so known globular clusters that roam the halo of our Milky Way galaxy. Though most star clusters consist of stars with the same age and composition, the enigmatic Omega Cen exhibits the presence of different stellar populations with a spread of ages and chemical abundances. In fact, Omega Cen may be the remnant core of a small galaxy merging with the Milky Way.

Thumbnail image of picture found for this day. APOD: 2011 May 3 - Globular Cluster M15 from Hubble
Explanation: Stars, like bees, swarm around the center of bright globular cluster M15. This ball of over 100,000 stars is a relic from the early years of our Galaxy, and continues to orbit the Milky Way's center. M15, one of about 150 globular clusters remaining, is noted for being easily visible with only binoculars, having at its center one of the densest concentrations of stars known, and containing a high abundance of variable stars and pulsars. This sharp image, taken by the Earth-orbiting Hubble Space Telescope, spans about 120 light years. It shows the dramatic increase in density of stars toward the cluster's center. M15 lies about 35,000 light years away toward the constellation of the Winged Horse (Pegasus). Recent evidence indicates that a massive black hole might reside as the center of M15.

Thumbnail image of picture found for this day. APOD: 2011 January 16 - Globular Star Cluster 47 Tuc
Explanation: Globular star cluster 47 Tucanae is a jewel of the southern sky. Also known as NGC 104, it roams the halo of our Milky Way Galaxy along with some 200 other globular star clusters. The second brightest globular cluster (after Omega Centauri) as seen from planet Earth, it lies about 13,000 light-years away and can be spotted naked-eye near the Small Magellanic Cloud in the constellation of the Toucan. The dense cluster is made up of several million stars in a volume only about 120 light-years across. Red giant stars on the outskirts of the cluster are easy to pick out as yellowish stars in this sharp telescopic portrait. Globular cluster 47 Tuc is also home to exotic x-ray binary star systems.

Thumbnail image of picture found for this day. APOD: 2010 October 9 - Globular Star Cluster NGC 6934
Explanation: Globular star clusters roam the halo of our Milky Way Galaxy. Gravitationally bound, these spherical groupings of typically several hundred thousand stars are ancient, older than the stars of the galactic disk. In fact, measurements of globular cluster ages constrain the age of the Universe (it must be older than the stars in it!) and accurate cluster distance determinations provide a rung on the astronomical distance ladder. Globular star cluster NGC 6934 itself lies about 50,000 light-years away in the constellation Delphinus. At that distance, this sharp image from Hubble's Advanced Camera for Surveys spans about 50 light-years. The cluster stars are estimated to be some 10 billion years old.

Thumbnail image of picture found for this day. APOD: 2010 July 5 - The Milky Way Over Pulpit Rock
Explanation: Can a picture of the sky be relaxing? A candidate for such a picture might be this image taken only last month from Cape Schank, Victoria, Australia. The frame is highlighted by a dreamlike lagoon, two galaxies, and tens of thousands of stars. The rock cropping on the left may appear from this angle like a human head, but the more famous rock structure is on the far right and known as Pulpit Rock. Across the top of the image runs a distant stream of bright stars and dark dust that is part of the disk of our spiral Milky Way Galaxy. On the right, just above Pulpit Rock, is the Milky Way's small neighboring galaxy the Small Magellanic Cloud (SMC). The bright white object just to the left of the SMC is a globular cluster of stars in the Milky Way known as 47 Tucana.

Thumbnail image of picture found for this day. APOD: 2010 May 27 - M13: The Great Globular Cluster in Hercules
Explanation: In 1716, English astronomer Edmond Halley noted, "This is but a little Patch, but it shews itself to the naked Eye, when the Sky is serene and the Moon absent." Of course, M13 is now modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. Telescopic views reveal the spectacular cluster's hundreds of thousands of stars. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter, but approaching the cluster core upwards of 100 stars could be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. Along with the cluster's dense core, the outer reaches of M13 are highlighted in this sharp color image. The cluster's evolved red and blue giant stars show up in yellowish and blue tints.

Thumbnail image of picture found for this day. APOD: 2010 May 24 - Rho Ophiuchi Wide Field
Explanation: The clouds surrounding the star system Rho Ophiuchi compose one of the closest star forming regions. Rho Ophiuchi itself is a binary star system visible in the light-colored region on the image right. The star system, located only 400 light years away, is distinguished by its colorful surroundings, which include a red emission nebula and numerous light and dark brown dust lanes. Near the upper right of the Rho Ophiuchi molecular cloud system is the yellow star Antares, while a distant but coincidently-superposed globular cluster of stars, M4, is visible between Antares and the red emission nebula. Near the image bottom lies IC 4592, the Blue Horsehead nebula. The blue glow that surrounds the Blue Horsehead's eye -- and other stars around the image -- is a reflection nebula composed of fine dust. On the above image left is a geometrically angled reflection nebula cataloged as Sharpless 1. Here, the bright star near the dust vortex creates the light of surrounding reflection nebula. Although most of these features are visible through a small telescope pointed toward the constellations of Ophiuchus, Scorpius, and Sagittarius, the only way to see the intricate details of the dust swirls, as featured above, is to use a long exposure camera.

Thumbnail image of picture found for this day. APOD: 2010 May 12 - M72: A Globular Cluster of Stars
Explanation: Globular clusters once ruled the Milky Way. Back in the old days, back when our Galaxy first formed, perhaps thousands of globular clusters roamed our Galaxy. Today, there are less than 200 left. Many globular clusters were destroyed over the eons by repeated fateful encounters with each other or the Galactic center. Surviving relics are older than any Earth fossil, older than any other structures in our Galaxy, and limit the universe itself in raw age. There are few, if any, young globular clusters in our Milky Way Galaxy because conditions are not ripe for more to form. Pictured above by the Hubble Space Telescope are about 100,000 of M72's stars. M72, which spans about 50 light years and lies about 50,000 light years away, can be seen with a small telescope toward the constellation of the Water Bearer (Aquarius).

Thumbnail image of picture found for this day. APOD: 2010 March 31 - Millions of Stars in Omega Centauri
Explanation: Featured in the sharp telescopic image, globular star cluster Omega Centauri (NGC 5139) is some 15,000 light-years away and 150 light-years in diameter. Packed with about 10 million stars much older than the Sun, Omega Cen is the largest of 200 or so known globular clusters that roam the halo of our Milky Way galaxy. Though most star clusters consist of stars with the same age and composition, the enigmatic Omega Cen exhibits the presence of different stellar populations with a spread of ages and chemical abundances. In fact, Omega Cen may be the remnant core of a small galaxy merging with the Milky Way.

Thumbnail image of picture found for this day. APOD: 2009 September 14 - The Center of Globular Cluster Omega Centauri
Explanation: What is left over after stars collide? To help answer this question, astronomers have been studying the center of the most massive ball of stars in our Milky Way Galaxy. In the center of globular cluster Omega Centauri, stars are packed in 10,000 times more densely than near our Sun. Pictured above, the newly upgraded Hubble Space Telescope has resolved the very center of Omega Centauri into individual stars. Visible are many faint yellow-white stars that are smaller than our Sun, several yellow-orange stars that are Red Giants, and an occasional blue star. When two stars collide they likely either combine to form one more massive star, or they stick, forming a new binary star system. Close binary stars interact, sometimes emitting ultraviolet or X-ray light when gas falls from one star onto the surface of a compact companion such as a white dwarf or neutron star. Two such binaries have now been located in Omega Centauri's center. The star cluster lies about 15,000 light-years away and is visible toward the constellation of Centaurus.

Thumbnail image of picture found for this day. APOD: 2009 June 17 - M13: A Great Globular Cluster of Stars
Explanation: M13 is one of the most prominent and best known globular clusters. Visible with binoculars in the constellation of Hercules, M13 is frequently one of the first objects found by curious sky gazers seeking celestials wonders beyond normal human vision. M13 is a colossal home to over 100,000 stars, spans over 150 light years across, lies over 20,000 light years distant, and is over 12 billion years old. At the 1974 dedication of Arecibo Observatory, a radio message about Earth was sent in the direction of M13. The reason for the low abundance of unusual blue straggler stars in M13 remains unknown.

Thumbnail image of picture found for this day. APOD: 2009 March 1 - Omega Centauri: The Largest Nearby Globular Cluster
Explanation: This huge ball of stars predates our Sun. Long before humankind evolved, before dinosaurs roamed, and even before our Earth existed, ancient globs of stars condensed and orbited a young Milky Way Galaxy. Of the 200 or so globular clusters that survive today, Omega Centauri is the largest, containing over ten million stars. Omega Centauri is also the brightest globular cluster, at apparent visual magnitude 3.9 it is visible to southern observers with the unaided eye. Cataloged as NGC 5139, Omega Centauri is about 18,000 light-years away and 150 light-years in diameter. Unlike many other globular clusters, the stars in Omega Centauri show several different ages and trace chemical abundances, indicating that the globular star cluster has a complex history over its 12 billion year age.

Thumbnail image of picture found for this day. APOD: 2009 January 23 - Globular Cluster NGC 2419
Explanation: Of three objects prominent in this thoughtful telescopic image, a view toward the stealthy constellation Lynx, two (the spiky ones) are nearby stars. The third is the remote globular star cluster NGC 2419, at distance of nearly 300,000 light-years. NGC 2419 is sometimes called "the Intergalactic Wanderer", an appropriate title considering that the distance to the Milky Way's satellite galaxy, the Large Magellanic Cloud, is only about 160,000 light-years. Roughly similar to other large globular star clusters like Omega Centauri, NGC 2419 is itself intrinsically bright, but appears faint because it is so far away. NGC 2419 may really have an extragalactic origin as, for example, the remains of a small galaxy captured and disrupted by the Milky Way. But its extreme distance makes it difficult to study and compare its properties with other globular clusters that roam the halo of our Milky Way galaxy.

Thumbnail image of picture found for this day. APOD: 2008 December 8 - The Dark Doodad Nebula
Explanation: What is that strange dark ribbon on the sky? When observing the great globular cluster NGC 4372, observers frequently take note of a strange dark streak nearly three degrees in length running near it. Unnamed, the streak, actually a long molecular cloud, has become known as the Dark Doodad Nebula. (Doodad is slang for a thingy or a whatchamacallit.) Pictured above in a rich and colorful star-field, the Dark Doodad Nebula can be found sweeping across the image center. The globular star cluster NGC 4372 is visible on the image left, while the bright star gamma Musca is seen to the cluster's right. The Dark Doodad Nebula can be found with strong binoculars toward the southern constellation of the Fly (Musca). The above image was compiled by consecutive 45 minutes exposures taken by a small telescope from the La Frontera region in Chile.

Thumbnail image of picture found for this day. APOD: 2008 September 6 - A Flock of Stars
Explanation: Only a few stars can be found within ten light-years of our lonely Sun, situated near an outer spiral arm of the Milky Way galaxy. But if the Sun were found within one of our galaxy's star clusters, thousands of stars might occupy a similar space. What would the night sky look like in such a densely packed stellar neighborhood? When Roger Hopkins took this picture at the Montezuma National Wildlife Refuge in the Finger Lakes region of western New York, USA, he was struck by this same notion. Appropriately, he had photographed a flock of starlings against the backdrop of a serene sunset. He then manipulated the image so that the black bird silhouettes were changed to white. The final picture dramatically suggests the tantalizing spectacle of approaching night in crowded skies above a cluster star world.

Thumbnail image of picture found for this day. APOD: 2008 August 26 - 47 Tuc: A Great Globular Cluster of Stars
Explanation: Stars come in bunches. Of the over 200 globular star clusters that orbit the center of our Milky Way Galaxy, 47 Tucanae is the second brightest globular cluster (behind Omega Centauri). Light takes about 13,000 years to reach us from 47 Tuc which can be seen on the sky near the Small Magellanic Cloud in the southern constellation of Tucana. Also known as NGC 104, the dense cluster is made up of several million stars in a volume only about 120 light-years across. The cluster's red giant stars are particularly easy to see in this picture. The globular cluster is also home to exotic x-ray binary star systems.

Thumbnail image of picture found for this day. APOD: 2008 August 6 - NGC 1818: A Young Globular Cluster
Explanation: Globular clusters once ruled the Milky Way. Back in the old days, back when our Galaxy first formed, perhaps thousands of globular clusters roamed our Galaxy. Today, there are perhaps 200 left. Many globular clusters were destroyed over the eons by repeated fateful encounters with each other or the Galactic center. Surviving relics are older than any Earth fossil, older than any other structures in our Galaxy, and limit the universe itself in raw age. There are few, if any, young globular clusters in our Milky Way Galaxy because conditions are not ripe for more to form. Things are different next door, however, in the neighboring LMC galaxy. Pictured above is a "young" globular cluster residing there: NGC 1818. Observations show it formed only about 40 million years ago - just yesterday compared to the 12 billion year ages of globular clusters in our own Milky Way

Thumbnail image of picture found for this day. APOD: 2008 May 1 - The Giants of Omega Centauri
Explanation: Globular star cluster Omega Centauri is some 15,000 light-years away and 150 light-years in diameter. Packed with about 10 million stars, Omega Cen is the largest of 200 or so known globular clusters that roam the halo of our Milky Way galaxy. This intriguing color picture combines a visible light image of the cluster in blue hues with infrared image data from the Spitzer Space Telescope. The Spitzer data includes images in two infrared bands, one shown in green and one in red. Both infrared bands are sensitive to light from the cool, giant stars in the cluster. Adding the red and green colors together creates yellow, showing off the cluster's giant stars as yellow spots. Of course, red spots also indicate cool, giant stars in the image, but some of the red spots are even more distant background galaxies. Also known simply as Red Giant Stars, they represent a stage in the life-cycle of stars more evolved than our own Sun, a stage the Sun will reach in about 5 billion years. Dust grains formed in the atmospheres of cool, giant stars are ultimately involved in the formation of other stars and planets.

Thumbnail image of picture found for this day. APOD: 2008 April 2- Globular Cluster M55 from CFHT
Explanation: The fifty-fifth entry in Charles Messier's catalog, M55 is a large and lovely globular cluster of around 100,000 stars. Only 20,000 light-years away in the constellation Sagittarius, M55 appears to earth-bound observers to be nearly 2/3 the size of the full moon. Globular star clusters like M55 roam the halo of our Milky Way Galaxy as gravitationally bound populations of stars known to be much older than stellar groups found in the Galactic disk. Astronomers who make detailed studies of globular cluster stars can accurately measure the cluster ages and distances. Their results ultimately constrain the age of the Universe (... it must be older than the stars in it!), and provide a fundamental rung on the astronomical distance ladder. This stunning color image was made with the 3.6 meter CFHT telescope and spans about 100 light-years across the globular cluster M55.

Thumbnail image of picture found for this day. APOD: 2007 November 15 - M13: The Great Globular Cluster in Hercules
Explanation: M13 is modestly recognized as the Great Globular Cluster in Hercules. A system of stars numbering in the hundreds of thousands, it is one of the brightest globular star clusters in the northern sky. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter, but approaching the cluster core over 100 stars would be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. This stunning view of the cluster combines recent telescopic images of the cluster's dense core with digitized photographic plates recorded between 1987 and 1991 using the Samuel Oschin Telescope, a wide-field survey instrument at Palomar Observatory. The resulting composite highlights both inner and outer reaches of the giant star cluster. Among the distant background galaxies also visible, NGC 6207 is above and to the left of the Great Globular Cluster M13.

Thumbnail image of picture found for this day. APOD: 2007 September 3 - The Colorful Clouds of Rho Ophiuchi
Explanation: The many spectacular colors of the Rho Ophiuchi (oh'-fee-yu-kee) clouds highlight the many processes that occur there. The blue regions shine primarily by reflected light. Blue light from the star Rho Ophiuchi and nearby stars reflects more efficiently off this portion of the nebula than red light. The Earth's daytime sky appears blue for the same reason. The red and yellow regions shine primarily because of emission from the nebula's atomic and molecular gas. Light from nearby blue stars - more energetic than the bright star Antares - knocks electrons away from the gas, which then shines when the electrons recombine with the gas. The dark regions are caused by dust grains - born in young stellar atmospheres - which effectively block light emitted behind them. The Rho Ophiuchi star clouds, well in front of the globular cluster M4 visible above on far lower left, are even more colorful than humans can see - the clouds emits light in every wavelength band from the radio to the gamma-ray.

Thumbnail image of picture found for this day. APOD: 2007 June 9 - Globular Star Cluster M3
Explanation: This immense ball of half a million stars older than the Sun lies over 30,000 light-years away. Cataloged as M3 (and NGC 5272), it is one of about 150 globular star clusters that roam the halo of our Milky Way Galaxy. Even in this impressively sharp image, individual stars are difficult to distinguished in the densely packed core, but colors are apparent for the bright stars on the cluster's outskirts. M3's many cool "red" giant stars take on a yellowish cast, while hotter giants and pulsating variable stars look light blue. A closer look at the deep telescopic view also reveals a host of background galaxies. Itself about 200 light-years across, the giant star cluster is a relatively bright, easy target for binoculars in the northern constellation Canes Venatici, The Hunting Dogs, and not far from Arcturus.

Thumbnail image of picture found for this day. APOD: 2007 May 18 - M13: The Great Globular Cluster in Hercules
Explanation: In 1714, Edmond Halley noted that M13 "shows itself to the naked eye when the sky is serene and the Moon absent." Of course, M13 is now modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. Telescopic views reveal the spectacular cluster's hundreds of thousands of stars. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter, but approaching the cluster core upwards of 100 stars could be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. Along with the cluster's dense core, the outer reaches of M13 are highlighted in this deep color image. A distant background galaxy, NGC 6207 is also visible above and to the right of the Great Globular Cluster M13.

Thumbnail image of picture found for this day. APOD: 2007 April 19 - NGC 5139: Omega Centauri
Explanation: Centaurus is one of the most striking constellations in the southern sky. The Milky Way flows through this celestial expanse whose wonders also include the closest star system to the Sun, Alpha Centauri, and the largest globular star cluster in our galaxy, Omega Centauri (aka NGC 5139). This sharp telescopic view of Omega Centauri shows off the central regions of the cluster of about 10 million stars. Omega Cen itself is about 15,000 light-years away and 150 light-years in diameter - the largest of 150 or so known globular star clusters that roam the halo of our galaxy. Though most star clusters consist of stars with the same age and composition, the enigmatic Omega Cen exhibits the presence of different stellar populations with a spread of ages and chemical abundances. In fact, Omega Cen may be the remnant core of a small galaxy merging with the Milky Way.

Thumbnail image of picture found for this day. APOD: 2007 April 15 - M3: Inconstant Star Cluster
Explanation: Star clusters appear constant because photographs of them are frozen in time. In reality, though, cluster stars swarm the center and frequently fluctuate in brightness. Although the time it takes for stars to cross a cluster is about 100,000 years, the time it takes for a star to fluctuate noticeably can be less than one night. In fact, the above time lapse movie of bright globular cluster M3 was taken over a single night. Most of the variable stars visible above are RR Lyrae stars, stars that can quickly double their brightness while becoming noticeably bluer. Furthermore, RR Lyrae stars vary their light in a distinctive pattern that allows unique identification. Lastly, since RR Lyrae stars all have the same intrinsic brightness, identifying them and measuring how dim they appear tells how far they are, since faintness means farness. These distances, in turn, help calibrate the scale of the entire universe.

Thumbnail image of picture found for this day. APOD: 2006 May 26 - Omega Centauri
Explanation: Centaurus, the Centaur, is one of the most striking constellations in the southern sky. The Milky Way flows through this celestial expanse whose wonders also include the closest star to the Sun, Alpha Centauri, and the largest globular star cluster in our galaxy, Omega Centauri. This gorgeous wide-field telescopic view of Omega Centauri shows off the cluster of about 10 million stars and the surrounding star field, with very faint dust clouds and distant background galaxies. Omega Cen itself is about 15,000 light-years away and 150 light-years in diameter - one of 150 or so known globular star clusters that roam the halo of our galaxy. The stars in globular clusters are much older, cooler, and less massive than our Sun.

Thumbnail image of picture found for this day. APOD: 2006 March 12 - Globular Cluster M3 from WIYN
Explanation: This huge ball of stars predates our Sun. Long before humankind evolved, before dinosaurs roamed, and even before our Earth existed, ancient globs of stars condensed and orbited a young Milky Way Galaxy. Of the 200 or so globular clusters that survive today, M3 is one of the largest and brightest, easily visible in the Northern hemisphere with binoculars. M3 contains about half a million stars, most of which are old and red. Light takes about 35,000 years to reach us from M3, which spans about 150 light years. The above picture is a composite of blue and red images.

Thumbnail image of picture found for this day. APOD: 2005 September 7 - The View from Husband Hill on Mars
Explanation: Scroll right to see a breathtaking panorama of Mars from the top of Husband Hill. The image was taken by the robotic rover Spirit now exploring the red planet. Spirit, situated in expansive Gusev Crater, has been exploring the Columbia Hills for some time including climbing Husband Hill over the last few months. On the way up, Spirit took in a color vista from Larry's Lookout. Visible in the above image is the vast eastern landscape previously blocked from view by the Columbia Hills themselves. The horizon is mostly defined by the rim of Thira crater visible some 15 kilometers in the distance. Spirit will now examine rocks and soil at the top of Husband Hill, looking for clues as to how the hills and local rocks formed in the distant past.

Thumbnail image of picture found for this day. APOD: 2005 September 5 - Globular Cluster 47 Tucanae from SALT
Explanation: Stars come in bunches. Of the over 200 globular star clusters that orbit the center of our Milky Way Galaxy, 47 Tucanae is the second brightest globular cluster, behind Omega Centauri. Known to some affectionately as 47 Tuc or NGC 104, it is only visible from Earth's Southern Hemisphere. It was therefore a fitting target for first light observations of the gigantic new 10-meter diameter Southern African Large Telescope (SALT) this past week. The resulting image is shown above. Light takes about 20,000 years to reach us from 47 Tuc which can be seen near the Small Magellanic Cloud toward the constellation of Tucana. The dynamics of stars near the center of 47 Tuc are not well understood, particularly why there are so few binary systems there.

Thumbnail image of picture found for this day. APOD: 2005 July 21 - X-Ray Stars of 47 Tuc
Explanation: Visible light images show the central region of globular cluster 47 Tucanae is closely packed, with stars less than a tenth of a light-year apart. This Chandra false-color x-ray view of central 47 Tuc also shows the cluster is a popular neighborhood for x-ray stars, many of which are "normal" stars co-orbiting with extremely dense neutron stars -- stars with the mass of the Sun but the diameter of Manhattan Island. One of the most remarkable of these exotic binary systems is cataloged as 47 Tuc W, a bright source near the center of this image. The system consists of a low mass star and a a neutron star that spins once every 2.35 milliseconds. Such neutron stars are known to radio astronomers as millisecond pulsars, believed to be driven to such rapid rotation by material falling from the normal star onto its dense companion. In fact, x-ray observations of the 47 Tuc W system link this spin-up mechanism observed to operate in other x-ray binary stars with fast rotating millisecond pulsars.

Thumbnail image of picture found for this day. APOD: 2005 July 9 - The Colorful Clouds of Rho Ophiuchi
Explanation: The many spectacular colors of the Rho Ophiuchi (oh'-fee-yu-kee) clouds highlight the many processes that occur there. The blue regions shine primarily by reflected light. Blue light from the star Rho Ophiuchi and nearby stars reflects more efficiently off this portion of the nebula than red light. The Earth's daytime sky appears blue for the same reason. The red and yellow regions shine primarily because of emission from the nebula's atomic and molecular gas. Light from nearby blue stars - more energetic than the bright star Antares - knocks electrons away from the gas, which then shines when the electrons recombine with the gas. The dark regions are caused by dust grains - born in young stellar atmospheres - which effectively block light emitted behind them. The Rho Ophiuchi star clouds, well in front of the globular cluster M4 visible above on far lower left, are even more colorful than humans can see - the clouds emits light in every wavelength band from the radio to the gamma-ray.

Thumbnail image of picture found for this day. APOD: 2005 June 27 - Globular Cluster M22 from CFHT
Explanation: The globular cluster M22, pictured above, contains over 100,000 stars. These stars formed together and are gravitationally bound. Stars orbit the center of the cluster, and the cluster orbits the center of our Galaxy. So far, about 140 globular clusters are known to exist in a roughly spherical halo around the Galactic center. Globular clusters do not appear spherically distributed as viewed from the Earth, and this fact was a key point in the determination that our Sun is not at the center of our Galaxy. Globular clusters are very old. There is a straightforward method of determining their age, and this nearly matches the 13.7 billion-year age of our entire universe.

Thumbnail image of picture found for this day. APOD: 2004 October 14 - Glimpse of a Globular Star Cluster
Explanation: Not a glimpse of this cluster of stars can be seen in the inset visible light image (lower right). Still, the infrared view from the Spitzer Space Telescope reveals a massive globular star cluster of about 300,000 suns in an apparently empty region of sky in the constellation Aquila. When astronomers used infrared cameras to peer through obscuring dust in the plane of our Milky Way galaxy, they were rewarded with the surprise discovery of the star cluster, likely one of the last such star clusters to be found. Globular star clusters normally roam the halo of the Milky Way, ancient relics of our galaxy's formative years. Yet the Spitzer image shows this otherwise hidden cluster crossing through the middle of the galactic plane some 10,000 light-years away. At that distance, the picture spans only about 20 light-years. In the false color infrared image, the red streak is a dust cloud which seems to lie behind the cluster core.

Thumbnail image of picture found for this day. APOD: 2004 October 12 - M3: Inconstant Star Cluster
Explanation: Star clusters appear constant because photographs of them are frozen in time. In reality, though, cluster stars swarm the center and frequently fluctuate in brightness. Although the time it takes for stars to cross a cluster is about 100,000 years, the time it takes for a star to fluctuate noticeably can be less than one night. In fact, the above time lapse movie of bright globular cluster M3 was taken over a single night. Most of the variable stars visible above are RR Lyrae stars, stars that can quickly double their brightness while becoming noticeably bluer. Furthermore, RR Lyrae stars vary their light in a distinctive pattern that allows unique identification. Lastly, since RR Lyrae stars all have the same intrinsic brightness, identifying them and measuring how dim they appear tells how far they are, since faintness means farness. These distances, in turn, help calibrate the scale of the entire universe.

Thumbnail image of picture found for this day. APOD: 2004 September 18 - M55: Globular Star Cluster
Explanation: The fifty-fifth entry in Charles Messier's catalog, M55 is a large and lovely globular cluster of around 100,000 stars. Only 20,000 light-years away in the constellation Sagittarius, M55 appears to earth-bound observers to be nearly 2/3 the size of the full moon. Globular star clusters like M55 roam the halo of our Milky Way Galaxy as gravitationally bound populations of stars known to be much older than stellar groups found in the galactic disk. Astronomers who make detailed studies of globular cluster stars can accurately measure the cluster ages and distances. Their results ultimately constrain the age of the Universe (... it must be older than the stars in it! ), and provide a fundamental rung on the astronomical distance ladder. This stunning three-color image made with astronomical (BVI) filters spans about 100 light-years across the globular cluster M55.

Thumbnail image of picture found for this day. APOD: 2004 July 15 - Stars and Dust in Corona Australis
Explanation: A cosmic dust cloud sprawls across a rich field of stars in this gorgeous wide field telescopic vista looking toward Corona Australis, the Southern Crown. Probably less than 500 light-years away and effectively blocking light from more distant, background stars in the Milky Way, the densest part of the dust cloud is about 8 light-years long. At its tip (lower left) is a series of lovely blue nebulae cataloged as NGC 6726, 6727, 6729, and IC 4812. Their characteristic blue color is produced as light from hot stars is reflected by the cosmic dust. The tiny but intriguing yellowish arc visible near the blue nebulae marks young variable star R Coronae Australis. Magnificent globular star cluster NGC 6723 is seen here below and left of the nebulae. While NGC 6723 appears to be just outside Corona Australis in the constellation Sagittarius, it actually lies nearly 30,000 light-years away, far beyond the Corona Australis dust cloud.

Thumbnail image of picture found for this day. APOD: 2004 June 2 - The Colorful Clouds of Rho Ophiuchi
Explanation: The many spectacular colors of the Rho Ophiuchi (oh'-fee-yu-kee) clouds highlight the many processes that occur there. The blue regions shine primarily by reflected light. Blue light from the star Rho Ophiuchi and nearby stars reflects more efficiently off this portion of the nebula than red light. The Earth's daytime sky appears blue for the same reason. The red and yellow regions shine primarily because of emission from the nebula's atomic and molecular gas. Light from nearby blue stars - more energetic than the bright star Antares - knocks electrons away from the gas, which then shines when the electrons recombine with the gas. The dark regions are caused by dust grains - born in young stellar atmospheres - which effectively block light emitted behind them. The Rho Ophiuchi star clouds, well in front of the globular cluster M4 visible above on far lower left, are even more colorful than humans can see - the clouds emits light in every wavelength band from the radio to the gamma-ray.

Thumbnail image of picture found for this day. APOD: 2004 May 11 - M13: The Great Globular Cluster in Hercules
Explanation: M13 is one of the most prominent and best known globular clusters. Visible with binoculars in the constellation of Hercules, M13 is frequently one of the first objects found by curious sky gazers seeking celestials wonders beyond normal human vision. M13 is a colossal home to over 100,000 stars, spans over 150 light years across, lies over 20,000 light years distant, and is over 12 billion years old. At the 1974 dedication of Arecibo Observatory, a radio message about Earth was sent in the direction of M13. The reason for the low abundance of unusual blue straggler stars in M13 is currently unknown.

Thumbnail image of picture found for this day. APOD: 2004 May 9 - Antares and Rho Ophiuchi
Explanation: Why is the sky near Antares and Rho Ophiuchi so colorful? The colors result from a mixture of objects and processes. Fine dust illuminated from the front by starlight produces blue reflection nebulae. Gaseous clouds whose atoms are excited by ultraviolet starlight produce reddish emission nebulae. Backlit dust clouds block starlight and so appear dark. Antares, a red supergiant and one of the brighter stars in the night sky, lights up the yellow-red clouds on the upper left. Rho Ophiuchi lies at the center of the blue nebula on the right. The distant globular cluster M4 is visible just below Antares, and to the left of the red cloud engulfing Sigma Scorpii. These star clouds are even more colorful than humans can see, emitting light across the electromagnetic spectrum.

Thumbnail image of picture found for this day. APOD: 2003 December 13 - A Flock of Stars
Explanation: Only a few stars can be found within ten light-years of our lonely Sun, situated near an outer spiral arm of the Milky Way galaxy. But if the Sun were found within one of our galaxy's star clusters, thousands of stars might occupy a similar space. What would the night sky look like in such a densely packed stellar neighborhood? When Roger Hopkins took this picture at the Montezuma National Wildlife Refuge in the Finger Lakes region of western New York, USA, he was struck by this same notion. Appropriately, he had photographed a flock of starlings against the backdrop of a serene sunset. He then manipulated the image so that the black bird silhouettes were changed to white. The final picture dramatically suggests the tantalizing spectacle of approaching night in crowded skies above a cluster star world.

Thumbnail image of picture found for this day. APOD: 2003 December 9 - NGC 604: Giant Stellar Nursery
Explanation: Stars are sometimes born in the midst of chaos. About 3 million years ago in the nearby galaxy M33, a large cloud of gas spawned dense internal knots which gravitationally collapsed to form stars. NGC 604 was so large, however, it could form enough stars to make a globular cluster. Many young stars from this cloud are visible in the above image from the Hubble Space Telescope, along with what is left of the initial gas cloud. Some stars were so massive they have already evolved and exploded in a supernova. The brightest stars that are left emit light so energetic that they create one of the largest cloud of ionized hydrogen gas known, comparable to the Tarantula Nebula in our Milky Way's close neighbor, the Large Magellanic Cloud.

Thumbnail image of picture found for this day. APOD: 2003 September 15 - Globular Cluster M3
Explanation: This huge ball of stars predates our Sun. Long before humankind evolved, before dinosaurs roamed, and even before our Earth existed, ancient globs of stars condensed and orbited a young Milky Way Galaxy. Of the 200 or so globular clusters that survive today, M3 is one of the largest and brightest, easily visible in the Northern hemisphere with binoculars. M3 contains about half a million stars, most of which are old and red. Light takes about 100,000 years to reach us from M3, which spans about 150 light years. The above picture is a composite of blue and red images.

Thumbnail image of picture found for this day. APOD: 2003 August 8 - Blue Stragglers in NGC 6397
Explanation: In our neck of the Galaxy stars are too far apart to be in danger of colliding, but in the dense cores of globular star clusters star collisions may be relatively common. In fact, researchers have evidence that the closely spaced blue stars near the center of the above image taken by the orbiting Hubble Space Telescope were formed when stars directly collided. Pictured is the central region of NGC 6397, a globular cluster about 6,000 light-years distant, whose stars all formed at about the same time. NGC 6397's massive stars have long since evolved off the main sequence, exhausting their central supplies of nuclear fuel. This should leave the cluster with only old low mass stars; faint red main sequence stars and brighter blue and red giants. However, spectroscopic data show that the indicated stars, descriptively dubbed blue stragglers, are clearly main sequence stars which are too blue and too massive to still be there. Suggestively the stragglers appear to be two and occasionally three times as massive as the lower mass cluster stars otherwise present, supporting evidence for their formation from two and even three star collisions.

Thumbnail image of picture found for this day. APOD: 2003 July 18 - The Planet, the White Dwarf, and the Neutron Star
Explanation: A planet, a white dwarf, and a neutron star orbit each other in the giant globular star cluster M4, some 5,600 light-years away. The most visible member of the trio is the white dwarf star, indicated above in an image from the Hubble Space Telescope, while the neutron star is detected at radio frequencies as a pulsar. A third body was known to be present in the pulsar/white dwarf system and a detailed analysis of the Hubble data has indicated it is indeed a planet with about 2.5 times the mass of Jupiter. In such a system, the planet is likely to be about 13 billion years old. Compared to our solar system's tender 4.5 billion years and other identified planets of nearby stars, this truly ancient world is by far the oldest planet known, almost as old as the Universe itself. Its discovery as part of an evolved cosmic trio suggests that planet formation spans the age of the Universe and that this newly discovered planet is likely only one of many formed in the crowded environs of globular star clusters.

Thumbnail image of picture found for this day. APOD: 2003 April 11 - London at Night
Explanation: Do you recognize this intriguing globular cluster of stars? It's actually the constellation of city lights surrounding London, England, planet Earth, as recorded with a digital camera from the International Space Station. Taken in February 2003, north is toward the top and slightly left in this nighttime view. The encircling "London Orbital" highway by-pass, the M25 (... but not Messier 25), is easiest to pick out south of the city. Even farther south are the lights of Gatwick airport and just inside the western (left hand) stretch of the Orbital is Heathrow. The darkened Thames river estuary fans out to the city's east. In particular, two small "dark nebulae" - Hyde Park and Regents Park - stand out slightly west of the densely packed lights at the city's core.

Thumbnail image of picture found for this day. APOD: 2003 January 25 - Palomar 13's Last Stand
Explanation: Globular star cluster Palomar 13 has roamed the halo of our Milky Way Galaxy for the last 12 billion years. The apparently sparse cluster of stars just left of center in this composite color digital image, it is one of the smallest, faintest globular clusters known. (The bright foreground star near bottom is unrelated and creates the spiky imaging artifacts.) Observations spanning forty years indicate that Palomar 13's galactic halo orbit is a highly eccentric one which, every one or two billion years, brings it relatively close to the galactic center. With each close approach to the Milky Way's central regions, gravitational tidal forces strip away the delicately bound cluster stars. In fact, detailed present day studies offer evidence for a dramatic end to this dwindling cluster's tidal tug of war. Palomar 13's latest close approach was only about 70 million years ago. But, when Palomar 13 again approaches the galaxy, it could well turn out to be the cluster's last stand.

Thumbnail image of picture found for this day. APOD: 2002 December 29 - NGC 1818: A Young Globular Cluster
Explanation: Globular clusters once ruled the Milky Way. Back in the old days, back when our Galaxy first formed, perhaps thousands of globular clusters roamed our Galaxy. Today, there are perhaps 200 left. Many globular clusters were destroyed over the eons by repeated fateful encounters with each other or the Galactic center. Surviving relics are older than any Earth fossil, older than any other structures in our Galaxy, and limit the universe itself in raw age. There are few, if any, young globular clusters in our Milky Way Galaxy because conditions are not ripe for more to form. Things are different next door, however, in the neighboring LMC galaxy. Pictured above is a "young" globular cluster residing there: NGC 1818. Observations show it formed only about 40 million years ago - just yesterday compared to the 12 billion year ages of globular clusters in our own Milky Way

Thumbnail image of picture found for this day. APOD: 2002 April 16 - Millions of Stars in Omega Centauri
Explanation: Pictured above is the largest ball of stars in our Galaxy. About 10 million stars orbit the center of this globular cluster - named Omega Centauri - as this giant globular cluster orbits our Galactic center. Recent evidence indicates that Omega Centauri is by far the most massive of the about 150 known globular clusters in the Milky Way. Omega Centauri, cataloged as NGC 5139, spans about 150 light years across, lies about 15,000 light years away, and can be seen without visual aid toward the constellation of Centaurus. The stars in globular clusters are generally older, redder and less massive than our Sun. Studying globular clusters tells us not only about the history of our Galaxy but also limits the age of the universe.

Thumbnail image of picture found for this day. APOD: 2002 February 20 - Oddities of Star Cluster NGC 6397
Explanation: One of these stars is blinking. This star, a member of globular cluster NGC 6397, is noteworthy not just because it blinks, but because it blinks so fast and because its companion star is so atypical. Speculation holds that this might be a neutron star spun up to a rate of 274 rotations each second by the bloated red star it orbits. Matter gravitationally pulled from the bloated star likely orbits the millisecond pulsar, making it spin faster when it crashes onto the surface. The odd system might have resulted when the neutron star captured a normal star after a near collision near the globular cluster's dense center. Other collisions near the center of NGC 6397 are thought to have produced other oddities -- blue straggler stars. The Hubble Space Telescope took the above image of the colorful globular cluster.

Thumbnail image of picture found for this day. APOD: 2001 December 10 - Globular Cluster M15
Explanation: Stars, like bees, swarm around the center of bright globular cluster M15. This ball of over 100,000 stars is a relic from the early years of our Galaxy, and continues to orbit the Milky Way's center. M15, one of about 150 globular clusters remaining, is noted for being easily visible with only binoculars, having at its center one of the densest concentrations of stars known, and containing a high abundance of unusual variable stars and pulsars. The above image, taken in ultraviolet light with the WIYN Telescope, spans about 120 light years and shows the gradual increase in stars toward the cluster's center. M15 lies about 35,000 light years away toward the constellation of Pegasus. Recent evidence indicates that a massive black hole might reside as the center of M15.

Thumbnail image of picture found for this day. APOD: 2001 October 15 - The Earth and Moon Planetary System
Explanation: How similar in size are the Earth and the Moon? A dramatic visual answer to this question is found by combining photographs taken by the Mariner 10 spacecraft that headed out toward Venus and Mercury in 1973. The Moon can be seen to have a diameter over one quarter that of Earth, relatively large compared to its planetary companion. In our Solar System, only Pluto and Charon are closer together in size. Striking features of the Earth visible to the passing spacecraft include blue oceans and white clouds, showing the Earth to be truly a water world.

Thumbnail image of picture found for this day. APOD: 2001 October 10 - The Center of Globular Cluster Omega Centauri
Explanation: What is left over after stars collide? To help answer this question, astronomers have been studying the center of the most massive ball of stars in our Milky Way Galaxy. In the center of globular cluster Omega Centauri, stars are packed in 10,000 times more densely than near our Sun. Pictured above, the Hubble Space Telescope has resolved the very center of Omega Centauri into individual stars. Visible are many faint yellow-white stars that are smaller than our Sun, several yellow-orange stars that are Red Giants, and an occasional blue star. When two stars collide they likely either combine to form one more massive star, or they stick, forming a new binary star system. Close binary stars interact, sometimes emitting ultraviolet or X-ray light when gas falls from one star onto the surface of a compact companion such as a white dwarf or neutron star. Two such binaries have now been located in Omega Centauri's center. The star cluster lies about 15,000 light-years away and is visible toward the constellation of Centaurus.

Thumbnail image of picture found for this day. APOD: 2001 September 20 - X Ray Stars in M15
Explanation: Side by side, two x-ray stars greeted astronomers in this false-color Chandra Observatory x-ray image of a region near the core of globular star cluster M15. The greeting was a pleasant surprise, as all previous x-ray images of the cluster showed only one such source where Chandra's sharper x-ray vision now reveals two. These x-ray sources are modeled as neutron star binary systems. Each is a city-sized neutron star in close orbit with a normal stellar companion. X-rays are generated as matter from the normal star falls onto the compact neutron star. This break through explains why observations of the previously recognized lone neutron star binary system in M15 were difficult to reconcile with any single model. It also suggests that other globular star clusters which roam the halo of our Milky Way galaxy and seem to contain only one such neutron star x-ray source may in fact contain more. An optical Hubble Space Telescope image of the dense M15 cluster is inset at the upper right.

Thumbnail image of picture found for this day. 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.

Thumbnail image of picture found for this day. APOD: 2001 July 3 - Unusual Flashes Toward Globular Cluster M22
Explanation: What is causing the unusual flashes behind globular cluster M22? This teeming ball of stars is the brightest globular cluster visible from Earth's northern hemisphere. M22, shown in full in the inset, spans about 50 light-years and lies 8,500 light-years away toward the constellation of Sagittarius. M22's center was recently imaged repeatedly by the high resolution Hubble Space Telescope. Behind M22 are many more stars near the center of our Galaxy. Unexpectedly, several stars near the Galactic center -- well behind M22 -- appeared to nearly double in brightness and return to normal within 20 hours. One hypothesis posed to explain these quick brightness changes is the gravitational lens effect of large planets roaming freely in the cluster. One problem with this is that no such planetary population was previously known! Future observations are planned to better understand these mysterious flashes.

Thumbnail image of picture found for this day. APOD: 2001 June 11 - Globular Cluster M2
Explanation: Beneath the south pole of our Milky Way Galaxy lies a ball of over 100,000 stars. M2, the second object on Charles Messier's eighteenth century list of bright diffuse sky objects, is known as a globular cluster, and orbits the center of our Galaxy like nearly 200 other globular clusters left over from the early days of our universe. M2, pictured above, spans over 150 light-years, lies about 50,000 light-years away, and can be seen with binoculars towards the constellation of Aquarius. Determining the distances and ages to globular clusters like M2 constrains the scale and age of our entire universe.

Thumbnail image of picture found for this day. APOD: 2001 May 24 - X-Ray Stars of 47 Tucanae
Explanation: A deep optical image (left) of 47 Tucanae shows an ancient globular star cluster so dense and crowded that individual stars can not be distinguished in its closely packed core. An x-ray image of its central regions (inset right) from the Chandra Observatory reveals a wealth of x-ray stars hidden there. Color-coded by energy, low energies are red, medium are green, and high energy cosmic x-ray sources are blue, while whitish sources are bright across the x-ray energy bands. The x-ray stars here are double stars or "compact" binary star systems. They are so called because one of the pair of stellar companions is a normal star and the other a compact object -- a white dwarf, neutron star, or possibly a black hole. Chandra's x-ray vision detects the presence of an unexpectedly large number of these exotic star systems within 47 Tucanae, but it also indicates the apparent absence of a large central black hole. The finding suggests that compact binary star systems of 47 Tucanae may be ejected from the cluster before coalescing to form a large black hole at its core.

Thumbnail image of picture found for this day. APOD: 2001 April 22 - Globular Cluster 47 Tucanae
Explanation: Stars come in bunches. Of the over 200 globular star clusters that orbit the center of our Milky Way Galaxy, 47 Tucanae is the second brightest globular cluster (behind Omega Centauri). Known to some affectionately as 47 Tuc or NGC 104, it is only visible from the Southern Hemisphere. Light takes about 20,000 years to reach us from 47 Tuc which can be seen near the SMC in the constellation of Tucana. Red Giant stars are particularly easy to see in this picture. The dynamics of stars near the center of 47 Tuc are not well understood, particularly why there are so few binary systems there.

Thumbnail image of picture found for this day. APOD: 2001 March 11 - NGC 1818: A Young Globular Cluster
Explanation: Globular clusters once ruled the Milky Way. Back in the old days, back when our Galaxy first formed, perhaps thousands of globular clusters roamed our Galaxy. Today, there are perhaps 200 left. Many globular clusters were destroyed over the eons by repeated fateful encounters with each other or the Galactic center. Surviving relics are older than any Earth fossil, older than any other structures in our Galaxy, and limit the universe itself in raw age. There are few, if any, young globular clusters in our Milky Way Galaxy because conditions are not ripe for more to form. Things are different next door, however, in the neighboring LMC galaxy. Pictured above is a "young" globular cluster residing there: NGC 1818. Observations show it formed only about 40 million years ago - just yesterday compared to the 12 billion year ages of globular clusters in our own Milky Way

Thumbnail image of picture found for this day. APOD: 2001 February 23 - M55 Color Magnitude Diagram
Explanation: This color "picture" of globular star cluster M55 may not look like any star cluster you've ever seen. Still, it shows a most fundamental view for students of stellar astronomy. In the picture, a Color Magnitude Diagram (CMD), M55's individual stars are represented as dots whose color indicates relative temperature, red (cool) to blue (hot). Position in the CMD does not correspond to a star's location in the sky, though. Instead, it corresponds to a measured astronomical color, (B-V color) read off the bottom scale, and a brightness in magnitudes (M) on the left hand scale. The temperature for each star can also be found by reading the equivalent scale at the top, where the Sun would have a temperature of 6,000 kelvins (K). Brightness relative to the Sun's luminosity (Sun = 1) is given on the scale at the right. The globular cluster stars clearly fall into distinct groups dramatically visible in this CMD. The broad swath extending diagonally from the lower right is the cluster's main sequence. A sharp turn toward the upper right hand corner follows the red giant branch while the blue giants are found grouped in the upper left. M55's stars were formed at the same time and at first were all located along the main sequence by mass, lower mass stars at the lower right. Over time, higher mass stars have evolved off the main sequence into red, then blue giants and beyond. The exact position of the sharp turn-off from the main sequence to the red giant branch measures the cluster's age.

Thumbnail image of picture found for this day. APOD: 2000 December 24 - NGC 1850: Gas Clouds and Star Clusters
Explanation: There's nothing like it in our own Galaxy. Globular clusters as young as NGC 1850 don't exist here. Globular clusters only 40 millions of years old can still be found in the neighboring LMC galaxy, though, but perhaps none so unusual as NGC 1850. Close inspection of the above photograph will reveal two clusters. Below and right of the main group of stars known as NGC 1850A is a smaller, still younger group dubbed NGC 1850B. This cluster is made of stars only about four million years old. The large red cloud of gas surrounding the clusters may have been predominantly created by supernovae explosions of stars in the younger cluster. The red supernova remnant N57D is visible on the upper left.

Thumbnail image of picture found for this day. APOD: 2000 November 30 - Palomar 13's Last Stand
Explanation: Globular star cluster Palomar 13 has roamed the halo of our Milky Way Galaxy for the last 12 billion years. The apparently sparse cluster of stars just left of center in this composite color digital image, it is one of the smallest, faintest globular clusters known. (The bright foreground star near bottom is unrelated and creates the spiky imaging artifacts.) Observations spanning forty years indicate that Palomar 13's galactic halo orbit is a highly eccentric one which, every one or two billion years, brings it relatively close to the galactic center. With each close approach to the Milky Way's central regions, gravitational tidal forces strip away the delicately bound cluster stars. In fact, detailed present day studies offer evidence for a dramatic end to this dwindling cluster's tidal tug of war. Palomar 13's latest close approach was only about 70 million years ago. But, when Palomar 13 again approaches the galaxy, it could well turn out to be the cluster's last stand.

Thumbnail image of picture found for this day. APOD: 2000 October 15 - Globular Cluster Omega Centauri
Explanation: Does an old, red globular cluster have any hot, blue stars? The rightmost picture, taken by the Ultraviolet Imaging Telescope in ultraviolet light, shows that indeed it does. Pictured, Omega Centauri is the largest known globular cluster of over 200 in our Galaxy, containing well over a million stars. Many of these stars are evident in the visible light photograph on the left. When photographed in ultraviolet light, however, different and less numerous stars emerge, as evident on the photograph on the right. Most of these stars are thought to have evolved past the current stage of our Sun. These stars no longer fuse hydrogen to helium in their core but rather fuse helium into carbon. These stars will soon shed their outer envelopes and end up as smoldering carbon embers known as white dwarf stars.

Thumbnail image of picture found for this day. APOD: 2000 September 22 - M55: Globular Star Cluster
Explanation: The fifty-fifth entry in Charles Messier's catalog, M55 is a large and lovely globular cluster of around 100,000 stars. Only 20,000 light-years away in the constellation Sagittarius, M55 appears to earth-bound observers to be nearly 2/3 the size of the full moon. Globular star clusters like M55 roam the halo of our Milky Way Galaxy as gravitationally bound populations of stars known to be much older than stellar groups found in the galactic disk. Astronomers who make detailed studies of globular cluster stars can accurately measure the cluster ages and distances. Their results ultimately constrain the age of the Universe (... it must be older than the stars in it! ), and provide a fundamental rung on the astronomical distance ladder. This stunning three-color image made with astronomical (BVI) filters spans about 100 light-years across the globular cluster M55.

Thumbnail image of picture found for this day. APOD: 2000 September 10 - White Dwarf Stars Cool
Explanation: Diminutive by stellar standards, white dwarf stars are also intensely hot ... but they are cooling. No longer do their interior nuclear fires burn, so they will continue to cool until they fade away. This Hubble Space Telescope image covers a small region near the center of a globular cluster known as M4. Here, researchers have discovered a large concentration of white dwarf stars (circled above). This was expected - low mass stars, including the Sun, are believed to evolve to the white dwarf stage. Studying how these stars cool could lead to a better understanding of their ages, of the age of their parent globular cluster, and even the age of our universe.

Thumbnail image of picture found for this day. APOD: 2000 August 4 - M15: Dense Globular Star Cluster
Explanation: Life might get dull at the core of M15 but the sky would always be bright with stars! In fact, only 40,000 light-years away in the constellation Pegasus, M15 is one of the most densely packed globular star clusters in our Milky Way Galaxy. This stunning Hubble Space Telescope image of M15 shows thousands of individual stars across the central 10 or so light-years of the cluster, also cataloged as NGC 7078. Yet even the Hubble's sharp vision can't clearly separate the stars at this cluster's core. Globular star clusters harbor from a hundred thousand up to a million stars and roam the Milky Way halo. Like most globulars, M15 is filled with ancient stars, about 12 billion years old compared to the Sun's estimated 4.5 billion years. Its cool red giant stars appear yellowish in this color composite image. Unlike most globulars, M15 displays a planetary nebula, the briefly visible gaseous shroud of a dying star. Can you pick it out? Cataloged as Kuestner 648, M15's planetary nebula is the round pinkish cloud at the upper left.

Thumbnail image of picture found for this day. APOD: 2000 July 19 - Globular Cluster M19
Explanation: M19 appears to be a typical globular cluster of stars - except for its shape. If one looks closely at the cluster, pictured above, it appears to be longer (top to bottom) than it is wide. In fact, M19 is the most aspherical globular cluster of the approximately 160 known orbiting the center of our Milky Way Galaxy. M19 lies about 27,000 light-years away, measures about 60 light-years across, and is home to over 100,000 stars. The cluster can be found with binoculars towards the constellation of Ophiuchus. The reason for the clusters' odd shape remains unknown, but might be related to the clusters' close (5000 light-year) proximity to the Galactic Center. Alternatively, the shape might be an illusion created by an unusual lane of dark absorbing dust on one side of the cluster.

Thumbnail image of picture found for this day. APOD: 2000 June 22 - Blue Stragglers In NGC 6397
Explanation: In our neck of the Galaxy stars are too far apart to be in danger of colliding, but in the dense cores of globular star clusters star collisions may be relatively common. In fact, researchers have evidence that the row of six closely spaced blue stars just below the label in this Hubble Space Telescope image were formed when stars directly collided. Pictured is the central region of NGC 6397, a globular cluster about 6,000 light-years distant, whose stars all formed at about the same time. NGC 6397's massive stars have long since evolved off the main sequence, exhausting their central supplies of nuclear fuel. This should leave the cluster with only old low mass stars; faint red main sequence stars and brighter blue and red giants. However, spectroscopic data show that the indicated stars, descriptively dubbed blue stragglers, are clearly main sequence stars which are too blue and too massive to still be there. Suggestively the stragglers appear to be two and occasionally three times as massive as the lower mass cluster stars otherwise present, supporting evidence for their formation from two and even three star collisions.

Thumbnail image of picture found for this day. APOD: 2000 May 23 - M4: The Closest Known Globular Cluster
Explanation: M4 is a globular cluster visible in dark skies about one degree west of the bright star Antares in the constellation Scorpius. M4 is perhaps the closest globular cluster at 7000 light years, meaning that we see M4 only as it was 7000 years ago, near the dawn of recorded human history. Although containing hundreds of thousands of stars and spanning over 50 light-years, M4 is one of the smallest and sparsest globular clusters known. A particularly unusual aspect for a globular cluster is M4's central bar of stars. M4, pictured above, is one of the oldest objects for which astronomers can estimate age directly. Cluster white dwarfs appear to be at least nine billion years old - so ancient they limit the youth of our entire universe.

Thumbnail image of picture found for this day. APOD: 2000 May 21 - Antares and Rho Ophiuchi
Explanation: Why is the sky near Antares and Rho Ophiuchi so colorful? The colors result from a mixture of objects and processes. Fine dust illuminated from the front by starlight produces blue reflection nebulae. Gaseous clouds whose atoms are excited by ultraviolet starlight produce reddish emission nebulae. Backlit dust clouds block starlight and so appear dark. Antares, a red supergiant and one of the brighter stars in the night sky, lights up the yellow-red clouds on the upper left. Rho Ophiuchi lies at the center of the blue nebula on the right. The distant globular cluster M4 is visible just below Antares, and to the left of the red cloud engulfing Sigma Scorpii. These star clouds are even more colorful than humans can see, emitting light across the electromagnetic spectrum.

Thumbnail image of picture found for this day. APOD: 2000 March 1 - M13: The Great Globular Cluster in Hercules
Explanation: M13 is one of the most prominent and best known globular clusters. Visible with binoculars in the constellation of Hercules, M13 is frequently one of the first objects found by curious sky gazers seeking celestials wonders beyond normal human vision. M13 is a colossal home to over 100,000 stars, spans over 150 light years across, lies over 20,000 light years distant, and is over 12 billion years old. At the 1974 dedication of Arecibo Observatory, a radio message about Earth was sent in the direction of M13. The reason for the low abundance of unusual blue straggler stars in M13 is currently unknown.

Thumbnail image of picture found for this day. APOD: September 17, 1999 - M3: Half A Million Stars
Explanation: This immense ball of half a million stars older than the sun lies 30,000 light-years above the plane of our Galaxy. Cataloged as M3 (and NGC 5272), it is one of about 250 globular star clusters which roam our galactic halo. Individual stars are difficult to distinguished in the densely packed core but colors are apparent for the bright stars on the cluster's outskirts. M3's many cool "red" giant stars take on a yellowish cast in this lovely composite image while hotter giants and pulsating variable stars look light blue.

Thumbnail image of picture found for this day. APOD: July 11, 1999 - Barringer Crater on Earth
Explanation: What happens when a meteor hits the ground? Usually nothing much, as most meteors are small, and indentations they make are soon eroded away. 49,000 years ago, however, a large meteor created Barringer Meteor Crater in Arizona, pictured above. Barringer is over a kilometer across. In 1920, it was the first feature on Earth to be recognized as an impact crater. Today, over 100 terrestrial impact craters have been identified. Recent computer modeling now indicates how some of the Canyon Diablo impactor melted during the impact that created Barringer.

Thumbnail image of picture found for this day. APOD: July 7, 1999 - M80: A Dense Globular Cluster
Explanation: If our Sun were part of M80, the night sky would glow like a jewel box of bright stars. M80, also known as NGC 6093, is one of about 250 globular clusters that survive in our Galaxy. Most of the stars in M80 are older and redder than our Sun, but some enigmatic stars appear to be bluer and younger. Young stars would contradict the hypothesis that all the stars in M80 formed at nearly the same time. These unusual stars are known as blue stragglers, and by analyzing pictures like the Hubble Space Telescope image above, astronomers have been able to find the largest population of blue stragglers yet. As blue stragglers are now thought to be due to stars coalescing, the collision and capture rate at the dense center of M80 must be very high.

Thumbnail image of picture found for this day. APOD: June 30, 1999 - NGC 6934 from Gemini North
Explanation: What's going on near the center of globular cluster NGC 6934? The blur caused by the Earth's atmosphere has prevented astronomers from discerning individual stars in this unusual environment. Telescopes in space can help, but the new Gemini North telescope took the above picture from the ground. In infrared light, Gemini was able to use its adaptive optic mirrors to resolve stars even near the globular cluster's center. NGC 6934 is a 15 billion-year-old ball of hundreds of thousands of stars. Dating stars in ancient globular clusters like NGC 6934 provide valuable constraints on the minimum age of the universe.

Thumbnail image of picture found for this day. APOD: February 25, 1999 - NGC 6712: Galactic Globular Cluster
Explanation: Following orbits which loop high above the galactic plane, globular star clusters are probably 12 to 14 billion years old - truly ancient denizens of our Milky Way Galaxy. After analyzing these new ESO/VLT images of portions of the globular cluster NGC 6712, astronomers report that this dense grouping of about 1 million stars seems to be slowly dissolving - steadily loosing fainter, lower mass stars into our Galaxy's halo. Their results offer strong evidence for gravitational stripping of stars from clusters which pass through the plane and central regions of the Galaxy. One of about 150 globular clusters known to be members of the Milky Way, NGC 6712 is thought to have crossed through the crowded galactic plane only a few million years ago. NGC 6712 is about 23,000 light-years away in the southern constellation Scutum.

Thumbnail image of picture found for this day. 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.

Thumbnail image of picture found for this day. APOD: January 17, 1999 - NGC 1818: A Young Globular Cluster
Explanation: Globular clusters once ruled the Milky Way. Back in the old days, back when our Galaxy first formed, perhaps thousands of globular clusters roamed our Galaxy. Today, there are perhaps 200 left. Many globular clusters were destroyed over the eons by repeated fateful encounters with each other or the Galactic center. Surviving relics are older than any earth fossil, older than any other structures in our Galaxy, and limit the universe itself in raw age. There are few, if any, young globular clusters in our Milky Way Galaxy because conditions are not ripe for more to form. But things are different next door - in the neighboring LMC galaxy. Pictured above is a "young" globular cluster residing there: NGC 1818. Recent observations show it formed only about 40 million years ago - just yesterday compared to the 12 billion year ages of globular clusters in our own Milky Way

Thumbnail image of picture found for this day. APOD: November 7, 1998 - Globular Cluster 47 Tucanae
Explanation: Stars come in bunches. Of the over 200 globular star clusters that orbit the center of our Milky Way Galaxy, 47 Tucanae is the second brightest globular cluster (behind Omega Centauri). Known to some affectionately as 47 Tuc or NGC 104, it is only visible from the Southern Hemisphere. Light takes about 20,000 years to reach us from 47 Tuc which can be seen near the SMC in the constellation of Tucana. Red Giant stars are particularly easy to see in this picture. The dynamics of stars near the center of 47 Tuc are not well understood, particularly why there are so few binary systems there.

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

Thumbnail image of picture found for this day. APOD: August 19, 1998 - M13: The Great Globular Cluster in Hercules
Explanation: M13 is one of the most prominent and best known globular clusters. Visible with binoculars in the constellation of Hercules, M13 is frequently one of the first steps beyond the ordinary visible to the casual sky gazer. M13 is a colossal home to over 100,000 stars, spans over 150 light years across, lies over 20,000 light years distant, and is over 12 billion years old. At the 1974 dedication of Arecibo Observatory, a radio message about Earth was sent in the direction of M13. The reason for the low abundance of unusual blue straggler stars in M13 is currently unknown.

Thumbnail image of picture found for this day. APOD: July 19, 1998 - Globular Cluster M3
Explanation: This huge ball of stars predates our Sun. Long before mankind evolved, before dinosaurs roamed, and even before our Earth existed, ancient globs of stars condensed and orbited a young Milky Way Galaxy. Of the 250 or so globular clusters that survive today, M3 is one of the largest and brightest, easily visible in the Northern hemisphere with binoculars. M3 contains about half a million stars, most of which are old and red. The existence of young blue stars in M3 once posed a mystery, but these blue stragglers are now thought to form via stellar interactions.

Thumbnail image of picture found for this day. APOD: March 7, 1998 - NGC 1818: A Young Globular Cluster
Explanation: Globular clusters once ruled the Milky Way. Back in the old days, back when our Galaxy first formed, perhaps thousands of globular clusters roamed our Galaxy. Today, there are perhaps 200 left. Many globular clusters were destroyed over the eons by repeated fateful encounters with each other or the Galactic center. Surviving relics are older than any earth fossil, older than any other structures in our Galaxy, and limit the universe itself in raw age. There are few, if any, young globular clusters in our Milky Way Galaxy because conditions are not ripe for more to form. But things are different next door - in the neighboring LMC galaxy. Pictured above is a "young" globular cluster residing there: NGC 1818. Recent observations show it formed only about 40 million years ago - just yesterday compared to the 12 billion year ages of globular clusters in our own Milky Way

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

Thumbnail image of picture found for this day. APOD: January 17, 1998 - At The Core Of M15
Explanation: Densely packed stars in the core of the globular cluster M15 are shown in this Hubble Space Telescope (HST) image. The star colors roughly indicate their temperatures - hot stars appear blue, cooler stars look reddish-orange. The region visible here is only about 1.6 light-years across, compared to the 4.3 light-year distance to our own Sun's nearest neighbor. Imagine the night sky viewed from a planet orbiting a star near this cluster's center! M15 has long been recognized as one of the densest clusters of stars in our galaxy outside of the galactic center itself. Even the unprecedented resolving power of the HST cameras could not separate the individual stars in its innermost regions. However, this HST image reveals that the density of stars continues to rise toward the cluster's core, suggesting that a sudden, runaway collapse due to the gravitational attraction of many closely packed stars or a single central massive object, perhaps a black hole, could account for the core's extreme density.

Thumbnail image of picture found for this day. APOD: December 24, 1997 - 30 Doradus Across the Spectrum
Explanation: 30 Doradus is lit up like a Christmas tree. Shining in light across the electromagnetic spectrum, 30 Doradus glows because of all the energetic processes that go on there. A distinctive region visible in a Milky Way satellite galaxy called the Large Magellanic Cloud (LMC), 30 Doradus is a hotbed of star formation, supernova explosions, and ionized plasma. The above image is a composite of three pictures taken in three different wavelength bands of light. Red represents X-ray emission created by gas as hot as 1 million degrees Kelvin. Green represents emission from ionized hydrogen gas, and blue represents ultraviolet radiation primarily emitted by hot stars. At the conclusion of this symphony of star formation and light in a few million years, astronomers expect that a new globular cluster will have formed.

Thumbnail image of picture found for this day. APOD: November 4, 1997 - Blue Stagglers in Globular Clusters
Explanation: This old dog is doing new tricks. On the left is ancient globular cluster 47 Tucanae which formed many billions of years ago. On the right is a closeup of its dense stellar center by the Hubble Space Telescope, released last week. Circled are mysterious stars called "blue stragglers." Stars as bright and blue as blue stragglers live short lives, much shorter than the age of the host globular cluster itself. But this contradicts evidence that globular cluster stars formed all at once. Although this problem has been known for almost 50 years, a mass and spin rate for a blue straggler was first published last Saturday. This new information indicates that BSS 19 was rejuvenated by two orbiting stars slowly coalescing , and not by a dramatic collision.

Thumbnail image of picture found for this day. APOD: November 2, 1997 - White Dwarf Stars Cool
Explanation: Diminutive by stellar standards, white dwarf stars are also intensely hot ... but they are cooling. No longer do their interior nuclear fires burn, so they will continue to cool until they fade away. This Hubble Space Telescope image covers a small region near the center of a globular cluster known as M4. Here, researchers have discovered a large concentration of white dwarf stars (circled above). This was expected - low mass stars, including the Sun, are believed to evolve to the white dwarf stage. Studying how these stars cool could lead to a better understanding of their ages, of the age of their parent globular cluster, and even the age of our universe!

Thumbnail image of picture found for this day. 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.

Thumbnail image of picture found for this day. APOD: September 22, 1997 - Antares and Rho Ophiuchi
Explanation: Why is the sky near Antares and Rho Ophiuchi so colorful? The colors result from a mixture of objects and processes. Fine dust illuminated from the front by starlight produces blue reflection nebulae. Gaseous clouds whose atoms are excited by ultraviolet starlight produce reddish emission nebulae. Backlit dust clouds block starlight and so appear dark. Antares, a red supergiant and one of the brighter stars in the night sky, lights up the yellow-red clouds on the upper left. Rho Ophiuchi lies at the center of the blue nebula on the right. The distant globular cluster M4 is visible just below Antares, and to the left of the red cloud engulfing Sigma Scorpii. These star clouds are even more colorful than humans can see, emitting light across the electromagnetic spectrum.

Thumbnail image of picture found for this day. APOD: September 19, 1997 - Globular Cluster 47 Tucanae
Explanation: Stars come in bunches. Of the over 200 globular star clusters that orbit the center of our Milky Way Galaxy, 47 Tucanae is the second brightest globular cluster (behind Omega Centauri). Known to some affectionately as 47 Tuc or NGC 104, it is only visible from the Southern Hemisphere. Light takes about 20,000 years to reach us from 47 Tuc which can be seen near the SMC in the constellation of Tucana. Red Giant stars are particularly easy to see in the above photograph. The dynamics of stars near the center of 47 Tuc are not well understood, particularly why there are so few binary systems there.

Thumbnail image of picture found for this day. APOD: July 19, 1997 - The Small Cloud of Magellan
Explanation: The southern sky contains wonders almost unknown in the north. These wonders include the Large and Small Magellanic Clouds: small irregular galaxies orbiting our own larger Milky Way spiral galaxy. The Small Magellanic Cloud (SMC), pictured here, is about 250,000 light years away. The SMC contains many young, hot, blue stars indicating it has undergone a recent period of star formation, possibly due to a collision with the LMC 500 million years ago. The bright object on the right is a globular cluster near the outskirts of the Milky Way.

Thumbnail image of picture found for this day. APOD: June 2, 1997 - Bright Star Knots in NGC 4038
Explanation: This galaxy is having a bad millennium. In fact, the past 100 million years haven't been so good, and probably the next billion or so should be quite tumultuous. NGC 4039 was a normal spiral galaxy, minding its own business, when NGC 4038 crashed into it. The evolving wreckage, known as the "Antennae", is pictured above. As gravity pulls each galaxy apart, clouds of gas slam into each other and bright blue knots are formed. These knots are large clusters of stars imbedded in vast regions of ionized hydrogen gas. The high abundance of relatively dim star clusters is quite unlike our Milky Way's globular cluster system, though. Perhaps some of these young star clusters will go on to form globular clusters, while others will disperse through close gravitational encounters. The above picture is centered around the larger of the two interacting galaxies: NGC 4038. The diagonal streak across the upper left is unrelated to the colliding galaxies. The color contrast in the above three-color mosaic was chosen to highlight extended features.

Thumbnail image of picture found for this day. APOD: March 5, 1997 - In the Center of NGC 604
Explanation: Stars are sometimes born in the midst of chaos. About 3 million years ago in the nearby galaxy M33, a large cloud of gas spawned dense internal knots which gravitationally collapsed to form stars. But NGC 604 was so large, it could form enough stars to make a globular cluster. Many young stars from this cloud are visible above, along with what is left of the initial gas cloud. Some stars were so massive they have already evolved and exploded in a supernova. The brightest stars that are left emit light so energetic that they create one of the largest cloud of ionized hydrogen gas known, second only to the 30 Doradus Cluster in Milky Way's close neighbor, the Large Magellanic Cloud.

Thumbnail image of picture found for this day. APOD: February 14, 1997 - NGC 1818: A Young Globular Cluster
Explanation: Globular clusters once ruled the Milky Way. Back in the old days, back when our Galaxy first formed, perhaps thousands of globular clusters roamed our Galaxy. Today, there are perhaps 200 left. Many globular clusters were destroyed over the eons by repeated fateful encounters with each other or the Galactic center. Surviving relics are older than any earth fossil, older than any other structures in our Galaxy, and limit the universe itself in raw age. There are few, if any, young globular clusters in our Milky Way Galaxy because conditions are not ripe for more to form. But things are different next door - in the neighboring LMC galaxy. Pictured above is a "young" globular cluster residing there: NGC 1818. Recent observations show it formed only about 40 million years ago - just yesterday compared to the 12 billion year ages of globular clusters in our own Milky Way

Thumbnail image of picture found for this day. APOD: December 6, 1996 - Globular Cluster M3
Explanation: This huge ball of stars predates our Sun. Long before mankind evolved, before dinosaurs roamed, and even before our Earth existed, ancient globs of stars condensed and orbited a young Milky Way Galaxy. Of the 250 or so globular clusters that survive today, M3 is one of the largest and brightest, easily visible in the Northern hemisphere with binoculars. M3 contains about half a million stars, most of which are old and red. The existence of young blue stars in M3 once posed a mystery, but these blue stragglers are now thought to form via stellar interactions.

Thumbnail image of picture found for this day. APOD: November 12, 1996 - Comet Hale-Bopp Passes M14
Explanation: Comet Hale-Bopp continues its slow trek across the night sky, and can now be seen superposed near the bright globular cluster M14. Will Comet Hale-Bopp become as bright in early 1997 as Comet Hyakutake did in early 1996? It is still too early to tell. Currently Hale-Bopp is curiously holding at about 5th magnitude - just barely bright enough to see without binoculars from a dark location. Because of the size of coma, some speculate that the nucleus of Hale-Bopp is unusually large. The actual nucleus is obscured, however, and recent speculation includes that the nucleus is comparable in size to Comet Halley - about 10-15 km across.

Thumbnail image of picture found for this day. APOD: October 4, 1996 - Globular Cluster Omega Centauri
Explanation: Does an old, red globular cluster have any hot, blue stars? The rightmost picture, taken by the Ultraviolet Imaging Telescope in ultraviolet light, shows that indeed it does. Pictured, Omega Centauri is the largest known globular cluster in our Galaxy, containing well over a million stars. Many of these stars are evident in the visible light photograph on the left. When photographed in ultraviolet light, however, different and less numerous stars emerge, as evident on the photograph on the right. Most of these stars are thought to have evolved past the current stage of our Sun. These stars no longer fuse hydrogen to helium in their core but rather fuse helium into carbon. These stars will soon shed their outer envelopes and end up as smoldering carbon embers known as white dwarf stars.

Thumbnail image of picture found for this day. APOD: July 13, 1996 - M81: A Bulging Spiral Galaxy
Explanation: Few stars are still forming in the old giant spiral galaxy M81. The blue regions in this picture - representing ultraviolet light - highlight regions of bright young stars and star formation and appear rare than in M74 and M33. The red regions - representing the visible light - show a large population of older, less massive stars. M81 is therefore classified as spiral galaxy type "Sab" on the Hubble Sequence of Galaxies. One distinguishing feature of these types of galaxies is the relatively large central bulge surrounding the center of the galaxy. A massive density wave circulates around the center of spiral galaxies. It is not well understood why the bulge of M81 glows as bright as it does in ultraviolet light. Speculation includes that this may be due to hot evolved stars such as those found in the ancient globular cluster Omega Centauri.

Thumbnail image of picture found for this day. 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.

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

Thumbnail image of picture found for this day. APOD: March 12, 1996 - The Colorful Clouds of Rho Ophiuchi
Explanation: The many spectacular colors of the Rho Ophiuchi (oh'-fee-yu-kee) clouds highlight the many processes that occur there. The blue regions shine primarily by reflected light. Blue light from the star rho Ophiuchi and nearby stars reflects more efficiently off this portion of the nebula than red light. The Earth's daytime sky appears blue for the same reason. The red and yellow regions shine primarily because of emission of the nebula's atomic and molecular gas. Light from nearby stars - particularly the bright star Antares in this case - knocks electrons away from the gas, which then shines when the electrons recombine with the gas. The dark regions are caused by dust grains - born in young stellar atmospheres - which effectively block light emitted behind them. The Rho Ophiuchi star clouds, well in front of the globular cluster M4 visible on far lower left, are even more colorful than humans can see - the clouds emits light in every wavelength band from the radio to the gamma-ray.

Thumbnail image of picture found for this day. APOD: February 21, 1996 - Millions of Stars in Omega Centauri
Explanation: Pictured above is the largest ball of stars in our Galaxy. About 10 million stars orbit the center of this globular cluster - named Omega Centauri - as this giant globular cluster orbits the center of our Galaxy. Recent evidence indicates that Omega Centauri is by far the most massive of the about 160 globular clusters in the Milky Way. The stars in globular clusters are generally older, redder and less massive than our Sun. Studying globular clusters tells us about the history of our Galaxy and the age of the universe.

Thumbnail image of picture found for this day. APOD: November 20, 1995 - At the Core of M15
Explanation: Densely packed stars in the core of the globular cluster M15 are shown in this Hubble Space Telescope (HST) image taken in April of 1994. The stars revealed are contained in an area 1.6 light years across and their colors roughly indicate their temperatures - hot stars appear blue, cooler stars look reddish-orange. M15 has long been recognized as one of the densest cluster of stars in our galaxy outside of the galactic center itself. Even the unprecedented resolving power of the HST cameras could not separate the individual stars in its innermost regions. However, this HST image reveals that the density of stars continues to rise toward the cluster's core, suggesting that a sudden, runaway collapse due to the gravitational attraction of many closely packed stars or a single central massive object, perhaps a black hole, could account for the core's extreme density.

Thumbnail image of picture found for this day. APOD: October 19, 1995 - Globular Cluster M5
Explanation: The globular cluster M5, pictured above, contains roughly 100,000 stars. These stars formed together and are gravitationally bound. Stars orbit the center of the cluster, and the cluster orbits the center of our Galaxy. So far, about 160 globular clusters are known to exist in a roughly spherical halo around the Galactic center. Globular clusters do not appear spherically distributed as viewed from the Earth, and this fact was a key point in the determination that our Sun is not at the center of our Galaxy. Globular clusters are very old. There is a straightforward method of determining their age, and this provides a very interesting lower limit on the age of our universe of about 14 billion years.

Thumbnail image of picture found for this day. APOD: September 19, 1995 - The Small Cloud of Magellan
Explanation: Almost unknown to casual observers in the northern hemisphere, the southern sky contains two diffuse wonders known as the Magellanic Clouds. The Magellanic Clouds are small irregular galaxies orbiting our own larger Milky Way spiral galaxy. The Small Magellanic Cloud (SMC), pictured here, is about 250,000 light years away and contains a preponderance of young, hot, blue stars indicating it has undergone a recent period of star formation. There is evidence that the SMC is actually two galaxies superposed to appear as one. The bright blob near the right hand edge of the frame is a globular cluster near the outskirts of the Milky Way.

Thumbnail image of picture found for this day. APOD: September 10, 1995 - White Dwarfs Cool
Explanation: The circled stars in the above picture are from a class that is hard to see in the cosmos: white dwarfs. The entire photo covers a small region near the center of a globular cluster known as M4. Researchers using the Hubble Space Telescope discovered a large concentration of white dwarfs in M4. This was expected - low mass stars, including the Sun, are known to evolved to the white dwarf stage. White dwarfs do not usually evolve further, they just gradually cool down from their high temperatures. It is hoped that studying how these stars cool could lead to a better understanding of their ages, of the age of their parent globular cluster, and hopefully even the age of our universe!

Thumbnail image of picture found for this day. APOD: July 26, 1995 - M15: A Great Globular Cluster
Explanation: A globular cluster is a system of about one million stars that together orbit a galaxy. One of the brightest globular clusters in our Milky Way galaxy is the pictured M15, the fifteenth object on Messier's list of diffuse objects on our sky. Most stars in globular clusters are older and redder than our Sun, which is about 5 billion years old.


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