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Astronomy Picture of the Day
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Thumbnail image of picture found for this day. APOD: 2014 January 24 - Bright Supernova in M82
Explanation: Astronomers really don't find supernovae by looking for the arrows. But in this image taken January 23rd, an arrow does point to an exciting, new supernova, now cataloged as SN 2014J, in nearby bright galaxy M82. Located near the Big Dipper in planet Earth's sky, M82 is also known as the Cigar Galaxy, a popular target for telescopes in the northern hemisphere. In fact, SN 2014J was first spotted as an unfamiliar source in the otherwise familiar galaxy by teaching fellow Steve Fossey and astronomy workshop students Ben Cooke, Tom Wright, Matthew Wilde and Guy Pollack at the University College London Observatory on the evening of January 21. M82 is a mere 12 million light-years away (so the supernova explosion did happen 12 million years ago, that light just now reaching Earth), making supernova SN 2014J one of the closest to be seen in recent decades. Spectra indicate it is a Type Ia supernova caused by the explosion of a white dwarf accreting matter from a companion star. By some estimates one week away from its maximum brightness, SN 2014J is already the brightest part of M82 and visible in small telescopes in the evening sky.

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

Thumbnail image of picture found for this day. APOD: 2013 October 11 - NGC 891 Edge On
Explanation: This sharp cosmic portrait features NGC 891. The spiral galaxy spans about 100 thousand light-years and is seen almost exactly edge-on from our perspective. In fact, about 30 million light-years distant in the constellation Andromeda, NGC 891 looks a lot like our Milky Way. At first glance, it has a flat, thin, galactic disk and a central bulge cut along the middle by regions of dark obscuring dust. The combined image data also reveal the galaxy's young blue star clusters and telltale pinkish star forming regions. And remarkably apparent in NGC 891's edge-on presentation are filaments of dust that extend hundreds of light-years above and below the center line. The dust has likely been blown out of the disk by supernova explosions or intense star formation activity. Faint neighboring galaxies can also be seen near this galaxy's disk.

Thumbnail image of picture found for this day. APOD: 2013 October 3 - M106 Close Up
Explanation: Close to the Great Bear (Ursa Major) and surrounded by the stars of the Hunting Dogs (Canes Venatici), this celestial wonder was discovered in 1781 by the metric French astronomer Pierre Mechain. Later, it was added to the catalog of his friend and colleague Charles Messier as M106. Modern deep telescopic views reveal it to be an island universe: a spiral galaxy around 30 thousand light-years across located only about 21 million light-years beyond the stars of the Milky Way. Along with prominent dust lanes and a bright central core, this colorful composite image highlights youthful blue star clusters and reddish stellar nurseries that trace the galaxy's spiral arms. The high resolution galaxy portrait is a mosaic of data from Hubble's sharp ACS camera combined with groundbased color image data. M106 (aka NGC 4258) is a nearby example of the Seyfert class of active galaxies, seen across the spectrum from radio to X-rays. Energetic active galaxies are powered by matter falling into a massive central black hole.

Thumbnail image of picture found for this day. APOD: 2013 September 5 - M1: The Incredible Expanding Crab
Explanation: The Crab Nebula is cataloged as M1, the first on Charles Messier's famous list of things which are not comets. In fact, the Crab is now known to be a supernova remnant, an expanding cloud of debris from the explosion of a massive star. The violent birth of the Crab was witnessed by astronomers in the year 1054. Roughly 10 light-years across today, the nebula is still expanding at a rate of over 1,000 kilometers per second. Want to watch the Crab Nebula expand? Check out this video (vimeo) animation comparing an image of M1 taken in 1999 at the European Southern Observatory, with this one, taken in 2012 at the Mt. Lemmon Sky Center. Background stars were used to register the two images. The Crab Nebula lies about 6,500 light-years away in the constellation Taurus.

Thumbnail image of picture found for this day. APOD: 2013 August 1 - Moon Over Andromeda
Explanation: The Great Spiral Galaxy in Andromeda (aka M31), a mere 2.5 million light-years distant, is the closest large spiral to our own Milky Way. Andromeda is visible to the unaided eye as a small, faint, fuzzy patch, but because its surface brightness is so low, casual skygazers can't appreciate the galaxy's impressive extent in planet Earth's sky. This entertaining composite image compares the angular size of the nearby galaxy to a brighter, more familiar celestial sight. In it, a deep exposure of Andromeda, tracing beautiful blue star clusters in spiral arms far beyond the bright yellow core, is combined with a typical view of a nearly full Moon. Shown at the same angular scale, the Moon covers about 1/2 degree on the sky, while the galaxy is clearly several times that size. The deep Andromeda exposure also includes two bright satellite galaxies, M32 and M110 (bottom).

Thumbnail image of picture found for this day. APOD: 2013 July 7 - NGC 2170: Still Life with Reflecting Dust
Explanation: In this beautiful celestial still life composed with a cosmic brush, dusty nebula NGC 2170 shines at the upper left. Reflecting the light of nearby hot stars, NGC 2170 is joined by other bluish reflection nebulae, a compact red emission region, and streamers of obscuring dust against a backdrop of stars. Like the common household items still life painters often choose for their subjects, the clouds of gas, dust, and hot stars pictured here are also commonly found in this setting - a massive, star-forming molecular cloud in the constellation of the Unicorn (Monoceros). The giant molecular cloud, Mon R2, is impressively close, estimated to be only 2,400 light-years or so away. At that distance, this canvas would be about 15 light-years across.

Thumbnail image of picture found for this day. APOD: 2013 May 30 - One Armed Spiral Galaxy NGC 4725
Explanation: While most spiral galaxies, including our own Milky Way, have two or more spiral arms, NGC 4725 has only one. In this sharp color composite image, the solo spira mirabilis seems to wind from a prominent ring of bluish, newborn star clusters and red tinted star forming regions. The odd galaxy also sports obscuring dust lanes a yellowish central bar structure composed of an older population of stars. NGC 4725 is over 100 thousand light-years across and lies 41 million light-years away in the well-groomed constellation Coma Berenices. Computer simulations of the formation of single spiral arms suggest that they can be either leading or trailing arms with respect to a galaxy's overall rotation. Also included in the frame, a more traditional looking spiral appears as a smaller background galaxy.

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

Thumbnail image of picture found for this day. APOD: 2012 October 26 - Reflection Nebula vdB1
Explanation: Every book has a first page and every catalog a first entry. And so this lovely blue cosmic cloud begins the van den Bergh Catalog (vdB) of stars surrounded by reflection nebulae. Interstellar dust clouds reflecting the light of the nearby stars, the nebulae usually appear blue because scattering by the dust grains is more effective at shorter (bluer) wavelengths. The same type of scattering gives planet Earth its blue daytime skies. Van den Bergh's 1966 list contains a total of 158 entries more easily visible from the northern hemisphere, including bright Pleiades cluster stars and other popular targets for astroimagers. Less than 5 light-years across, VdB1 lies about 1,600 light-years distant in the constellation Cassiopeia. Also on this scene, two intriguing nebulae at the right show loops and outflow features associated with the energetic process of star formation. Within are extremely young variable stars V633 Cas (top) and V376 Cas.

Thumbnail image of picture found for this day. APOD: 2012 October 8 - Spherical Planetary Nebula Abell 39
Explanation: Ghostly in appearance, Abell 39 is a remarkably simple, spherical nebula about five light-years across. Well within our own Milky Way galaxy, the cosmic sphere is roughly 7,000 light-years distant toward the constellation Hercules. Abell 39 is a planetary nebula, formed as a once sun-like star's outer atmosphere was expelled over a period of thousands of years. Still visible, the nebula's central star is evolving into a hot white dwarf. Although faint, the nebula's simple geometry has proven to be a boon to astronomers exploring the chemical abundances and life cycles of stars. In this deep image recorded under dark night skies, very distant background galaxies can be found -- some visible right through the nebula itself.

Thumbnail image of picture found for this day. APOD: 2012 August 17 - Spiral Galaxy NGC 5033
Explanation: Magnificent island universe NGC 5033 lies some 40 million light-years away in the well-trained northern constellation Canes Venatici. This telescopic portrait reveals striking details of dust lanes winding near the galaxy's bright core and majestic but relatively faint spiral arms. Speckled with pink star forming regions and massive blue star clusters, the arms span over 100,000 light-years, similar in size to our own spiral Milky Way. A well-studied example of the class of Seyfert active galaxies, NGC 5033 has a core that is very bright and variable. The emission is likely powered by a supermassive black hole. The bright nucleus and rotational center of the galaxy also seem to be slightly offset, suggesting NGC 5033 is the result of an ancient galaxy merger.

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 29 - Dark Clouds in Aquila
Explanation: Part of a dark expanse that splits the crowded plane of our Milky Way galaxy, the Aquila Rift arcs through the northern hemisphere's summer skies near bright star Altair and the Summer Triangle. In silhouette against the Milky Way's faint starlight, its dusty molecular clouds likely contain raw material to form hundreds of thousands of stars and astronomers eagerly search the clouds for telltale signs of star birth. This telescopic close-up looks toward the region at a fragmented Aquila dark cloud complex identified as LDN 673, stretching across a field of view slightly wider than the full moon. In the scene, visible indications of energetic outflows associated with young stars include the small red tinted nebulosity RNO 109 at top left and Herbig-Haro object HH32 above and right of center. The dark clouds in Aquila are estimated to be some 600 light-years away. At that distance, this field of view spans about 7 light-years.

Thumbnail image of picture found for this day. APOD: 2012 May 3 - M106 Close Up
Explanation: Close to the Great Bear (Ursa Major) and surrounded by the stars of the Hunting Dogs (Canes Venatici), this celestial wonder was discovered in 1781 by the metric French astronomer Pierre Mechain. Later, it was added to the catalog of his friend and colleague Charles Messier as M106. Modern deep telescopic views reveal it to be an island universe: a spiral galaxy around 30 thousand light-years across located only about 21 million light-years beyond the stars of the Milky Way. Along with prominent dust lanes and a bright central core, this colorful composite image highlights youthful blue star clusters and reddish stellar nurseries that trace the galaxy's spiral arms. The high resolution galaxy portrait is a mosaic of data from Hubble's sharp ACS camera combined with groundbased color image data. M106 (aka NGC 4258) is a nearby example of the Seyfert class of active galaxies, seen across the spectrum from radio to X-rays. Energetic active galaxies are powered by matter falling into a massive central black hole.

Thumbnail image of picture found for this day. APOD: 2012 March 22 - M95 with Supernova
Explanation: Barred spiral galaxy M95 is about 75,000 light-years across, comparable in size to our own Milky Way and one of the larger galaxies of the Leo I galaxy group. In fact, it is part of a not quite so famous trio of Leo galaxies with neighbors M96 and M105, about 38 million light-years distant. In this sharp and colorful cosmic portrait, a bright, compact ring of star formation surrounds the galaxy's core. Surrounding the prominent yellowish bar are tightly wound spiral arms traced by dust lanes, young blue star clusters, and telltale pinkish star forming regions. As a bonus, follow along the spiral arm unwinding down and to the right and you'll soon get to M95's latest supernova SN 2012aw, discovered on March 16 and now identified as the explosion of a massive star. A good target for small telescopes, the supernova stands out in this video feature (vimeo) comparing the recent image with a deep image of M95 without supernova taken in 2009.

Thumbnail image of picture found for this day. APOD: 2012 March 9 - NGC 1579: Trifid of the North
Explanation: Colorful NGC 1579 resembles the better known Trifid Nebula, but lies much farther north in planet Earth's sky, in the heroic constellation Perseus. About 2,100 light-years away and 3 light-years across, NGC 1579 is, like the Trifid, a study in contrasting blue and red colors, with dark dust lanes prominent in the nebula's central regions. In both, dust reflects starlight to produce beautiful blue reflection nebulae. But unlike the Trifid, in NGC 1579 the reddish glow is not emission from clouds of glowing hydrogen gas excited by ultraviolet light from a nearby hot star. Instead, the dust in NGC 1579 drastically diminishes, reddens, and scatters the light from an embedded, extremely young, massive star, itself a strong emitter of the characteristic red hydrogen alpha light.

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

Thumbnail image of picture found for this day. APOD: 2011 December 8 - Sh2-239: Celestial Impasto
Explanation: The cosmic brush of star formation composed this alluring mix of dust and dark nebulae. Cataloged as Sh2-239 and LDN 1551, the region lies near the southern end of the Taurus molecular cloud complex some 450 light-years distant. Stretching for nearly 3 light-years, the canvas abounds with signs of embedded young stellar objects driving dynamic outflows into the surrounding medium. Included near the center of the frame, a compact, tell-tale red jet of shocked hydrogen gas is near the position of infrared source IRS5, known to be a system of protostars surrounded by dust disks. Just below it are the broader, brighter wings of HH 102, one of the region's many Herbig-Haro objects, nebulosities associated with newly born stars. Estimates indicate that the star forming LDN 1551 region contains a total amount of material equivalent to about 50 times the mass of the Sun.

Thumbnail image of picture found for this day. APOD: 2011 November 5 - GK Per: Nova of 1901
Explanation: Early in the 20th century, GK Persei briefly became one of the brightest stars in planet Earth's sky, an event known as Nova Persei 1901. Documented in this modern day composite of two images from 2003 and 2011 the ejecta from the explosion, popularly called the Firework Nebula, continues to expand into space. These images are part of a time lapse video tracking the nebula's expansion over the last 17 years. About 1500 light-years away, the nebula is still just under a light-year in diameter. GK Per and similar cataclysmic variable stars known as classical novae are understood to be binary systems consisting of a compact white dwarf star and swollen cool giant star in a close orbit. The build up of mass transferred to the surface of the white dwarf from the giant star through an accretion disk eventually triggers a thermonuclear outburst, blasting the stellar material into space without destroying the white dwarf star. With a 2 day orbital period, the GK Per system has produced much smaller outbursts in recent years.

Thumbnail image of picture found for this day. APOD: 2011 October 31 - Ghost of the Cepheus Flare
Explanation: Spooky shapes seem to haunt this starry expanse, drifting through the night in the royal constellation Cepheus. Of course, the shapes are cosmic dust clouds faintly visible in dimly reflected starlight. Far from your own neighborhood on planet Earth, they lurk at the edge of the Cepheus Flare molecular cloud complex some 1,200 light-years away. Over 2 light-years across the ghostly nebula and relatively isolated Bok globule, also known as vdB 141 or Sh2-136, is near the center of the field. The core of the dark cloud on the right is collapsing and is likely a binary star system in the early stages of formation. Even so, if the spooky shapes could talk, they might well wish you a happy Halloween.

Thumbnail image of picture found for this day. APOD: 2011 March 26 - T Tauri and Hind's Variable Nebula
Explanation: The yellowish star near center in this remarkable telescopic skyview is T Tauri, prototype of the class of T Tauri variable stars. Nearby it is a dusty yellow cosmic cloud historically known as Hind's Variable Nebula (NGC 1555). Over 400 light-years away, at the edge of a molecular cloud, both star and nebula are seen to vary significantly in brightness but not necessarily at the same time, adding to the mystery of the intriguing region. T Tauri stars are now generally recognized as young (less than a few million years old), sun-like stars still in the early stages of formation. To further complicate the picture, infrared observations indicate that T Tauri itself is part of a multiple system and suggest that the associated Hind's Nebula may also contain a very young stellar object. The naturally colored image spans about 4 light-years at the estimated distance of T Tauri.

Thumbnail image of picture found for this day. APOD: 2011 February 24 - NGC 1999: South of Orion
Explanation: South of the large star-forming region known as the Orion Nebula, lies bright blue reflection nebula NGC 1999. Also at the edge of the Orion molecular cloud complex some 1,500 light-years distant, NGC 1999's illumination is provided by the embedded variable star V380 Orionis. The nebula is marked with a dark sideways T-shape near center in this broad cosmic vista that spans over 10 light-years. The dark shape was once assumed to be an obscuring dust cloud seen in silhouette against the bright reflection nebula. But recent infrared images indicate the shape is likely a hole blown through the nebula itself by energetic young stars. In fact, this region abounds with energetic young stars producing jets and outflows that create luminous shock waves. Cataloged as Herbig-Haro (HH) objects, named for astronomers George Herbig and Guillermo Haro, the shocks appear bright red in this view that includes HH1 and HH2 just below NGC 1999. The stellar jets and outflows push through the surrounding material at speeds of hundreds of kilometers per second.

Thumbnail image of picture found for this day. APOD: 2011 January 1 - Fireworks Galaxy NGC 6946
Explanation: Celebrate the New Year with the Fireworks Galaxy! Also known as NGC 6946, the big, beautiful spiral galaxy is located just 10 million light-years away, behind a veil of foreground dust and stars in the high and far-off constellation of Cepheus. From our vantage point in the Milky Way Galaxy, we see NGC 6946 face-on. In this colorful cosmic portrait, the galaxy's colors change from the yellowish light of old stars in the core to young blue star clusters and reddish star forming regions along the loose, fragmented spiral arms. NGC 6946 is bright in infrared light and rich in gas and dust, exhibiting a furious rate of star formation. Nearly 40,000 light-years across, the nearby spiral is fittingly referred to as the Fireworks Galaxy. Over the last 100 years, at least nine supernovae, the death explosions of massive stars, were discovered in NGC 6946. By comparison, the average rate for supernovae in the Milky Way is about 1 per century.

Thumbnail image of picture found for this day. APOD: 2010 December 30 - Still Life with NGC 2170
Explanation: In this beautiful celestial still life composed with a cosmic brush, dusty nebula NGC 2170 shines at the upper left. Reflecting the light of nearby hot stars, NGC 2170 is joined by other bluish reflection nebulae, a compact red emission region, and streamers of obscuring dust against a backdrop of stars. Like the common household items still life painters often choose for their subjects, the clouds of gas, dust, and hot stars pictured here are also commonly found in this setting - a massive, star-forming molecular cloud in the constellation Monoceros. The giant molecular cloud, Mon R2, is impressively close, estimated to be only 2,400 light-years or so away. At that distance, this canvas would be about 15 light-years across.

Thumbnail image of picture found for this day. APOD: 2010 November 26 - Flame Nebula Close-Up
Explanation: Of course, the Flame Nebula is not on fire. Also known as NGC 2024, the nebula's suggestive reddish color is due to the glow of hydrogen atoms at the edge of the giant Orion molecular cloud complex some 1,500 light-years away. The hydrogen atoms have been ionized, or stripped of their electrons, and glow as the atoms and electrons recombine. But what ionizes the hydrogen atoms? In this close-up view, the central dark lane of absorbing interstellar dust stands out in silhouette against the hydrogen glow and actually hides the true source of the Flame Nebula's energy from optical telescopes. Behind the dark lane lies a cluster of hot, young stars, seen at infrared wavelengths through the obscuring dust. A young, massive star in that cluster is the likely source of energetic ultraviolet radiation that ionizes the hydrogen gas in the Flame Nebula.

Thumbnail image of picture found for this day. APOD: 2010 July 2 - Galaxies on a String
Explanation: Galaxies NGC 5216 (top) and NGC 5218 really do look like they are connected by a string. Of course, that string is a cosmic trail of gas, dust, and stars about 22,000 light-years long. Also known as Keenan's system (for its discoverer) and Arp 104, the interacting galaxy pair is some 17 million light-years away in the constellation Ursa Major. The debris trail that joins them, along with NGC 5218's comma-shaped extension and the distorted arms of NGC 5216, are a consequence of mutual gravitational tides. The tides disrupt the galaxies as they repeatedly swing close to one another. Drawn out over billions of years, the encounters will likely result in their merger into a single galaxy of stars. Such spectacular galactic mergers are now understood to be a normal part of the evolution of galaxies, including our own Milky Way.

Thumbnail image of picture found for this day. APOD: 2010 May 20 - M87: Elliptical Galaxy with Jet
Explanation: In spiral galaxies, majestic winding arms of young stars, gas, and dust rotate in a flat disk around a bulging galactic nucleus. But elliptical galaxies seem to be simpler. Lacking gas and dust to form new stars, their randomly swarming older stars, give them an ellipsoidal (egg-like) shape. Still, elliptical galaxies can be very large. Centered in this telescopic view and over 120,000 light-years in diameter, larger than our own Milky Way, elliptical galaxy M87 (NGC 4486) is the dominant galaxy of the Virgo Galaxy Cluster. Some 50 million light-years away, M87 is likely home to a supermassive black hole responsible for a high-energy jet of particles emerging from the giant galaxy's central region. In this well-processed image, M87's jet is near the one o'clock position. Other galaxies are also in the field of view, including large Virgo Cluster ellipticals NGC 4478 right of center and NGC 4476 near the right edge.

Thumbnail image of picture found for this day. APOD: 2010 January 30 - Messier 88
Explanation: Charles Messier described the 88th entry in his 18th century catalog of Nebulae and Star Clusters as a spiral nebula without stars. Of course the gorgeous M88 is now understood to be a galaxy full of stars, gas, and dust, not unlike our own Milky Way. In fact, M88 is one of the brightest galaxies in the Virgo Galaxy Cluster some 50 million light-years away. M88's beautiful spiral arms are easy to trace in this colorful cosmic portait. The arms are lined with young blue star clusters, pink star-forming regions, and obscuring dust lanes extending from a yellowish core dominated by an older population of stars. Spiral galaxy M88 spans over 100,000 light-years.

Thumbnail image of picture found for this day. APOD: 2009 August 2 - Stars, Dust and Nebula in NGC 6559
Explanation: When stars form, pandemonium reigns. A textbook case is the star forming region NGC 6559. Visible above are red glowing emission nebulas of hydrogen, blue reflection nebulas of dust, dark absorption nebulas of dust, and the stars that formed from them. The first massive stars formed from the dense gas will emit energetic light and winds that erode, fragment, and sculpt their birthplace. And then they explode. The resulting morass can be as beautiful as it is complex. After tens of millions of years, the dust boils away, the gas gets swept away, and all that is left is a naked open cluster of stars.

Thumbnail image of picture found for this day. APOD: 2009 July 7 - The Trifid Nebula in Stars and Dust
Explanation: Unspeakable beauty and unimaginable bedlam can be found together in the Trifid Nebula. Also known as M20, this photogenic nebula is visible with good binoculars towards the constellation of Sagittarius. The energetic processes of star formation create not only the colors but the chaos. The red-glowing gas results from high-energy starlight striking interstellar hydrogen gas. The dark dust filaments that lace M20 were created in the atmospheres of cool giant stars and in the debris from supernovae explosions. Which bright young stars light up the blue reflection nebula is still being investigated. The light from M20 we see today left perhaps 3,000 years ago, although the exact distance remains unknown. Light takes about 50 years to cross M20.

Thumbnail image of picture found for this day. APOD: 2009 May 2 - The Whale Galaxy
Explanation: NGC 4631 is a big beautiful spiral galaxy. Seen edge-on, it lies only 25 million light-years away in the well-trained northern constellation Canes Venatici. The galaxy's slightly distorted wedge shape suggests to some a cosmic herring and to others its popular moniker, The Whale Galaxy. Either way, it is similar in size to our own Milky Way. In this gorgeous color image, the galaxy's yellowish core, dark dust clouds, bright blue star clusters, and red star forming regions are easy to spot. A companion galaxy, the small elliptical NGC 4627, is just above the Whale Galaxy. Out of view, off the lower edge of the picture lies another distorted galaxy, hockey stick-shaped NGC 4656. The distortions and mingling trails of gas and dust detected at other wavelengths suggest that all three galaxies have had close encounters with each other in their past. The Whale Galaxy is also known to have spouted a halo of hot gas glowing in x-rays.

Thumbnail image of picture found for this day. APOD: 2009 April 14 - M101: The Pinwheel Galaxy
Explanation: Why do many galaxies appear as spirals? A striking example is M101, shown above, whose relatively close distance of about 27 million light years allows it to be studied in some detail. Recent evidence indicates that a close gravitational interaction with a neighboring galaxy created waves of high mass and condensed gas which continue to orbit the galaxy center. These waves compress existing gas and cause star formation. One result is that M101, also called the Pinwheel Galaxy, has several extremely bright star-forming regions (called HII regions) spread across its spiral arms. M101 is so large that its immense gravity distorts smaller nearby galaxies.

Thumbnail image of picture found for this day. APOD: 2008 November 26 - The Horsehead Nebula in Orion
Explanation: One of the most identifiable nebulae in the sky, the Horsehead Nebula in Orion, is part of a large, dark, molecular cloud. Also known as Barnard 33, the unusual shape was first discovered on a photographic plate in the late 1800s. The red glow originates from hydrogen gas predominantly behind the nebula, ionized by the nearby bright star Sigma Orionis. A blue reflection nebula dubbed NGC 2023 surrounds the bright star at the lower left. The darkness of the Horsehead is caused mostly by thick dust, although the lower part of the Horsehead's neck casts a shadow to the left. Streams of gas leaving the nebula are funneled by a strong magnetic field. Bright spots in the Horsehead Nebula's base are young stars just in the process of forming. Light takes about 1500 years to reach us from the Horsehead Nebula. The above image was taken earlier this month with a 0.6-meter telescope at the Mt. Lemmon SkyCenter in Arizona, USA.

Thumbnail image of picture found for this day. APOD: 2008 November 15 - Arp 273
Explanation: The two prominent stars in the foreground of this colorful skyscape are well within our own Milky Way Galaxy. Their spiky appearance is due to diffraction in the astronomer's telescope. But the two eye-catching galaxies in view lie far beyond the Milky Way, at a distance of about 200 million light-years. Their distorted appearance is due to gravitational tides as the pair engage in close encounters. From our perspective, the bright cores of the galaxies are separated by about 80,000 light-years. Cataloged as Arp 273 (also as UGC 1810), the galaxies do look peculiar, but interacting galaxies are now understood to be common in the universe. In fact, the nearby large spiral Andromeda Galaxy is known to be some 2 million light-years away and approaching the Milky Way. Arp 273 may offer an analog of their far future encounter. Repeated galaxy encounters on a cosmic timescale can ultimately result in a merger into a single galaxy of stars.

Thumbnail image of picture found for this day. APOD: 2008 August 19 - NGC 6960: The Witch's Broom Nebula
Explanation: Ten thousand years ago, before the dawn of recorded human history, a new light must suddenly have appeared in the night sky and faded after a few weeks. Today we know this light was an exploding star and record the colorful expanding cloud as the Veil Nebula. Pictured above is the west end of the Veil Nebula known technically as NGC 6960 but less formally as the Witch's Broom Nebula. The expanding debris cloud gains its colors by sweeping up and exciting existing nearby gas. The supernova remnant lies about 1400 light-years away towards the constellation of Cygnus. This Witch's Broom actually spans over three times the angular size of the full Moon. The bright star 52 Cygni is visible with the unaided eye from a dark location but unrelated to the ancient supernova.

Thumbnail image of picture found for this day. APOD: 2008 February 14 - Long Stem Rosette
Explanation: The Rosette Nebula (aka NGC 2237) is not the only cosmic cloud of gas and dust to evoke the imagery of flowers. But it is the one most often suggested as a suitable astronomy image for Valentine's Day. Of the many excellent Rosette Nebula pictures submitted to APOD editors, this view seemed most appropriate, with a long stem of glowing hydrogen gas in the region included in the composition. At the edge of a large molecular cloud in Monoceros, some 5,000 light years away, the petals of this rose are actually a stellar nursery whose lovely, symmetric shape is sculpted by the winds and radiation from its central cluster of hot young stars. The stars in the energetic cluster, cataloged as NGC 2244, are only a few million years old, while the central cavity in the Rosette Nebula is about 50 light-years in diameter. Happy Valentine's Day!

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 15 - Messier 96
Explanation: Dust lanes seem to swirl around the core of Messier 96 in this colorful, detailed portrait of the beautiful island universe. Of course M96 is a spiral galaxy, and counting the faint arms extending beyond the brighter central region it spans 100 thousand light-years or so, about the size of our own Milky Way. M96 is known to be 38 million light-years distant, a dominant member of the Leo I galaxy group. Background galaxies and smaller Leo I group members can be found by examining the picture, but accomplished astro-imager Adam Block notes he is most intrigued by the edge-on spiral galaxy that apparently lies behind the outer spiral arm near the 10 o'clock position. The edge-on spiral appears to be about 1/5 the size of M96. If the spiral is similar in actual size to M96, then it lies about 5 times farther away.

Thumbnail image of picture found for this day. APOD: 2007 April 12 - The Cone Nebula Neighborhood
Explanation: Cosmic clouds of hydrogen gas and dust abound in this gorgeous skyscape, stretching through Monoceros in the neighborhood of The Cone Nebula. A dark, obscuring dust cloud, the simple, sculpted shape of the Cone Nebula is near the lower left edge. Surrounded by the red glow of hydrogen gas, the cone points up, toward bright, blue-white S Monocerotis, a quadruple system of very massive, hot stars. S Mon itself is also surrounded by intriguing red emission nebulae characteristic of star forming regions while above and to the right of S Mon, expansive dark markings on the sky are silhouetted by a larger region of fainter emission. Yellowish open star cluster Trumpler 5 is below picture center, with the striking blue reflection nebula, IC 2169 near the center right edge. The curious compact cometary shape near the bottom edge is Hubble's Variable Nebula. Over 31 hours of exposure time went into this truly remarkable, 2.5 degree wide, color mosaic.

Thumbnail image of picture found for this day. APOD: 2006 December 28 - Moon Over Andromeda
Explanation: The Great Spiral Galaxy in Andromeda (aka M31), a mere 2.5 million light-years distant, is the closest large spiral to our own Milky Way. Andromeda is visible to the unaided eye as a small, faint, fuzzy patch, but because its surface brightness is so low, casual skygazers can't appreciate the galaxy's impressive extent in planet Earth's sky. This entertaining composite image compares the angular size of the nearby galaxy to a brighter, more familiar celestial sight. In it, a deep exposure of Andromeda, tracing beautiful blue star clusters in spiral arms far beyond the bright yellow core, is combined with a typical view of a nearly full Moon. Shown at the same angular scale, the Moon covers about 1/2 degree on the sky, while the galaxy is clearly several times that size. The deep Andromeda exposure also includes two bright satellite galaxies, M32 and M110 (bottom).

Thumbnail image of picture found for this day. APOD: 2006 October 31 - SH2 136: A Spooky Nebula
Explanation: The dark nebula SH2-136 appears to be celebrating Halloween all of the time. The complex process of star formation create dust clouds of many shapes and sizes -- it is human perception that might identify a ghoulish creature, on the right of the above image, chasing humans. Halloween's modern celebration retains historic roots in dressing to scare away the spirits of the dead. Since the fifth century BC, Halloween has been celebrated as a cross-quarter day, a day halfway between an equinox (equal day / equal night) and a solstice (minimum day / maximum night in the northern hemisphere). With our modern calendar, however, the real cross-quarter day will occur next week. Other cross-quarter markers include Groundhog Day and Walpurgis Night.

Thumbnail image of picture found for this day. APOD: 2006 August 31 - Extra Galaxies
Explanation: Careful inspection of the full field of view for this sharp composite image reveals a surprising number of galaxies both near and far toward the constellation Ursa Major. The most striking is clearly NGC 3718, a warped spiral galaxy found near picture center. NGC 3718's faint spiral arms look twisted and extended, its bright central region crossed by obscuring dust lanes. A mere 150 thousand light-years to the right is another large spiral galaxy, NGC 3729. The two are likely interacting gravitationally, accounting for the peculiar appearance of NGC 3718. While this galaxy pair lies about 52 million light-years away, the remarkable Hickson Group 56 can also be seen clustered just below NGC 3718. Hickson Group 56 consists of five interacting galaxies and lies over 400 million light-years away.

Thumbnail image of picture found for this day. APOD: 2006 April 3 - Stars and Dust Across 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: 2006 March 11 - Colors of Comet Pojmanski
Explanation: Comet Pojmanski flew by planet Earth last weekend on a surprise trip through the inner solar system. Then an easy binocular target for morning skygazers, Pojmanski ultimately showed off a long tail, but it also presented some lovely green-blue hues as gas molecules in its tenuous coma and tail fluoresced in the sunlight. Astronomers Adam Block and Jay GaBany recorded this colorful high-resolution view on March 3rd in the darkness just before twilight. The picture spans about one full moon on the sky. Comet Pojmanski (C/2006 A1) is outward bound and fading now, still visible in binoculars for northern hemisphere observers.

Thumbnail image of picture found for this day. APOD: 2005 November 19 - NGC 2359: Thor's Helmet
Explanation: NGC 2359 is a striking emission nebula with an impressive popular name - Thor's Helmet. Sure, its suggestive winged appearance might lead some to refer to it as the "duck nebula", but if you were a nebula which name would you choose? By any name NGC 2359 is a bubble-like nebula some 30 light-years across, blown by energetic winds from an extremely hot star seen near the center and classified as a Wolf-Rayet star. Wolf-Rayet stars are rare massive blue giants which develop stellar winds with speeds of millions of kilometers per hour. Interactions with a nearby large molecular cloud are thought to have contributed to this nebula's more complex shape and curved bow-shock structures. NGC 2359 is about 15,000 light-years distant toward the constellation Canis Major.

Thumbnail image of picture found for this day. APOD: 2005 July 30 - M106 in Canes Venatici
Explanation: Close to the Great Bear (Ursa Major) and surrounded by the stars of the Hunting Dogs (Canes Venatici), this celestial nebula was discovered in 1781 by the metric French astronomer Pierre Mechain. Later, it was added to the catalog of his friend and colleague Charles Messier as M106. Modern deep telescopic views reveal it to be an island universe -- a spiral galaxy around 30 thousand light-years across located only about 21 million light-years beyond the stars of the Milky Way. Youthful blue star clusters and reddish stellar nurseries trace the striking spiral arms of M106. Seen so clearly in this beautiful image, the galaxy's bright core is also visible across the spectrum from radio to x-rays, making M106 a nearby example of the Seyfert class of active galaxies. The bright core of a Seyfert galaxy is believed to be powered by matter falling into a massive central black hole.

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

Thumbnail image of picture found for this day. APOD: 2005 June 3 - M27: The Dumbbell Nebula
Explanation: The first hint of what will become of our Sun was discovered inadvertently in 1764. At that time, Charles Messier was compiling a list of diffuse objects not to be confused with comets. The 27th object on Messier's list, now known as M27 or the Dumbbell Nebula, is a planetary nebula, the type of nebula our Sun will produce when nuclear fusion stops in its core. M27 is one of the brightest planetary nebulae on the sky, and can be seen in the constellation Vulpecula with binoculars. It takes light about 1000 years to reach us from M27, shown above, digitally sharpened, in three standard colors. Understanding the physics and significance of M27 was well beyond 18th century science. Even today, many things remain mysterious about bipolar planetary nebula like M27, including the physical mechanism that expels a low-mass star's gaseous outer-envelope, leaving an X-ray hot white dwarf.

Thumbnail image of picture found for this day. APOD: 2005 May 24 - Swirls and Stars in IC 4678
Explanation: Swirls of gas and dust enrich this little observed starfield toward the constellation of Sagittarius. Just to the side of the more often photographed Lagoon Nebula (M8) and the Trifid Nebula (M20) lies this busy patch of sky dubbed IC 4678. Prominent in the above image are large emission nebulas of red glowing gas highlighted by unusually bright red filaments. On the left, a band of thin dust preferentially reflects the blue light of a bright star creating a small reflection nebula. On the right and across the bottom, swaths of thicker dust appear as dark absorption nebulas, blocking the light from stars farther in the distance. IC 4678 spans about 25 light years and lies about 5,000 light years distant.

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

Thumbnail image of picture found for this day. APOD: 2005 March 21 - Orion's Horsehead Nebula
Explanation: The Horsehead Nebula is one of the most famous nebulae on the sky. It is visible as the dark indentation to the red emission nebula seen above and to the right of center in the above photograph. The bright star on the left is located in the belt of the familiar constellation of Orion. The horse-head feature is dark because it is really an opaque dust cloud which lies in front of the bright red emission nebula. Like clouds in Earth's atmosphere, this cosmic cloud has assumed a recognizable shape by chance. After many thousands of years, the internal motions of the cloud will alter its appearance. The emission nebula's red color is caused by electrons recombining with protons to form hydrogen atoms. Also visible in the picture are blue reflection nebulae, which preferentially reflect the blue light from nearby stars.

Thumbnail image of picture found for this day. APOD: 2004 December 13 - Announcing Comet Machholz
Explanation: A comet discovered only this summer is brightening quickly and already visible to the unaided eye. Comet C/2004 Q2 (Machholz) is currently best visible in Earth's Southern Hemisphere where some observers report it brighter than magnitude 5. The comet is moving rapidly to northern skies and should continue to brighten until early January. By coincidence, Comet Machholz will be easy to view as it will be nearly opposite the Sun when appearing its brightest. How bright Comet Machholz will become then remains uncertain, but it will surely stay in northern skies for much of 2005, even approaching Polaris in early March. Pictured above, Comet Machholz was captured in early December already sporting a bright surrounding coma, a white oblong dust tail fading off toward the bottom, and a long wispy ion tail toward the right with a kink near the end.

Thumbnail image of picture found for this day. APOD: 2004 November 27 - NGC 2683: Spiral Edge On
Explanation: This gorgeous island universe, cataloged as NGC 2683, lies a mere 16 million light-years distant in the northern constellation Lynx. A spiral galaxy comparable to our own Milky Way, NGC 2683 is seen nearly edge-on in this cosmic vista, with more distant galaxies scattered in the background. Blended light from a large population of old yellowish stars forms the remarkably bright galactic core. Starlight silhouettes the dust lanes along winding spiral arms, dotted with the telltale pink glow of ionized hydrogen gas from this galaxy's star forming regions.

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

Thumbnail image of picture found for this day. APOD: 2004 June 29 - In the Center of NGC 6559
Explanation: Bright gas and dark dust permeate the space between stars in the center of a nebula known as NGC 6559. The gas, primarily hydrogen, is responsible for the diffuse red glow of the emission nebula. As energetic light from neighboring stars ionizes interstellar hydrogen, protons and electrons recombine to emit light of very specific colors, including the red hue observed. Small dust particles reflect blue starlight efficiently and so creates the blue reflection nebulosity seen near two of the bright stars. Dust also absorbs visible light, causing the dark clouds and filaments visible. NGC 6559 lies about 5000 light-years away toward the constellation of Sagittarius.

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 April 22 - Comet C/2002 T7 (LINEAR)
Explanation: Discovered by the the Lincoln Near Earth Asteroid Research (LINEAR) project in October of 2002, comet C/2002 T7 is now visiting the inner solar system, making its closest approach (see animation by L. Koehn) to the Sun tomorrow, April 23rd. Emerging from the solar glare, the comet is now just visible to the unaided eye in the constellation Pisces, near the eastern horizon in morning twilight. In this gorgeous telescopic view recorded before dawn yesterday, the clearly active comet has developed an extensive, complex tail extending over 2 degrees in the anti-sunward direction, and a pronounced anti-tail or anomalous tail. Later next month this comet should appear brighter, making its closest approach to planet Earth on May 19th. In fact, it could share southern skies with another naked-eye comet, also anticipated to brighten in May, designated C/2001 Q4 (NEAT).

Thumbnail image of picture found for this day. APOD: 2004 April 9 - NGC 4565: Galaxy on the Edge
Explanation: Magnificent spiral galaxy NGC 4565 is viewed edge-on from planet Earth. Also known as the Needle Galaxy for its narrow profile, bright NGC 4565 is a stop on many springtime telescopic tours of the northern sky as it lies in the faint but well-groomed constellation Coma Berenices. This sharp color image reveals the galaxy's bulging central core dominated by light from a population of older, yellowish stars. The core is dramatically cut by obscuring dust lanes which lace NGC 4565's thin galactic plane. A large island universe similar to our own Milky Way Galaxy, NGC 4565 is only about 30 million light-years distant, but over 100,000 light-years in diameter. In fact, some consider NGC 4565 to be a prominent celestial masterpiece Messier missed.

Thumbnail image of picture found for this day. APOD: 2004 February 19 - McNeil's Nebula
Explanation: It was a clear, cold western Kentucky night on January 23rd as seasoned amateur astronomer Jay McNeil tried out his recently acquired 3-inch refracting telescope by imaging the area around a familiar object, the M78 reflection nebula in Orion. Days later while processing the images, he noted a substantial but totally unfamiliar nebulosity in the region! With a little help from his friends, his amazing discovery is now recognized as a newly visible reflection nebula surrounding a newborn star -- McNeil's Nebula. Pictured here at the center of this close-up, McNeil's Nebula with its illuminating young star at the tip, do not appear in images of the area before September 2003. The emergence of McNeil's Nebula is a rare event to witness and astronomers are eagerly following its development, but Orion will soon lie too close to the Sun in the sky, interrupting further observations for several months. The Orion nebula complex itself is around 1,500 light-years away. At that distance, the above image spans less than 10 light-years.

Thumbnail image of picture found for this day. APOD: 2004 January 23 - NGC 4631: The Whale Galaxy
Explanation: NGC 4631 is a big beautiful spiral galaxy seen edge-on only 25 million light-years away towards the small northern constellation Canes Venatici. This galaxy's slightly distorted wedge shape suggests to some a cosmic herring and to others the popular moniker of The Whale Galaxy. Either way, it is similar in size to our own Milky Way. In this gorgeous color image, the Whale's dark interstellar dust clouds, young bright blue star clusters, and purplish star forming regions are easy to spot. A companion galaxy, the small elliptical NGC 4627 appears above the Whale Galaxy. Out of view off the lower left corner of the picture lies another distorted galaxy, the hockey stick-shaped NGC 4656. The distortions and mingling trails of gas and dust detected at other wavelengths suggest that all three galaxies have had close encounters with each other in their past. The Whale Galaxy is also known to have spouted a halo of hot gas glowing in x-rays.

Thumbnail image of picture found for this day. APOD: 2003 September 1 - A Beautiful Trifid
Explanation: The beautiful Trifid Nebula (aka M20), a photogenic study in cosmic contrasts, lies about 5,000 light-years away toward the nebula rich constellation Sagittarius. A star forming region in the plane of our galaxy, the Trifid alone illustrates three basic types of astronomical nebulae; red emission nebulae dominated by light from hydrogen atoms, blue reflection nebulae produced by dust reflecting starlight, and dark absorption nebulae where dense dust clouds appear in silhouette. The bright emission nebula on the right, separated into three parts by obscuring dust lanes, lends the nebula its popular name. Many details are apparent in this gorgeous high-resolution image of the Trifid. For example, light-year long pillars and jets sculpted by newborn stars - visible here in the upper right-hand corner of the emission nebula - appear in Hubble Space Telescope close-up images of the region.

Thumbnail image of picture found for this day. APOD: 2003 June 17 - The Bubble Nebula from NOAO
Explanation: It's the bubble versus the cloud. NGC 7635, the Bubble Nebula, is being pushed out by the stellar wind of massive central star BD+602522. Next door, though, lives a giant molecular cloud, visible above to the lower right. At this place in space, an irresistible force meets an immovable object in an interesting way. The cloud is able to contain the expansion of the bubble gas, but gets blasted by the hot radiation from the bubble's central star. The radiation heats up dense regions of the molecular cloud causing it to glow. The Bubble Nebula, pictured above as a color negative to help bring up contrast, is about 10 light-years across and part of a much larger complex of stars and shells. The Bubble Nebula can be seen with a small telescope towards the constellation of Cassiopeia.

Thumbnail image of picture found for this day. APOD: 2003 June 14 - The Planetary Nebula Show
Explanation: What do the Owl, the Cat's Eye, the Ghost of Jupiter, and Saturn have in common? They're all planetary nebulae of course, glowing gaseous shrouds shed by dying sun-like stars as they run out of nuclear fuel. Beautiful to look at, the symmetric, planet-like shapes of these cosmic clouds, typically 1,000 times the size of our solar system, evoke their popular names. Flipping through digital pictures made by participants in the Kitt Peak National Observatory Visitor Center's Advanced Observing Program, astronomer Adam Block created this delightful animation. Ten different planetary nebula images are presented, each registered on the central star. In order, their catalog designations are NGC 1535, NGC 3242 (Ghost of Jupiter), NGC 6543 (Cat's Eye), NGC 7009 (Saturn Nebula), NGC 2438, NGC 6772, Abell 39, NGC 7139, NGC 6781, and M97 (Owl Nebula). This glorious final phase in the life of a star lasts only about 10,000 years.

Thumbnail image of picture found for this day. APOD: 2003 April 17 - M106 in Canes Venatici
Explanation: Close to the Great Bear (Ursa Major) and surrounded by the stars of the Hunting Dogs (Canes Venatici), this celestial nebula was discovered in 1781 by the metric French astronomer Pierre Mechain and later added to the catalog of his friend and colleague Charles Messier as M106. Modern deep telescopic views reveal it to be an island universe -- a spiral galaxy around 30 thousand light-years across located only about 21 million light-years beyond the stars of the Milky Way. Youthful blue star clusters and reddish stellar nurseries trace the striking spiral arms of M106. Seen so clearly in this beautiful image, the galaxy's bright core is also visible across the spectrum from radio to x-rays, making M106 a nearby example of the Seyfert class of active galaxies. The bright core of a Seyfert galaxy is believed to be powered by matter falling into a massive central black hole.

Thumbnail image of picture found for this day. APOD: 2002 December 5 - NGC 2359: Thor's Helmet
Explanation: NGC 2359 is a striking emission nebula with an impressive popular name - Thor's Helmet. Sure, its suggestive winged appearance might lead some to refer to it as the "duck nebula", but if you were a nebula which name would you choose? By any name NGC 2359 is a bubble-like nebula some 30 light-years across, blown by energetic winds from an extremely hot star seen near the center and classified as a Wolf-Rayet star. Wolf-Rayet stars are rare massive blue giants which develop stellar winds with speeds of millions of kilometers per hour. Interactions with a nearby large molecular cloud are thought to have contributed to this nebula's more complex shape and curved bow-shock structures. NGC 2359 is about 15,000 light-years distant toward the constellation Canis Major.

Thumbnail image of picture found for this day. APOD: 2002 July 30 - A Star Cluster in Motion
Explanation: Star clusters are a swarm of complex motions. The stars that compose globular clusters and many open clusters all orbit the cluster center, occasionally interacting, gravitationally, with a close-passing star. The orbits of stars around the cluster are typically not as circular as the orbits of planets in our solar system. Cluster stars frequently fall more directly toward the center and many times trace out unusual and complex loops. The vast space inside a cluster results in stars colliding only rarely. The above computer animation, derived from a type of computer code called an N-body simulation, shows 100 identical stars in a time-lapse movie where hundreds of years pass in one second.

Thumbnail image of picture found for this day. APOD: 2002 March 29 - NGC 4631: The Whale Galaxy
Explanation: NGC 4631 is a big beautiful spiral galaxy seen edge-on only 25 million light-years away towards the small northern constellation Canes Venatici. This galaxy's slightly distorted wedge shape suggests to some a cosmic herring and to others the popular moniker of The Whale Galaxy. Either way, it is similar in size to our own Milky Way. In this gorgeous color image, the Whale's dark interstellar dust clouds, young bright blue star clusters, and purplish star forming regions are easy to spot. A companion galaxy, the small elliptical NGC 4627 appears above the Whale Galaxy. Out of view off the lower left corner of the picture lies another distorted galaxy, the hockey stick-shaped NGC 4656. The distortions and mingling trails of gas and dust detected at other wavelengths suggest that all three galaxies have had close encounters with each other in their past. The Whale Galaxy is also known to have spouted a halo of hot gas glowing in x-rays.

Thumbnail image of picture found for this day. APOD: 2001 December 27 - The Incredible Expanding Crab
Explanation: The Crab Nebula is cataloged as M1, the first on Charles Messier's famous list of things which are not comets. In fact, the Crab is now known to be a supernova remnant, an expanding cloud of debris from the explosion of a massive star. The violent birth of the Crab was witnessed by astronomers in the year 1054. Roughly 10 light-years across today, the nebula is still expanding at a rate of over 1,000 kilometers per second. Flipping between two images made nearly 30 years apart, this animation clearly demonstrates the expansion. The smaller Crab was recorded as a photographic image made in 1973 using the Kitt Peak National Observatory 4-meter telescope in 1973. The expanded Crab was made this year with the Kitt Peak Visitor Center's 0.4-meter telescope and digital camera. Background stars were used to register the two images.

Thumbnail image of picture found for this day. APOD: 2001 October 3 - The Planetary Nebula Show
Explanation: What do the Owl, the Cat's Eye, the Ghost of Jupiter, and Saturn have in common? They're all planetary nebulae of course(!), glowing gaseous shrouds shed by dying sun-like stars as they run out of nuclear fuel. Beautiful to look at, the symmetric, planet-like shapes of these cosmic clouds, typically 1,000 times the size of our solar system, evoke their popular names. Flipping through digital pictures made by participants in the Kitt Peak National Observatory Visitor Center's Advanced Observing Program, astronomer Adam Block created this delightful animation. Ten different planetary nebula images are presented, each registered on the central star. In order, their catalog designations are NGC 1535, NGC 3242 (Ghost of Jupiter), NGC 6543 (Cat's Eye), NGC 7009 (Saturn Nebula), NGC 2438, NGC 6772, Abell 39, NGC 7139, NGC 6781, and M97 (Owl Nebula). This glorious final phase in the life of a star lasts only about 10,000 years.

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

Thumbnail image of picture found for this day. APOD: 2000 July 17 - Lightning on Earth
Explanation: Nobody knows what causes lightning. It is known that charges slowly separate in some clouds causing rapid electrical discharges (lightning), but how electrical charges get separated in clouds remains a topic of much research. Nevertheless, lightning bolts are common in clouds during rainstorms, and on average 6000 lightning bolts occur between clouds and the Earth every minute. Above, several lightning strokes were photographed behind Kitt Peak National Observatory in Arizona. Lightning has also been found on the planets Venus, Jupiter, Saturn, and Uranus. NASA launched the TRMM mission in 1997 that continues to measure rainfall and lightning on planet Earth.


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