juliensimon/nasa-apod huggingface dataset:
Inside the Coma Cluster of Galaxies (2008-06-16)

Almost every object in the above photograph is a galaxy.  The Coma Cluster of Galaxies pictured above is one of the densest clusters known - it contains thousands of galaxies.  Each of these galaxies houses billions of stars - just as our own Milky Way Galaxy does.  Although nearby when compared to most other clusters, light from the Coma Cluster still takes hundreds of millions of years to reach us.  In fact, the Coma Cluster is so big it takes light millions of years just to go from one side to the other!  The above mosaic of images of a small portion of Coma was taken in unprecedented detail by the Hubble Space Telescope to investigate how galaxies in rich clusters form and evolve.  Most galaxies in Coma and other clusters are ellipticals, although some imaged here are clearly spirals.   The spiral galaxy on the upper left of the above image can also be found as one of the bluer galaxies on the upper left of this wider field image. In the background thousands of unrelated galaxies are visible far across the universe.    Note: APOD started thirteen years ago today.
Eta Carinae and the Homunculus Nebula (2008-06-17)

How did the star Eta Carinae create this unusual nebula?  No one knows for sure.  About 165 years ago, the southern star Eta Carinae mysteriously became the second brightest star in the night sky.   In 20 years, after ejecting more mass than our Sun, Eta Car unexpectedly faded.  This outburst appears to have created the Homunculus Nebula, pictured above in a composite image from the Hubble Space Telescope taken last decade.  Visible in the above image center is purple-tinted light reflected from the violent star Eta Carinae itself.  Surrounding this star are expanding lobes of gas laced with filaments of dark dust.  Jets bisect the lobes emanating from the central star. Surrounding these lobes are red-tinted debris captured only by its glow in a narrow band of red light.  This debris is expanding most quickly of all, and includes streaming whiskers and bow shocks caused by collisions with previously existing material. Eta Car still undergoes unexpected outbursts, and its high mass and volatility make it a candidate to explode in a spectacular supernova sometime in the next few million years.
Pyramid Ice Crystal Halos Over Finland (2008-06-18)

What if the atmosphere above you became one gigantic lens? This actually happens when a nearly transparent sheet of pyramid shaped ice crystals falls from the sky in a common orientation.  These ice-crystals act together like millions of miniature ice mirrors, with external and internal reflections from different faces creating arcs and halos of different radii.  An amazing display of pyramid ice crystal halos was captured on June 5 above Tampere, Finland.  Visible above are very unusual sun halos of 9, 18, 20, 23, and 24 degrees. In contrast, thin and flat falling ice crystals will produce a halo of 22 degrees only.  The high clouds containing the ice crystals are faintly visible, as are some sundogs.  The usual Sun image was covered behind a light post, and the above image was significantly digitally sharpened.  It is not currently known how large areas of nearly uniform pyramidal ice crystals form.

Image credit: Kari Nyman
The Star Streams of NGC 5907 (2008-06-19)

Grand tidal streams of stars seem to surround galaxy NGC 5907. The arcing structures form tenuous loops extending more than 150,000 light-years from the narrow, edge-on spiral, also known as the Splinter or Knife Edge Galaxy. Recorded only in very deep exposures, the streams likely represent the ghostly trail of a dwarf galaxy -- debris left along the orbit of a smaller satellite galaxy that was gradually torn apart and merged with NGC 5907 over four billion years ago. Ultimately this remarkable discovery image, from a small robotic observatory in New Mexico, supports the cosmological scenario in which large spiral galaxies, including our own Milky Way, were formed by the accretion of smaller ones. NGC 5907 lies about 40 million light-years distant in the northern constellation Draco.

Image credit: R Jay Gabany
(Blackbird Observatory) - collaboration;
D.Martínez-Delgado(IAC,
MPIA),

J.Peñarrubia
(U.Victoria)
I. Trujillo (IAC)
S.Majewski (U.Virginia),
M.Pohlen (Cardiff),
Solstice Moonrise, Cape Sounion (2008-06-20)

Today's solstice marks the northernmost point of the Sun's annual motion through planet Earth's sky and the astronomical beginning of the northern hemisphere's summer. But only two days ago, the Full Moon nearest the solstice rose close to the ecliptic plane opposite the Sun, near its southernmost point for the year. Astronomer Anthony Ayiomamitis recorded this dramatic picture of the solstice Full Moon rising above Cape Sounion, Greece. The twenty-four hundred year old Temple of Poseidon lies in the foreground, also visible to sailors on the Aegean Sea. In this well-planned single exposure, a telescopic lens makes the Moon loom large, but even without optical aid casual skygazers often find the Full Moon looking astonishingly large when seen near the horizon. That powerful visual effect is known as the Moon Illusion.

Image credit: Anthony Ayiomamitis
Vanishing Act (2008-06-21)

Compare these two close-up pictures taken on sol 20 (left) and sol 24 of a trench dug in the Martian surface by NASA's Phoenix Lander. Those sols of the Phoenix Mission (a sol is a Martian day), correspond to June 15 and 18 on planet Earth. Light-colored, dice-sized chunks, visible in the lower left shadow region of the trench in the sol 20 image have vanished by sol 24 -- a strong indication that the chunks were ice uncovered by digging the shallow trench. The vanishing act likely demonstrates the sublimation of ice in the trench, a process similar to evaporation, in which the ice went directly from solid to gas after it was exposed to sunlight and the thin, dry Martian atmosphere.
Barred Spiral Galaxy NGC 1300 (2008-06-22)

Big, beautiful, barred spiral galaxy NGC 1300 lies some 70 million light-years away on the banks of the constellation Eridanus. This Hubble Space Telescope composite view of the gorgeous island universe is one of the largest Hubble images ever made of a complete galaxy. NGC 1300 spans over 100,000 light-years and the Hubble image reveals striking details of the galaxy's dominant central bar and majestic spiral arms. In fact, on close inspection the nucleus of this classic barred spiral itself shows a remarkable region of spiral structure about 3,000 light-years across. Unlike other spiral galaxies, including our own Milky Way, NGC 1300 is not presently known to have a massive central black hole.
The International Space Station Expands Again (2008-06-23)

The developing International Space Station (ISS) has changed its appearance again.  Earlier this month, the Space Shuttle orbiter Discovery visited the ISS and added components that included Japan's Kibo Science Laboratory. The entire array of expansive solar panels is visible in this picture taken by the Discovery Crew after leaving the ISS to return to Earth.  The world's foremost space outpost can be seen developing over the past several years by comparing the above image to past images.  Also visible above are many different types of modules, a robotic arm, another impressive set of  solar panels, and a supply ship.  Construction began on the ISS in 1998.
Ithaca Chasma: The Great Rift on Saturn's Tethys (2008-06-24)

What created the Great Rift on Saturn's moon Tethys? No one is sure.  More formally named Ithaca Chasma, the long canyon running across the right of the above image extends about 2,000 kilometers long and spreads as much as 100 kilometers wide. The above image was captured by the Saturn-orbiting robotic Cassini spacecraft as it zoomed by the icy moon last month.  Hypotheses for the formation of Ithaca Chasma include cracking of Tethy's outer crust as the moon cooled long ago, and that somehow the rift is related to the huge Great Basin impact crater named Odysseus, visible elsewhere on the unusual moon.  Cassini has now been orbiting Saturn for about four years and is scheduled to continue to probe and photograph Saturn for at least two more years.
What is Hanny's Voorwerp? (2008-06-25)

What is that green thing? A volunteer sky enthusiast surfing through online Galaxy Zoo images has discovered something really strange. The mystery object is unusually green, not of any clear galaxy type, and situated below relatively normal looking spiral galaxy IC 2497. Dutch schoolteacher Hanny van Arkel, discovered the  strange green "voorwerp" (Dutch for "object") last year. The Galaxy Zoo project encourages sky enthusiasts to browse through SDSS images and classify galaxy types.  Now known popularly as Hanny's Voorwerp, subsequent observations have shown that the mysterious green blob has the same distance as neighboring galaxy IC 2497.  Research is ongoing, but one leading hypothesis holds that Hanny's Voorwerp is a small galaxy that acts like a large reflection nebula, showing the reflected light of a bright quasar event that was visible in the center of IC 2497 about 100,000 years ago.   Pictured above, Hanny's Voorwerp was imaged recently by the 2.5-meter Isaac Newton Telescope in the Canary Islands by Dan Smith, Peter Herbert and Chris Lintott (Univ. Hertfordshire). Other collaboration members include Matt Jarvis, Kevin Schawinski, and William Keel.
M27: Not A Comet (2008-06-26)

Born on June 26th in 1730, astronomer Charles Messier scanned 18th century French skies for comets. To avoid confusion and aid his comet hunting, he diligently recorded this object as number 27 on his list of things which are definitely not comets. In fact, 21st century astronomers would classify it as a Planetary Nebula, but it's not a planet either, even though it may appear round and planet-like in a small telescope. Messier 27 (M27) is now known to be an excellent example of a gaseous emission nebula created as a sun-like star runs out of nuclear fuel in its core. The nebula forms as the star's outer layers are expelled into space, with a visible glow generated by atoms excited by the dying star's intense but invisible ultraviolet light. Known by the popular name of the Dumbbell Nebula, the beautifully symmetric interstellar gas cloud is over 2.5 light-years across and about 1,200 light-years away in the constellation Vulpecula. This impressive color composite highlights subtle jet features in the nebula. It was recorded with a robotic telescope sited in Hawaii using narrow band filters sensitive to emission from oxygen atoms (shown in green) and hydrogen atoms. The hydrogen emission is seen as red (H-alpha) and fainter bluish hues (H-beta).

Image credit: Nik Szymanek
M81: Feeding a Black Hole (2008-06-27)

This impressive color composite shows spiral galaxy M81 across the electromagnetic spectrum. It combines X-ray data (blue) from the Chandra Observatory, infrared data (pink) from the Spitzer Space Telescope, and an ultraviolet image (purple) from the GALEX satellite, with a visible light (green) Hubble image. The inset highlights X-rays from some of M81's black holes, including black holes in binary star systems with about 10 times the mass of the sun, as well as the central, supermassive black hole of over 70 million solar masses. Comparing computer models of the giant black hole's energy output to the multiwavelength data suggests that feeding that monster is relatively simple -- energy and radiation is generated as material in the central region swirls inwards forming an accretion disk. In fact, the process otherwise appears to be just like the accretion process feeding M81's stellar mass black holes, even though the central black hole is millions of times more massive. M81 itself is about 70,000 light-years across and only 12 million light-years away in the northern constellation Ursa Major.
Fireball at Ayers Rock (2008-06-28)

A weekend trip for astrophotography in central Australia can result in gorgeous skyscapes. In this example recorded in March of 2006, the center of our Milky Way Galaxy rises over planet Earth's horizon and the large sandstone formation called Uluru, also known as Ayers Rock. After setting up two cameras to automatically image this celestial scene in a series of exposures, one through a wide-angle and the other through a telephoto lens, photographer Joseph Brimacombe briefly turned his back to set up other equipment. To his surprise, the ground around him suddenly lit up with the brilliant flash of a fireball meteor. To his delight, both cameras captured the bright meteor streak. Highlighted in the telephoto view (inset), the fireball trail shines through cloud banks, just left of Ayers Rock.

Image credit: Joseph Brimacombe
Shadow of a Martian Robot (2008-06-29)

What if you saw your shadow on Mars and it wasn't human?  Then you might be the  Opportunity rover currently exploring Mars.  Opportunity and sister robot Spirit have been probing the red planet since early 2004, finding evidence of ancient water, and sending breathtaking images across the inner Solar System.  Pictured above, Opportunity looks opposite the Sun into Endurance Crater and sees its own  shadow.  Two wheels are visible on the lower left and right, while the floor and walls of the unusual crater are visible in the background.  Opportunity and Spirit have now spent over four years exploring the red world, find new clues into the wet ancient past of our Solar System's second most habitable planet.
In the Center of the Trifid Nebula (2008-06-30)

Clouds of glowing gas mingle with dust lanes in the Trifid Nebula, a star forming region toward the constellation of Sagittarius.  In the center, the three prominent dust lanes that give the Trifid its name all come together. Mountains of opaque dust appear on the right, while other dark filaments of dust are visible threaded throughout the nebula.  A single massive star visible near the center causes much of the Trifid's glow.  The Trifid, also known as M20, is only about 300,000 years old, making it among the youngest emission nebulae known.  The nebula lies about 9,000 light years away and the part pictured here spans about 10 light years.  This image was created with the 0.8-meter IAC80 telescope on the Canary Islands of Spain.

Image credit: Daniel Lopez
(Observatorio del
Teide)
Pickering's Triangle from Kitt Peak (2008-07-01)

Wisps like this are all that remain visible of a Milky Way star.  About 7,500 years ago that star exploded in a supernova leaving the Veil Nebula, also known as the Cygnus Loop. At the time, the expanding cloud was likely as bright as a crescent Moon, remaining visible for weeks to people living at the dawn of recorded history.  Today, the resulting supernova remnant has faded and is now visible only through a small telescope directed toward the constellation of Cygnus. The remaining Veil Nebula is physically huge, however, and even though it lies about 1,400 light-years distant, it covers over five times the size of the full Moon.  In images of the complete Veil Nebula, studious readers should be able to identify the Pickering's Triangle component pictured above, a component named for a famous astronomer and the wisp's approximate shape.  The above image is a mosaic from the 4-meter Mayall telescope at the Kitt Peak National Observatory located in Arizona, USA.

Image credit: T. Rector
Night Shinings (2008-07-02)

In the early morning hours of June 30th, ghostly clouds hovered in the east in this view of near dawn skies over western France. The noctilucent or night-shining clouds lie near the edge of space, reflecting sunlight from about 80 kilometers above Earth's surface. Usually spotted above the poles in summer, they are now seen with increasing frequency farther from the poles, in this case extending to the photographer's latitude of about 48 degrees north. The trend could be a telltale sign of global changes in the atmosphere. Another 400,000 kilometers away, the Moon's sunlit crescent shines brightly, its night side illuminated by Earthshine. Of course, as a bonus for early risers June's old crescent Moon was followed closely across the sky by the lovely Pleiades star cluster, surrounded by cosmic dust clouds and shining from a mere 400 light-years away.

Image credit: PixHeaven.net
Hoodoo Sky (2008-07-03)

The strange-looking rock formations in the foreground of this skyscape are called hoodoos. Towers of weathered, eroded sedimentary rock, hoodoos are found in arid regions of planet Earth and are particularly abundant in an area known as Bryce Canyon National Park in southern Utah, USA. The more familiar night sky pictured here was recorded early Monday morning and includes bright star Capella, alpha star of the constellation Auriga, left of center. On the far right, a very over exposed crescent Moon dominates the sky in close conjunction with the sister stars of the Pleiades cluster. The curious shapes of the two tall, illuminated hoodoos suggest their popular monikers; Thor's Hammer (right) and The Temple of Osiris.

Image credit: Wally Pacholka
SN 1006 Supernova Remnant (2008-07-04)

A new star, likely the brightest supernova in recorded human history, lit up planet Earth's sky in the year 1006 AD. The expanding debris cloud from the stellar explosion, found in the southerly constellation of Lupus, still puts on a cosmic light show across the electromagnetic spectrum. In fact, this composite view includes X-ray data in blue from the Chandra Observatory, optical data in yellowish hues, and radio image data in red. Now known as the SN 1006 supernova remnant, the debris cloud appears to be about 60 light-years across and is understood to represent the remains of a white dwarf star. Part of a binary star system, the compact white dwarf gradually captured material from its companion star. The buildup in mass finally triggered a thermonuclear explosion that destroyed the dwarf star. Because the distance to the supernova remnant is about 7,000 light-years, that explosion actually happened 7,000 years before the light reached Earth in 1006. Shockwaves in the remnant accelerate particles to extreme energies and are thought to be a source of the mysterious cosmic rays.
Comet Between Fireworks and Lightning (2008-07-05)

Sometimes the sky itself is the best show in town.  In January 2007, people from Perth, Australia  gathered on a local beach to watch a sky light up with delights near and far.  Nearby, fireworks exploded as part of Australia Day celebrations.  On the far right, lightning from a thunderstorm flashed in the distance.  Near the image center, though, seen through clouds, was the most unusual sight of all: Comet McNaught.  The photogenic comet was so bright that it even remained visible though the din of Earthly flashes.  Comet McNaught has now returned to the outer Solar System and is now only visible with a large telescope. The above image is actually a three photograph panorama digitally processed to reduce red reflections from the exploding firework.

Image credit: Antti Kemppainen
Apollo 17: VIP Site Anaglyph (2008-07-06)

Get out your red/blue glasses and check out this stereo scene from Taurus-Littrow valley on the Moon! The color anaglyph features a detailed 3D view of Apollo 17's Lunar Rover in the foreground -- behind it lies the Lunar Module and distant lunar hills. Because the world was  going to be able to watch the Lunar Module's ascent stage liftoff via the rover's TV camera, this parking place was also known as the VIP Site. In December of 1972, Apollo 17 astronauts Eugene Cernan and Harrison Schmitt spent about 75 hours on the Moon, while colleague Ronald Evans orbited overhead. The crew returned with 110 kilograms of rock and soil samples, more than from any of the other lunar landing sites. Cernan and Schmitt are still the last to walk (or drive) on the Moon.
The Southern Cross in a Southern Sky (2008-07-07)

This breathtaking patch of sky would be above you were you to stand at the South Pole of the Earth.  On the upper left of this image are the four stars that mark the boundaries of the famous Southern Cross.  At the top of this constellation, also known as The Crux, is the orange star Gamma Crucis.  The band of stars, dust, and gas crossing the middle of the photograph is part our Milky Way Galaxy.  Just below the Southern Cross on the far left is the dark Coal Sack Nebula, and the bright nebula on the far right is the Carina Nebula.  The Southern Cross is such a famous constellation that it is depicted on the national flag of Australia.

Image credit: Yuri Beletsky
(ESO)
In the Heart of the Virgo Cluster (2008-07-08)

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

Image credit: G�nter Kerschhuber 
(Gahberg Observatory)
Planets and Fire by Moonlight (2008-07-09)

On July 5th, wandering planets, bright stars, and a young crescent Moon graced western skies after sunset. Arrayed along the solar system's ecliptic plane, the three celestial beacons forming this skyscape's eye-catching line-up with the Moon are Saturn (upper left), then Mars, and finally Regulus, alpha star of the constellation Leo. Of course planet Earth itself lies in the foreground, a scene dominated by the city lights of Santa Barbara, California. The smoky haze hanging over the city is from threatening wild fires still burning along the hill at the right. On Thursday evening, Saturn and Mars can be seen in a much closer pairing or conjunction, separated by only about 3/4 degree on the sky.

Image credit: Dmitrii Zagorodnov
Enhanced Color Caloris (2008-07-10)

The sprawling Caloris basin on Mercury is one of the solar system's largest impact basins. Created during the early history of the solar system by the impact of a large asteroid-sized body, the basin spans about 1,500 kilometers and is seen in yellowish hues in this enhanced color mosaic. The image data is from the January 14th flyby of the MESSENGER spacecraft, captured with the MDIS instrument. Orange splotches around the basin's perimeter are now thought to be volcanic vents, new evidence that Mercury's smooth plains are indeed lava flows. Other discoveries at Mercury by NASA's MESSENGER mission include evidence that Mercury, like planet Earth, has a global magnetic field generated by a dynamo process in its large core, and that Mercury's surface has contracted significantly as its core cooled.