juliensimon/nasa-apod huggingface dataset:
The Gamma Ray Sky (2002-01-12)

What if you could see gamma rays? If you could, the sky would seem to be filled with a shimmering high-energy glow from the most exotic and mysterious objects in the Universe. In the early 1990s NASA's orbiting Compton Observatory, produced this premier vista of the entire sky in gamma rays, photons with more than 40 million times the energy of visible light. The diffuse gamma-ray glow from the plane of our Milky Way Galaxy runs horizontally through the false-color image. The brightest spots in the galactic plane (right of center) are pulsars, spinning magnetized neutron stars formed in the violent crucibles of stellar explosions. Above and below the plane, quasars, believed to be powered by supermassive black holes, produce gamma-ray beacons at the edges of the universe. The nature of many of the fainter sources remains unknown.
Hypatia of Alexandria (2002-01-13)

Sixteen hundred years ago, Hypatia became one of the world's leading scholars in mathematics and astronomy.  Hypatia's legendary knowledge, modesty, and public speaking ability flourished during the era of the Great Library of Alexandria. Hypatia is credited with contributions to geometry and astrometry, and she is thought instrumental in the development of the sky-measuring astrolabe. "Reserve your right to think, for even to think wrongly is better than not to think at all," Hypatia is credited with saying.  "To teach superstitions as truth is a most terrible thing."
Sun Halo at Winter Solstice (2002-01-14)

Sometimes it looks like the Sun is being viewed through a large lens.  In the above case, however, there are actually millions of lenses: ice crystals.  As water freezes in the upper atmosphere, small, flat, six-sided, ice crystals might be formed.  As these crystals flutter to the ground, much time is spent with their faces flat, parallel to the ground.  An observer may pass through the same plane as many of the falling ice crystals near sunrise or sunset.  During this alignment, each crystal can act like a miniature lens, refracting sunlight into our view and creating phenomena like parhelia, the technical term for sundogs.  The above image was taken in the morning of the 2000 Winter Solstice near Ames, Iowa, USA.  Visible in the image center is the Sun, while two bright sundogs glow prominently from both the left and the right.  Also visible behind neighborhood houses and trees are the 22 degree halo, three sun pillars, and the upper tangent arc, all created by sunlight reflecting off of atmospheric ice crystals.

Image credit: Philip Appleton 
(SIRTF Science Center), 
Caltech
Red Auroral Corona (2002-01-15)

Few auroras show this level of detail.  This unusual display of an auroral corona occurred on Earth three days after an unusual solar event -- the fifth most powerful explosion yet recorded on the Sun.  An X14-class solar flare on April 15 sent a tremendous Coronal Mass Ejection (CME) into the Solar System.  This CME did not directly impact the Earth.  The Solar-System wide shock wave it created probably did, however, causing a G3-class geomagnetic storm and a night filled with colorful auroras across much of northern North America.  The unusual red color of this Michigan aurora is caused by solar ions striking oxygen molecules 300 kilometers high in Earth's atmosphere.  More typical green auroras are caused by oxygen recombining only 100 kilometers high.

Image credit: Duane Clausen
Abell 2597's Cosmic Cavities (2002-01-16)

Typical of large galaxy clusters billions of light-years away, Abell 2597 features hundreds of galaxies embedded in a cloud of multimillion degree gas which glows in x-rays. This Chandra Observatory x-ray image shows the hot gas in this cluster's central regions and also reveals two large dark cavities within the x-ray glow; one below and right of center, the other above and left. Not a comment on dental health, Abell 2597's cavities are about 60,000 light-years across. They are thought to be remnants of a 100 million year old explosion originating from a supermassive black hole at the cluster's core. But the dim ghost cavities are not completely empty or they would have collapsed long ago. Instead they are likely filled with hotter gas, high energy particles, and magnetic fields and are moving away from the cluster center, like bubbles rising in champagne. Over the life of a galaxy cluster such explosions may happen over and over, creating a series of cavities which transport magnetic fields away from the cluster center. In fact, radio observations suggest another explosion has since occurred in the center of Abell 2597.
Pick a Galaxy, Any Galaxy (2002-01-17)

Pick a galaxy, any galaxy. In the top panel you can choose from a myriad of distant galaxies revealed in a deep Hubble Space Telescope image of a narrow slice of the cosmos toward the constellation Hercules. If you picked the distorted reddish galaxy indicated by the yellow box, then you've chosen one a team of infrared astronomers has recently placed at a distance of 9 billion light-years. Classified as an ERO (Extremely Red Object), this galaxy is from a time when the Universe was only one third its present age. Along the bottom panel, this galaxy's appearance in filters ranging from visible to infrared wavelengths (left to right) is presented as a series of negative images. The brightness of the galaxy in the infrared compared to the visible suggests that light from intense star formation activity, reddened by dust clouds within the galaxy itself, is responsible for the extremely red color. Astronomers estimate that this galaxy has around 100 billion stars and may in fact be a very distant mirror -- an analog of our own Milky Way Galaxy in its formative years.
Saturn and Vesta in Taurus (2002-01-18)

Last November, while skygazing toward the constellation Taurus, astrophotographer Joe Orman arranged this time exposure to include the lovely Hyades and Pleiades star clusters in the field of his telephoto lens. A distance of 400 light-years for the close-knit Pleiades and 150 light-years for the V-shaped Hyades puts these clusters in the general galactic neighborhood of the Sun. Punctuating the Hyades' appearance, bright yellow Aldebaran, 60 light-years away, is not actually a member of the cluster, but it is Taurus' brightest star. Above Aldebaran a yellower, even brighter Saturn is is seen about 1.2 light-hours from our fair planet. Last and least massive, one of the faint specks below Aldebaran is main-belt asteroid Vesta, a mere 13 light-minutes away. Still cruising through Taurus, Vesta is steadily approaching a close alignment or conjunction with Saturn on March 19. Need a program to follow the players? Click on the image for a labeled version.

Image credit: Joe Orman
Stars Without Galaxies (2002-01-19)

Galaxies are made up of stars, but are all stars found within galaxies? Using the Hubble Space Telescope, researchers exploring the Virgo Cluster of galaxies have found about 600 red giant stars adrift in intergalactic space. Above is an artist's vision of the sky from a hypothetical planet of such a lonely sun. The night sky on a world orbiting an intergalactic star would be a stark contrast to Earth's - which features a spectacle of stars, all members of our own Milky Way Galaxy. As suggested by the illustration, a setting red sun would leave behind a dark sky flecked only with faint, fuzzy, apparitions of Virgo Cluster galaxies. Possibly ejected from their home galaxies during galaxy-galaxy collisions, these isolated suns may well represent part of a large, previously unseen stellar population, filling the space between Virgo Cluster galaxies.
Callisto Full Face (2002-01-20)

Callisto's surface shows its age.  While probably formed at the same time as Io, the difference between the surfaces of these two moons of Jupiter could hardly be greater.  Io's surface is young, shows practically no impact craters, and is continually being repaved by the lava exploding from its many large volcanoes.  Callisto's surface is old, shows the highest density of impact craters in the Solar System, and harbors no volcanoes or even any large mountains.  Callisto's surface is one large ice-field, laced with cracks and craters from billions of years of collisions with interplanetary debris. The above image was taken in 2001 May and is, so far, the only complete global color image taken by the Jupiter-orbiting Galileo spacecraft.
Volcano and Aurora in Iceland (2002-01-21)

Sometimes both heaven and Earth erupt.  In Iceland in 1991, the volcano Hekla erupted at the same time that auroras were visible overhead.  Hekla, one of the most famous volcanoes in the world, has erupted at least 20 times over the past millennium, sometimes causing great destruction.  The last eruption occurred only two years ago but caused only minor damage.  The green auroral band occurred fortuitously about 100 kilometers above the erupting lava.  Is Earth the Solar System's only planet with both auroras and volcanos?

Image credit: Sigurdur H. Stefnisson
Neutron Bounce Quantized in Earth Gravity (2002-01-22)

For the first time, Earth's gravity has been used to isolate quantum energy levels of the neutron.  The effect may be used in the future to test for slightly different effects of gravity on neutrally charged particles of different mass. In an experiment by Valery Nesvizhevsky and colleagues at the Laue-Langevin Institute, carefully dropped neutrons were seen to appear at only discrete heights.  The effect is also of interest because it involves the intersection of two branches of physics that remain formally separate. A theory known as Quantum Mechanics tells us about how the universe works on the smallest scales, while Einstein's General Theory of Relativity tells us about how gravity and the universe works on the largest scales.  The effect does not in itself, however, imply attributes of a possible quantum field nature of gravity. Pictured above is a false-color surface that might be created by the evolution of a one-dimensional string.  By describing fundamental particles as tiny strings, many physicists are working toward the creation of a truly quantum theory of gravity.
Local Group Galaxy NGC 6822 (2002-01-23)

Nearby galaxy NGC 6822 is irregular in several ways.  First, the galaxy's star distribution merits a formal classification of dwarf irregular, and from our vantage-point the small galaxy appears nearly rectangular.  What strikes astronomers as more peculiar, however, is NGC 6822's unusually high abundance of HII regions, locales of ionized hydrogen that surround young stars.  Large HII regions, also known as emission nebulas, are visible surrounding the small galaxy, particularly toward the upper right.  Toward the lower left are bright stars that are loosely grouped into an arm. Pictured above, NGC 6822, also known as Barnard's Galaxy, is located only about 1.5 million light years away and so is a member of our Local Group of Galaxies.  The galaxy, home to famous nebulas including Hubble V, is visible with a small telescope toward the constellation of Sagittarius.

Image credit: Local Group Galaxies Survey Team
Ski Enceladus (2002-01-24)

A small inner moon of Saturn, Enceladus is only about 500 kilometers in diameter. But the cold, distant world does reflect over 90 percent of the sunlight it receives, giving its surface about the same reflectivity as new-fallen snow. Seen here in a mosaic of Voyager 2 images from 1981, Enceladus shows a variety of surface features and very few impact craters - indicating that it is an active world even though this ice moon should have completely cooled off long ago. In fact the fresh, resurfaced appearance of Enceladus suggests that an internal mechanism, perhaps driven by tidal pumping, generates heat and supplies liquid water to geysers or water volcanos. Since Enceladus orbits within the tenuous outer E ring of Saturn, the moon's surface may be kept snow-bright as it is continuously bombarded with icy ring particles. Eruptions on Enceladus itself would in turn supply material to the E ring. Interplanetary ski bums take note: tiny Enceladus has only about one hundredth the surface gravity of planet Earth.
The Spiral Arms of NGC 4622 (2002-01-25)

While stirring a morning cup of coffee and thinking cosmic thoughts many astronomers would glance at this Hubble Space Telescope image of spiral galaxy NGC 4622 and assume that the galaxy was rotating counterclockwise in the picture. One hundred million light-years away in the constellation Centaurus, NGC 4622's gorgeous outer spiral arms, traced by bright bluish star clusters and dark dust lanes, should be winding up like ... well, like swirls in a cup of coffee. But a closer look at this galaxy reveals that a pronounced inner spiral arm winds in the opposite direction. So which way is this galaxy rotating? Recent evidence combining ground-based spectroscopy and the sharp Hubble image data surprisingly indicates that the galaxy is likely rotating clockwise in the picture, its outer spiral arms opening outward in the direction of rotation. There are further indications that a past collision with a smaller companion galaxy has contributed to this bizarre rotational arrangement of spiral arms, essentially unique among known large spiral galaxies, in NGC 4622.
Shuttle Engine Blast (2002-01-26)

The Space Shuttle Discovery's orbital maneuvering system (OMS) engine firing produced this dramatic flare as it cruised "upside down" in low Earth orbit. Discovery was named for a ship commanded by Captain James Cook RN, the 18th Century English astronomer and navigator. Cook's voyages of discovery established new standards in scientific exploration and brought extensive knowledge of the Pacific regions, including Australia, New Zealand, and the Hawaiian Island archipelago to Europeans. The Space Shuttle Endeavor, also named after one of Cook's ships, is the newest of NASA's four-orbiter shuttle fleet.
Earth Rise (2002-01-27)

During 1968, the Apollo 8 crew flew from the Earth to the Moon and back. The crew, consisting of Frank Borman, James Lovell, and William Anders, were launched atop a Saturn V rocket on December 21, circled the Moon ten times in their command module, and landed back on Earth on December 27.  The Apollo 8 mission's impressive list of firsts includes: the first humans to journey to the Earth's Moon, the first manned flight using the Saturn V, and the first to photograph the Earth from deep space.  The famous picture above, showing the Earth rising above the Moon's limb as seen from lunar orbit, was a marvelous gift to the world.
An Apollo 17 Panorama (2002-01-28)

What would it be like to stand on the surface of another world, to look all around you, and to try to figure out how this world got there?  To get an idea, scroll right.  In 1972 during the Apollo 17 mission, astronauts Harrison Schmitt and Eugene Cernan found out first hand.  In this case, the world was Earth's own Moon.  In one of the more famous panoramas taken on the Moon, the magnificent desolation of the barren Moon is apparent.  Visible are rocks, hills, craters, the lunar rover, and astronaut Schmitt preparing to take a soil sample.  A few days after this image was taken, humanity left the Moon and has yet to return.  An interactive version of the above image can be found here.
The Southern Sky in Warm Hydrogen (2002-01-29)

A robotic telescope with red sunglasses in Chile has been photographing the entire southern sky for years.  The result, shown above, is the most complete sky map of the most common visible light emitted from the most abundant element in our Galaxy: hydrogen.  A very specific red color emitted by warm ionized hydrogen was observed.  Although spectacular emission nebulas glow brightly in this type of red light (H-alpha), a diffuse amount of warm hydrogen is spread throughout our Galaxy and its glow nicely indicates not only where darker hydrogen and other gasses may be located, but also the sometimes- complex history of interstellar gas.  Gas tracking the plane of our Galaxy runs across the center, and huge gas clouds, some of which are the expanding shells of long dead stars, are also visible.  The above map was derived from the Southern H-Alpha Sky Survey Atlas (SHASSA) and shows that our entire Galaxy is one big emission nebula, albeit at a quite faint level.
Moonrise Over Seattle (2002-01-30)

Is the Moon larger when near the horizon?  No -- as shown above, the Moon appears to be very nearly the same size no matter its location on the sky.  Oddly, the cause or causes for the common Moon Illusion are still being debated. Two leading explanations both hinge on the illusion that foreground objects make a horizon Moon seem farther in the distance.  The historically most popular explanation then holds that the mind interprets more distant objects as wider, while a more recent explanation adds that the distance illusion may actually make the eye focus differently.  Either way, the angular diameter of the Moon is always about 0.5 degrees.  In the above time-lapse sequence taken near the end of last year, the Moon was briefly re-imaged every 2.5 minutes, with the last exposure of longer duration to bring up a magnificent panorama of the city of Seattle.

Image credit: Shay Stephens
EUVE Sky Map (2002-01-31)

The stars beyond the Sun and the distant galaxies should be undetectable at extreme ultraviolet wavelengths. At least that was the conventional wisdom when it was first realized that the space between the stars is filled with hydrogen, a strong absorber of extreme ultraviolet light. But this idea was seriously questioned as it became apparent that interstellar hydrogen was patchy in its distribution and the quest to explore the extreme ultraviolet sky culminated in 1992 with the launch of the EUVE (Extreme UltraViolet Explorer) spacecraft. An all-sky map, based on the satellite's first six months of operation, appears above showing brightness variations in the EUV sky in false color (north is up). EUVE's scanning by orbit gives the picture a striped look while other instrument artifacts are seen as crosses and data gaps are dark. Multiple images of the Moon combine to form the short, bright dashed lines wandering along the middle of the picture. The belt stars of the familiar constellation Orion are fairly easy to make out left of picture center. EUVE's science operations ceased in 2001 but it ultimately detected nearly 1,000 celestial objects, including over three dozen outside our own galaxy. EUVE's voyage of discovery is now complete, and the spacecraft re-entered the Earth's atmosphere yesterday at approximately 11:15 p.m. EST.
Balloon TIGER (2002-02-01)

Where does a two-ton tiger hang out? Well, in this case the Trans-Iron Galactic Element Recorder (TIGER) experiment hangs from a mobile crane on the far left in this panorama photo recorded last December near McMurdo Station, Antarctica. The helium-filled balloon which carried TIGER aloft for a record setting 31+ days is stretched out far to the right (scroll right) against the background of majestic Mt. Erebus, the southernmost active volcano in the world. While cruising with its two-ton payload above 100,000 feet, the scientific balloon's fully inflated internal volume was roughly the same as the Louisiana Superdome, but its walls were as thin as shrinkwrap. TIGER was designed to detect the unexplained galactic cosmic rays -- atomic nuclei moving at near light-speed which impinge on the Earth from outside our Solar System. By making the first sensitive measurements of cosmic rays with atomic numbers between 26 (Iron) and 40 (Zirconium), TIGER investigators will seek to identify the type of astrophysical environments which could be sources of the galactic cosmic-ray material and possible ways in which the nuclei are accelerated to such high speeds.
Centaurus A: The Galaxy Deep Inside (2002-02-02)

Deep inside Centaurus A, the closest active galaxy to Earth, lies ... another galaxy! Cen A is a giant elliptical galaxy a mere 10 million light-years distant with a central jumble of stars, dust, and gas that probably hides a massive black hole. This composite combines an optical picture of Cen A with dark lines tracing lobes of radio emission and an infrared image from the ISO satellite (in red). The ISO data maps out the dust in what appears to be a barred spiral galaxy about the size of the prominent nearby spiral M33. The discoverers believe that the giant elliptical's gravity helps this barred spiral galaxy maintain its shape. In turn, material funneled along the spiral's bar fuels the central black hole which powers the elliptical's radio lobes. This apparently intimate association between two distinct and dissimilar galaxies suggests a truly cosmic symbiotic relationship.
The Coma Cluster of Galaxies (2002-02-03)

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!  Most galaxies in Coma and other clusters are ellipticals, while most galaxies outside of clusters are spirals.  The nature of Coma's X-ray emission is still being investigated.

Image credit: O. Lopez-Cruz  
(INAOEP) et al., 
AURA,
NOAO, 
NSF
Comet LINEAR (WM1) Shines in the South (2002-02-04)

A new comet has brightened unexpectedly and is currently visible to unaided observers of southern skies.  Comet C/2000 WM1 (LINEAR) is now reported by some observers to be at third magnitude, making it brighter -- although more diffuse -- than most visible stars.  A dust tail as long as 3 degrees has also been reported.  Pictured above is the center of Comet LINEAR (WM1) taken the morning of February 1 from 300 km north of Sydney, Australia.  A bright coma and the start of the dust tail are visible despite a bright, nearly full Moon.  The comet has now passed its closest approach to the Sun (January) and the Earth (December) and will move toward northern skies as it fades.
Giant Storm Systems Battle on Jupiter (2002-02-05)

Two of the largest storm systems on Jupiter are colliding, and nobody is sure what will result.  The larger storm is the famous Great Red Spot, while the smaller is a large white oval.  Both are swirling cloud systems that circulate on Jupiter.  The white oval is part of a belt of clouds that circles Jupiter faster than the Great Red Spot.  The oval started being slowed by the Great Red Spot two weeks ago and the collision could last another month.  The oval will likely survive but could possibly be disrupted or absorbed.  The two storm systems went at it at least once before in 1975 causing the Spot's red color to fade for several years.  The passing Voyager 2 robot spacecraft took the above picture of Jupiter's Great Red Spot in 1979.  A different white oval was then visible below the Spot.
The Cosmic Infrared Background (2002-02-06)

What cosmic wallpaper is on the sky?  The answer depends on the type of light considered, and for some wavelengths, all the cluttering material in the foreground makes it still unknown.  Recently, however, the background in near infrared light was imaged at high resolution by the 2MASS telescope, and confirmed earlier estimates that it is over twice as bright as originally expected.  A small section of Cosmic Infrared Background (CIB) is shown above in representative colors.  Visible is light emitted from the very first stars and galaxies in the universe, emitted when the universe was less than half its current age.  The CIB is more uniform than the present universe since stars were more spread out at early times.  The brightness of the background indicates that many stars were forming in the universe between cosmological redshifts of one and seven.