juliensimon/nasa-apod huggingface dataset:
The GRB 110328A Symphony (2011-04-19)

A symphony of planet-wide observations began abruptly on March 28 when the Earth-orbiting Swift satellite detected a burst of high-frequency gamma-rays from GRB 110328A. When the same source flared again after a 45 minute pause it was clear this event was not a typical gamma-ray burst.  Twelve hours after the initial fanfare astronomers using the 2.5-meter Nordic Optical Telescope chimed in with a mid-range observation of the optical counterpart. Early the next day the explosion was picked up in baritone low-frequencies of radio waves by the EVLA radio dishes in the USA. Later many optical telescopes, including the 8-meter Gemini North telescope in Hawaii, began playing along by tracking the optical counterpart. The unusual source was spotted at a higher register in X-rays by the Chandra X-ray Observatory and was intermittently followed in the even more soprano-like gamma-ray range for a week. Joining the chorus, Hubble Space Telescope recorded this image in optical and infrared light, confirming that the flash was located along the path of a galaxy at redshift 0.351.  If associated with the galaxy, this explosion occurred when the universe was about two thirds of its present age. There is much speculation that the unusual gamma-ray burst was a star being ripped apart by a supermassive black hole in the center of a galaxy and the puzzling features of the distant detonation are still being explored.
Rio Morning Moonset (2011-04-20)

As the Sun rose, a nearly full Moon set in this serene seaside vista captured last Monday from Rio de Janeiro, Brazil. In the foreground, the reddened early morning sunlight illuminates a stretch of South Atlantic coastline. Looking toward the west, a scene that is a familiar one to Rio's Ipanema beach goers, the favela Vidigal is nestled below the twin peaks of Morro Dois Irmãos (Two Brothers Hill). This well-composed multiple-exposure image recorded the steady progress of the dramatic moonset with a 6.5 minute gap between each frame. Flying from their nests at the break of dawn, all the ocean birds appear only in the last single frame.

Image credit: Babak Tafreshi
Peculiar Galaxies of Arp 273 (2011-04-21)

The spiky stars in the foreground of this sharp cosmic portrait are well within our own Milky Way Galaxy. The two eye-catching galaxies lie far beyond the Milky Way, at a distance of over 300 million light-years. Their distorted appearance is due to gravitational tides as the pair engage in close encounters. 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. From our perspective, the bright cores of the Arp 273 galaxies are separated by only a little over 100,000 light-years. The release of this stunning vista celebrates the 21st anniversary of the Hubble Space Telescope in orbit.
Virgo Cluster Galaxies (2011-04-22)

Well over a thousand galaxies are known members of the Virgo Cluster, the closest large cluster of galaxies to our own local group. In fact, the galaxy cluster is difficult to appreciate all at once because it covers such a large area on the sky. Spanning about 5x3 degrees, this careful mosaic of telescopic images clearly records the central region of the Virgo Cluster through faint foreground dust clouds lingering above the plane of our own Milky Way galaxy. The cluster's dominant giant elliptical galaxy M87, is just below center in the frame. Above M87 is the famous interacting galaxy pair NGC 4438, also known as The Eyes. A closer examination of the image will reveal many Virgo cluster member galaxies as small fuzzy patches. Sliding your cursor over the image will label the larger galaxies using NGC catalog designations. Galaxies are also shown with Messier catalog numbers, including M84, M86, and prominent colorful spirals M88, M90, and M91. On average, Virgo Cluster galaxies are measured to be about 48 million light-years away. The Virgo Cluster distance has been used to give an important determination of the Hubble Constant and the scale of the Universe.  (Editor's Note: Labels courtesy of Astrometry.net.)

Image credit: Rogelio
Bernal Andreo
Shadows at the Lunar South Pole (2011-04-23)

What is it? It's a multi-temporal illumination map, of course. To make it, the wide angle camera on the Lunar Reconnaissance Orbiter spacecraft collected 1,700 images over a period of 6 lunar days (6 Earth months), repeatedly covering an area centered on the Moon's south pole. Converted to binary values (shadowed pixels set to 0, illuminated pixels set to 1) the images were stacked to produce a map representing the percentage of time each spot on the surface was illuminated by the Sun. Remaining convincingly in shadow, the floor of the 19 kilometer diameter Shackleton crater is seen near the center of the map. The lunar south pole itself is at about 9 o'clock on the crater's rim. Since the Moon's axis of rotation stays almost perpendicular to the ecliptic plane, crater floors near the lunar south and north poles can remain in permanent shadow and mountain tops in nearly continuous sunlight. Useful to future outposts, the shadowed crater floors could offer reservoirs of water ice, and the sunlit mountain tops ideal locations for solar power arrays.
The Cat's Eye Nebula from Hubble (2011-04-24)

Staring across interstellar space, the alluring Cat's Eye nebula lies three thousand light-years from Earth. A classic planetary nebula, the Cat's Eye (NGC 6543) represents a final, brief yet glorious phase in the life of a sun-like star. This nebula's dying central star may have produced the simple, outer pattern of dusty concentric shells by shrugging off outer layers in a series of regular convulsions. But the formation of the beautiful, more complex inner structures is not well understood. Seen so clearly in this sharp Hubble Space Telescope image, the truly cosmic eye is over half a light-year across. Of course, gazing into the Cat's Eye, astronomers may well be seeing the fate of our sun, destined to enter its own planetary nebula phase of evolution ... in about 5 billion years.
Monsters of IC 1396 (2011-04-25)

Is there a monster in IC 1396? Known to some as the Elephant's Trunk Nebula, parts of the glowing gas and dust clouds of this star formation region may appear to take on foreboding forms, some nearly human.  The entire nebula might even look like a face of a monster. The only real monster here, however, is a bright young star too far from Earth to be dangerous.  Energetic light from this star is eating away the dust of the dark cometary globule at the top right of the image.  Jets and winds of particles emitted from this star are also pushing away ambient gas and dust. Nearly 3,000 light-years distant, the IC 1396 complex is relatively faint and covers a region on the sky with an apparent width of more than 10 full moons. Recently, over 100 young stars have been discovered forming in the nebula.

Image credit: Geert Barentsen
& Jorick Vink
(Armagh Observatory)
& the IPHAS Collaboration
Hydrogen in the LMC (2011-04-26)

A satellite galaxy of our own Milky Way, the Large Magellanic Cloud (LMC) is an alluring sight in dark southern skies and the constellation Dorado. A mere 180,000 light-years distant, the LMC is seen in amazing detail in this very deep 4 frame mosaic of telescopic images, a view that reveals the Milky Way's satellite to have the appearance of a fledgling barred spiral galaxy. The mosaic includes image data taken through a narrow filter that transmits only the red light of hydrogen atoms. Ionized by energetic starlight, a hydrogen atom emits the characteristic red H-alpha light as its single electron is recaptured and transitions to lower energy states. As a result, this mosaic seems spattered with pinkish clouds of hydrogen gas surrounding massive, young stars. Sculpted by the strong stellar winds and ultraviolet radiation, the glowing hydrogen clouds are known as H II (ionized hydrogen) regions. Composed of many overlapping clouds, the sprawling Tarantula Nebula left of center, is by far the LMC's largest star forming region. The Large Magellanic Cloud is about 15,000 light-years across.

Image credit: Star Echoes
The Dark Tower in Scorpius (2011-04-27)

In silhouette against a crowded star field toward the constellation Scorpius, this dusty cosmic cloud evokes for some the image of an ominous dark tower. In fact, clumps of dust and molecular gas collapsing to form stars may well lurk within the dark nebula, a structure that spans almost 40 light-years across this gorgeous telescopic portrait. Known as a cometary globule, the swept-back cloud, extending from the lower right to the head (top of the tower) left and above center, is shaped by intense ultraviolet radiation from the OB association of very hot stars in NGC 6231, off the upper edge of the scene. That energetic ultraviolet light also powers the globule's bordering reddish glow of hydrogen gas. Hot stars embedded in the dust can be seen as bluish reflection nebulae. This dark tower, NGC 6231, and associated nebulae are about 5,000 light-years away.

Image credit: Don Goldman
Scintillating (2011-04-28)

On June 4, 2010 Regulus, alpha star of the constellation Leo, and wandering planet Mars were at about the same apparent brightness, separated on the sky by 1.5 degrees. An ingenious and creative 10 second exposure from a swinging camera recorded these gyrating trails of the celestial pairing. Can you tell which trail belongs to the star and which to the planet? Hint: atmospheric turbulence causes the image of the star to scintillate or vary in brightness and color more readily than the planet. The scintillation is more pronounced because the star is effectively a point source of light seen as a narrow bundle of light rays. Rapidly changing refraction due to turbulence along the line of sight affects different colors of light by different amounts and generally produces a twinkling effect for stars. But Mars is much closer than the distant stars and an extended source of light. Though tiny, its disk is seen as a bundle of light rays that is substantially broader compared to a star's and so, on average, less affected by small scale turbulence. The result is the varied, rainbow like trail for Regulus (left) and the steadier, consistently reddish trail for Mars.

Image credit: Juergen Michelberger
The Antennae (2011-04-29)

Some 60 million light-years away in the southerly constellation Corvus, two large galaxies collided. But the stars in the two galaxies, cataloged as NGC 4038 and NGC 4039, don't collide in the course of the ponderous event, lasting hundreds of millions of years. Instead, their large clouds of molecular gas and dust do, triggering furious episodes of star formation near the center of the cosmic wreckage. Spanning about 500 thousand light-years, this stunning view also reveals new star clusters and matter flung far from the scene of the accident by gravitational tidal forces. Of course, the suggestive visual appearance of the extended arcing structures gives the galaxy pair its popular name - The Antennae.

Image credit: stargazer-observatory
Molecular Cloud Barnard 163 (2011-05-01)

It may look to some like a duck, but it lays stars instead of eggs.  In the center of the above image lies Barnard 163, a nebula of molecular gas and dust so thick that visible light can't shine through it.  With a wing span measured in light years, Barnard 163's insides are surely colder than its exterior, allowing conditions where gas can clump and eventually form stars.  Barnard 163 lies about 3,000 light years from Earth toward the constellation of Cepheus the King. The red glow in the background results from IC 1396, a large emission nebula that houses the Elephant's Trunk Nebula.  Finding Barnard 163 in an image of its greater emission nebula IC 1396 can be a challenge, but it's possible.

Image credit: T. Rector
Jupiter's Great Red Spot from Voyager 1 (2011-05-02)

It is a hurricane twice the size of the Earth. It has been raging at least as long as telescopes could see it, and shows no signs of slowing. It is Jupiter's Great Red Spot, the largest swirling storm system in the Solar System. Like most astronomical phenomena, the Great Red Spot was neither predicted nor immediately understood after its discovery. Still today, details of how and why the Great Red Spot changes its shape, size, and color remain mysterious. A better understanding of the weather on Jupiter may help contribute to the better understanding of weather here on Earth. The above image is a recently completed digital enhancement of an image of Jupiter taken in 1979 by the Voyager 1 spacecraft as it zoomed by the Solar System's largest planet. At about 117 AU from Earth, Voyager 1 is currently the most distant human made object in the universe and expected to leave the entire solar heliosheath any time now.
Globular Cluster M15 from Hubble (2011-05-03)

Stars, like bees, swarm around the center of bright globular cluster M15.  This ball of over 100,000 stars is a relic from the early years of our Galaxy, and continues to orbit the Milky Way's center.  M15, one of about 150 globular clusters remaining, is noted for being easily visible with only binoculars, having at its center one of the densest concentrations of stars known, and containing a high abundance of variable stars and pulsars.  This sharp image, taken by the Earth-orbiting Hubble Space Telescope, spans about 120 light years. It shows the dramatic increase in density of stars toward the cluster's center.  M15 lies about 35,000 light years away toward the constellation of the Winged Horse (Pegasus).  Recent evidence indicates that a massive black hole might reside as the center of M15.
Celestial Trails over Greece (2011-05-04)

If you watch the horizon at just the right place and at just the right time, you can witness some spectacular juxtapositions between Earth and sky. In the above video, stars, the Moon, and even a partially eclipsed Sun were recorded rising and setting over photogenic landmarks in Greece. The inspiring video features effects on star trails that are pretty but somewhat deceiving, as stars will typically remain at nearly the same brightness as the Earth turns beneath them. Also, the time lapse nature of part of the video causes clouds to appear to jump about and fade in an unfamiliar fashion. Several stills of these scenes have appeared on APOD previously, including the partial solar eclipse over the Temple of Poseidon, cloud and star trails over a deserted church, and star trails over an old ship. Other foreground venues include an old church in Peloponnese, a stone bridge in Ioannina, a Frankish Castle in Euboea, and a picturesque sunset above Lycabettus Hill, Athens.

Image credit: Chris Kotsiopoulos
50 Years Ago: Freedom 7 Flies (2011-05-05)

Fifty years ago, near the dawn of the space age, NASA controllers "lit the candle" and sent Mercury astronaut Alan Shepard arcing into space atop a Redstone rocket. His cramped space capsule was dubbed Freedom 7. Broadcast live to a global television audience, the historic Mercury-Redstone 3 (MR-3) spacecraft was launched from Cape Canaveral Florida at 9:34 a.m. Eastern Time on May 5, 1961. The flight of Freedom 7 - the first space flight by an American - followed less than a month after the first human venture into space by Soviet Cosmonaut Yuri Gagarin. The 15 minute sub-orbital flight achieved an altitude of 116 miles and a maximum speed of 5,134 miles per hour. As Shepard looked back toward planet Earth near the peak of Freedom 7's trajectory, he could see the outlines of the west coast of Florida, Lake Okeechobe in central Florida, the Gulf of Mexico, and the Bahamas.
Farther Along (2011-05-06)

What is humanity's most distant spacecraft? Launched in 1977, Voyager 1 now holds that distinction at 17.5 billion kilometers from the Sun. That corresponds to 16 light-hours or 117 Astronomical Units (AU). This graphic shows the position of Voyager 1 relative to the outer solar system (top and side views) along with other distant spacecraft contenders. Next most distant, Pioneer 10 is about 15.4 billion kilometers from the Sun, though on the opposite side of the solar system from Voyager 1. Voyager 2 and Pioneer 11, both also well beyond the orbit of Pluto, are 14.2 billion and 12.4 billion kilometers from the Sun respectively. Still outbound for Pluto, the New Horizons spacecraft is presently 3 billion kilometers from the Sun and will encounter the Pluto system in July of 2015. All these spacecraft have used sling-shot style gravity assist maneuvers to increase their speeds through the outer solar system. Voyager 1 is moving the fastest, escaping the solar system at about 17 kilometers per second. Still operational, both Voyagers are headed towards the outer boundary of the solar system, in search of the heliopause and the beginning of interstellar space.
Dawn of the Planets (2011-05-07)

This month, four of the five naked-eye planets gather along the eastern horizon near dawn. The celestial grouping is seen here just before sunrise on May 5, from a beach near Buenos Aires, Argentina. Starting near the top of the frame, the brightest beacon is Venus. Mercury is below and right of Venus and brilliant Jupiter is lower still, near image center. Below Jupiter, Mars is relatively faint and struggles the most to shine through a thin cloud bank and the warming twilight glow. Watch, and as the month progresses the tantalizing configuration will change, with Mars and Jupiter moving higher while Venus and Mercury wander through the sky closer to the rising sun.

Image credit: Luis Argerich
Shadow of a Martian Robot (2011-05-08)

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.  Although the Spirit rover is now stuck, Opportunity is continuing on its long trek to expansive Endeavor crater.
Wonder and Mystery above the Very Large Telescopes (2011-05-09)

What's that bright orange dot above the large telescope on the right? Even seasoned sky enthusiasts might ponder the origin of the orange orb seen by scrolling across this panoramic image, taken last December. Perhaps identifying known objects will help.  To start, on the far left is a diagonal band of light known as zodiacal light, sunlight reflected off of dust orbiting in the inner Solar System. The bright white spot on the left, just above the horizon, is Venus, which also glows by reflected sunlight.  Rising diagonally from the ground to the right of Venus is the band of our Milky Way Galaxy. In the image, the band, which usually stretches dramatically overhead, appears to arch above the elevated Chilean landscape. Under the Milky Way arch, toward the left, lie both the Large and Small Magellanic Cloud galaxies, while toward the right lies the constellation of Orion surrounded by the red ring of Barnard's Loop. On the ground, each of the four Very Large Telescopes is busy keeping an eye on the distant universe. The orange spot -- it's the Moon.  The image was taken during a total lunar eclipse when the normally bright full moon turned into a faint orb tinted orange by the intervening Earth's atmosphere.
Gravity Probe B Confirms the Existence of Gravitomagnetism (2011-05-10)

Does gravity have a magnetic counterpart?  Spin any electric charge and you get a magnetic field.  Spin any mass and, according to Einstein, you should get a very slight effect that acts something like magnetism.  This effect is expected to be so small that it is beyond practical experience and ground laboratory measurement.  In a bold attempt to directly measure gravitomagnetism, NASA launched in 2004 the smoothest spheres ever manufactured into space to see how they spin.  These four spheres, each roughly the size of a ping-pong ball, are the key to the ultra-precise gyroscopes at the core of Gravity Probe B.  Last week, after accounting for persistent background signals, the results were announced -- the gyroscopes precessed at a rate consistent with the gravitational predictions of Einstein's General Theory of Relativity.  The results, which bolster existing findings, may have untold long term benefits as well as shorter term benefits such as better clocks and global positioning trackers.
The Southern Cliff in the Lagoon (2011-05-11)

Undulating bright ridges and dusty clouds cross this close-up of the nearby star forming region M8, also known as the Lagoon Nebula. A sharp, false-color composite of narrow band visible and broad band near-infrared data from the 8-meter Gemini South Telescope, the entire view spans about 20 light-years through a region of the nebula sometimes called the Southern Cliff. The highly detailed image explores the association of many newborn stars imbedded in the tips of the bright-rimmed clouds and Herbig-Haro objects. Abundant in star-forming regions, Herbig-Haro objects are produced as powerful jets emitted by young stars in the process of formation heat the surrounding clouds of gas and dust. The cosmic Lagoon is found some 5,000 light-years away toward constellation Sagittarius and the center of our Milky Way Galaxy. (Editor's Note: For location and scale, check out this image superimposing the close-up region shown in today's APOD on the larger Lagoon Nebula. Scale image is courtesy R. Barbá.)
Enceladus Looms (2011-05-12)

A sunlit crescent of Saturn's moon Enceladus looms above the night side of Saturn in this dramatic image from the Cassini spacecraft. Captured on August 13, 2010 looking in a sunward direction during a flyby of the icy moon, the view also traces layers in the upper atmosphere of Saturn scattering sunlight along the planet's bright limb. Closer to the spacecraft than Saturn, Enceladus is a mere 60,000 kilometers from Cassini's camera. The south polar region of the 500 kilometer-diameter moon is illuminated, including plumes of water vapor and icy particles spraying above the long fissures in the moon's surface. The fissures have been dubbed tiger stripes. First discovered in Cassini images from 2005, the plumes are strong evidence that liquid water exists near the surface of surprisingly active Enceladus.
A Beautiful Trifid (2011-05-13)

The beautiful Trifid Nebula is a cosmic study in colorful contrasts. Also known as M20, it 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 illustrates three different types of astronomical nebulae; red emission nebulae dominated by light emitted by hydrogen atoms, blue reflection nebulae produced by dust reflecting starlight, and dark nebulae where dense dust clouds appear in silhouette. The bright red emission region, roughly separated into three parts by obscuring, dark dust lanes, lends the Trifid its popular name. In this well met scene, the red emission is also juxtaposed with the telltale blue haze of reflection nebulae. Pillars and jets sculpted by newborn stars, below and left of the emission nebula's center, appear in Hubble Space Telescope close-up images of the region. The Trifid Nebula is about 40 light-years across.

Image credit: R Jay Gabany