juliensimon/nasa-apod huggingface dataset:
The M7 Open Star Cluster in Scorpius (2004-02-22)

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 in 1995 at the Burrell-Schmidt Telescope at Kitt Peak National Observatory in Arizona.  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.

Image credit: N. A. Sharp
Heaven on Earth (2004-02-23)

If sometimes it appears that the entire Milky Way Galaxy is raining down on your head, do not despair.  It happens twice a day.  As the Sun rises in the East, wonders of the night sky become less bright than the sunlight scattered by our own Earth's atmosphere, and so fade from view.  They will only rotate back into view when the Earth again eclipses our bright Sun at dusk.  This battle between heaven and Earth was captured dramatically in a digitally enhanced double-exposure over the Kofa Mountains in Arizona, USA in 2003 May.  Dark dust, millions of stars, and bright glowing red gas highlight the plane of our Milky Way Galaxy, which lies on average thousands of light years behind Earth's mountains.

Image credit: Richard Payne (Arizona Astrophotography)
X-Rays Indicate Star Ripped Up by Black Hole (2004-02-24)

What could rip a star apart?  A black hole.  Giant black holes in just the right mass range would pull on the front of a closely passing star much more strongly than on the back.  Such a strong tidal force would stretch out a star and likely cause some of the star's gasses to fall into the black hole.  The infalling gas has been predicted to emit just the same blast of X-rays that have recently been seen in the center of galaxy RX J1242-11.  Above, an artist's illustration depicts the sequence of destruction (assuming that image-distorting gravitational-lens effects of the black hole are somehow turned off).  Most of the stellar remains would be flung out into the galaxy.  Such events are rare, occurring perhaps only one in 10,000 years for typical black holes at the center of typical galaxies.
White Boat Rock on Mars (2004-02-25)

What caused this rock to have an unusual shape?  Earlier this month the robot Spirit rover on Mars stopped to examine a rock dubbed "white boat", named for its unusually light color and shape.  White boat, the large rock near the image center of the above color-composite image, was examined by Spirit just after Adirondack, a football-sized rock determined to be composed of volcanic basalt.  Spirit resumed scientific operations two weeks ago after recovering from a computer memory problem.  Spirit and its twin rover Opportunity, on the other side of Mars, continue to roam the red planet in search of clues to the ancient past of Earth's most hospitable neighbor.
Galaxy Cluster in the Early Universe (2004-02-26)

Long before medieval alchemists dreamed of transmuting base metals to gold, stellar furnaces in this massive cluster of galaxies - cataloged as RDCS 1252.9-2927 - had transformed light elements into heavy ones. In the false-color composite image individual cluster galaxies can be seen at optical and near-infrared wavelengths, shown in red, yellow, and green colors. X-ray data (in purple) reveal the hot intracluster gas, enriched in heavy elements. Attracting the attention of astronomers using the orbiting Chandra and XMM-Newton x-ray telescopes, as well as the Hubble Space Telescope and ground based VLT, the galaxy cluster lies nearly 9 billion light-years away ... and so existed at a time when the Universe was less than 5 billion years old. A measured mass of more than 200 trillion Suns makes this galaxy cluster the most massive object ever found when the Universe was so young. The cluster elemental abundances are consistent with the idea that most heavy elements were synthesized early on by massive stars, but current theories suggest that such a massive cluster should be rare in the early Universe.
Rumors of a Strange Universe (2004-02-27)

Only a few short years ago, when the APOD editors were in graduate school, the pervasive, cosmic Dark Energy was not even seriously discussed. Of course, it now appears that this strange energy dominates the cosmos (as well as lectures on cosmology) and provides a repulsive force accelerating the large scale expansion of the Universe. In fact, recent brightness measurements of distant and therefore ancient, stellar explosions or supernovae indicate that the universal expansion began to speed up in earnest four to six billion years ago, when the Dark Energy's repulsive force began to overcome the attractive force of gravity over cosmic distances. The Hubble Space telescope images above show a sample of the distant supernova explosions, billions of light-years away, in before (top) and after (bottom) pictures of their faint host galaxies. Hubble measured supernovae also hint that the Dark Energy's repulsive force is constant over cosmic time and so could be consistent with Einstein's original theory of gravitation. If the force actually changes with time, the Universe could still end in a Big Crunch or a Big Rip ... but not for at least an estimated 30 billion years.
POX 186: Not So Long Ago (2004-02-28)

Not so long ago and not so far, far away, a galaxy was born. Seen in this Hubble Space Telescope image, the island universe of stars, gas, and dust cataloged as POX 186 is a mere 68 million light-years distant toward an uncrowded region in the constellation Virgo. POX 186 is truly dwarfed by galaxies like our own Milky Way. The diminutive galaxy is about 900 light-years across with around 10 million stars, compared to the Milky Way's 100,000 light-year span and more than 200 billion stars. Cosmically speaking, POX 186 is also very young as the Hubble snapshot reveals a disturbed galaxy that is likely the result of a 100 million year old collision between two even smaller star systems. In fact, POX 186 observations suggest that such isolated, small galaxies may be the last to form, since the most massive galaxies in the universe seem to have formed billions of years ago.
Julius Caesar and Leap Days (2004-02-29)

Today, February 29th, is a leap day - a relatively rare occurrence. In 46 BC, Julius Caesar, pictured above in a self-decreed minted coin, created a calendar system that added one leap day every four years. Acting on advice by Alexandrian astronomer Sosigenes, Caesar did this to make up for the fact that the Earth's year is slightly more than 365 days. In modern terms, the time it takes for the Earth to circle the Sun is slightly more than the time it takes for the Earth to rotate 365 times (with respect to the Sun -- actually we now know this takes about 365.24219 rotations). So, if calendar years contained 365 days they would drift from the actual year by about 1 day every 4 years. Eventually July (named posthumously for Julius Caesar himself) would occur during the northern hemisphere winter! By adopting a leap year with an extra day every four years, the calendar year would drift much less. This Julian Calendar system was used until the year 1582 when Pope Gregory XIIIprovided further fine-tuning when he added that leap days should not occur in years ending in "OO", unless divisible by 400. This Gregorian Calendar system is the one in common use today.
Cassini Closes in on Saturn (2004-03-01)

Are they gone? They were not originally predicted to even be there.  The mystery revolves around strange shadow-like spokes that appeared on Saturn's large B-ring, the large middle ring in the complex system of particles that orbits Saturn.  The spokes were discovered 23 years ago by the passing Voyager spacecraft and attributed to very fine dust of unknown origin.  The missing spokes were noted in the above image, taken last month, from the robot Cassini spacecraft now approaching Saturn.  Launched in 1997, the distance remaining between Cassini and Saturn is now less than half that between the Earth and the Sun.  Cassini is expected to enter orbit around the ringed Jovian giant planet in July and drop a probe onto Titan, Saturn's largest moon.
NGC 6960: The Witch's Broom Nebula (2004-03-02)

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 rampaging gas gains its colors by impacting 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 Cygnus is visible with the unaided eye from a dark location but unrelated to the ancient supernova.

Image credit: T. A. Rector
Opportunity Rover Indicates Ancient Mars Was Wet (2004-03-03)

Was Mars ever wet enough to support life?  To help answer this question, NASA launched two rover missions to the red planet and landed them in regions that satellite images indicated might have been covered with water.  Yesterday, mounting evidence was released indicating that the Mars Opportunity rover had indeed uncovered indications that its landing site, Meridiani Planum, was once quite wet.  Evidence that liquid water once flowed includes the physical appearance of many rocks, rocks with niches where crystals appear to have grown, and rocks with sulfates.  Pictured above, Opportunity looks back on its now empty lander.  Visible is some of the light rock outcropping that yielded water indications, as well as the rim of the small crater where Opportunity landed.  The rover will continue to explore its surroundings and try to determine the nature and extent that water molded the region.
Cold Mountain Sky (2004-03-04)

This lovely celestial view is surely a familiar one to winter skygazers in the northern hemisphere. Lights silhouetting the trees are from nearby towns Morganton and Rutherford College, North Carolina, USA. But the scene may also look familiar to attentive fans of the movie Cold Mountain, whose fictional characters discuss this same factual starry sky. Sporting belt and sword, Orion the hunter is an easily identifiable constellation above center. Above and to his right lies a V-shaped arrangement of stars in the Hyades star cluster anchored by bright Aldebaran. Below and at the left of the hunter are his dogs Canis Major and Canis Minor, and their respective alpha stars Sirius and Procyon. Need a program to follow the stars? Just put your cursor over the Cold Mountain Sky.

Image credit: David Cortner
V838 Mon: Echoes from the Edge (2004-03-05)

Variable star V838 Monocerotis lies near the edge of our Milky Way Galaxy, about 20,000 light-years from the Sun. Still, ever since a sudden outburst was detected in January 2002, this enigmatic star has taken the center of an astronomical stage while researchers try to understand where it fits into the picture of stellar evolution. As light from the stellar flash echoes across pre-existing dust shells around V838 Mon, its appearance changes dramatically. Revealed in a sharp snapshot recorded in February by the Hubble's Advanced Camera for Surveys, this portion of the dust shell is about six light-years in diameter. But because light reflected from the dust follows only a slightly indirect path compared to the direct line-of-sight to the star, the light echoes visible now are only lagging about two years behind the outburst itself. Astronomers expect the expanding echoes to continue to light up the dusty environs of V838 Mon for at least the rest of the current decade.
N49's Cosmic Blast (2004-03-06)

Scattered debris from a cosmic supernova explosion lights up the sky in this gorgeous composited image based on data from the Hubble Space Telescope. Cataloged as N49, these glowing filaments of shocked gas span about 30 light-years in our neighboring galaxy, the Large Magellanic Cloud. Light from the original exploding star reached Earth thousands of years ago, but N49 also marks the location of another energetic outburst -- an extremely intense blast of gamma-rays detected by satellites only twenty-five years ago on March 5, 1979. That date was the beginning of an exciting journey in astrophysics which led researchers to the understanding of an exotic new class of stars. The source of the March 5th Event is now attributed to a magnetar - a highly magnetized, spinning neutron star also born in the ancient stellar explosion which created supernova remnant N49. The magnetar hurtles through the supernova debris cloud at over 1,200 kilometers per second.
An Anomalous SETI Signal (2004-03-07)

No one knows for sure what caused this signal.  There is a slight possibility that it just might originate from an extraterrestrial intelligence. The bright colors on the blue background indicate that an anomalous signal was received here on Earth by a radio telescope involved in a  Search for Extraterrestrial Intelligence (SETI).  A search for these signals is ongoing by  several groups including volunteer members of the SETI League.  Time labels the vertical axis of the above plot, and frequency marks the horizontal axis.  Although  this strong signal was never positively identified, astronomers have identified in it many attributes characteristic of a more mundane and ultimately terrestrial origin.  In this case, a leading possibility is that the signal originates from an unusual modulation between a GPS satellite and an unidentified Earth-based source. Many unusual signals from space remain unidentified.  No signal has yet been strong enough or run long enough to be unambiguously identified as originating from an extraterrestrial intelligence.

Image credit: SETI League
Moon and Venus over Corona Del Mar Beach (2004-03-08)

The crescent Moon passed nearly in front of Venus two weeks ago.  The close conjunction of the night sky's two brightest objects created a striking pose for many viewing the evening sky just after sunset.  Such a pose, shown above, was captured between clouds over Corona Del Mar Beach in California, USA.  To be precise, the Moon appeared to pass only about three degrees from Venus on February 23.  A similar conjunction will occur later this month, on March 24, when Venus appears near its furthest from the Sun while the Moon passes only about 2 degrees away.

Image credit: Wally Pacholka (Astropics)
The Hubble Ultra Deep Field (2004-03-09)

What did the first galaxies look like? To help answer this question, the Hubble Space Telescope has just finished taking the Hubble Ultra Deep Field (HUDF), the deepest image of the universe ever taken in visible light.  Pictured above, the HUDF shows a sampling of the oldest galaxies ever seen, galaxies that formed just after the dark ages, 13 billion years ago, when the universe was only 5 percent of its present age.  The Hubble Space Telescope's NICMOS and new ACS cameras took the image.  Staring nearly 3 months at the same spot, the HUDF is four times more sensitive, in some colors, than the original Hubble Deep Field (HDF). Astronomers the world over will likely study the HUDF for years to come to better understand how stars and galaxies formed in the early universe.
Humphrey Rock Indicates Ancient Martian Water (2004-03-10)

How prevalent was water on Mars?  Results from the Spirit rover now indicate that Gusev crater likely had a wet past, a result that comes shortly after Spirit's twin rover Opportunity uncovered clear evidence of past water at Meridiani Planum on the other side of Mars.  Evidence uncovered by Spirit and released last week focussed on a large rock of unusual shape nicknamed Humphrey, shown above near the image bottom.  Detailed inspection of the rock revealed a bright material filling internal cracks.  Such material may have crystallized from water trickling through the volcanic rock.  The amount of Mars once covered by ancient water remains unknown, as both rovers landed in regions thought likely to once be underwater.  Spirit continues to roll across Mars, recently passing the 300-meter mark on its way to Bonneville crater.
Henize 206: Cosmic Generations (2004-03-11)

Peering into a dusty nebula in nearby galaxy the Large Magellanic Cloud, infrared cameras on board the Spitzer Space Telescope recorded this detailed view of stellar nursery Henize 206 filled with newborn stars. The stars appear as white spots within the swirls of dust and gas in the false-color infrared image. Near the top, the sweeping telltale arcs of a supernova remnant are also visible, expanding debris from the final explosion of a massive star. The proximity of the ancient supernova indicates that the shockwave from that stellar death explosion itself likely triggered the formation of the new generation of emerging stars, compressing the gas and dust within Henize 206 and continuing the cosmic cycle of star death and star birth. At the distance of the Large Magellanic Cloud, about 163,000 light-years, this image covers an area about 1,000 light-years across.
X-Ray Saturn (2004-03-12)

Above, the ringed planet Saturn shines in x-rays. Otherwise beyond the range of human vision, the eerie x-ray view was created by overlaying a computer generated outline of the gas giant's disk and ring system on a false-color picture of smoothed, reconstructed x-ray data from the orbiting Chandra Observatory. The data represent the first clear detection of Saturn's disk at x-ray energies and held some surprises for researchers. For starters, the x-rays seem concentrated near the planet's equator rather than the poles, in marked contrast to observations of Jupiter, the only other gas giant seen at such high energies. And while Saturn's high energy emission is found to be consistent with the reflection of x-rays from the Sun, the intensity of the reflected x-rays was also found to be unusually strong. Outside the planet's disk, only a faint suggestion of x-rays from Saturn's magnificent ring system is visible at the left.
A Cerro Tololo Sky (2004-03-13)

High atop a Chilean mountain lies one of the premier observatories of the southern sky: the Cerro Tololo Inter-American Observatory (CTIO). Pictured above is the dome surrounding one of the site's best known instruments, the 4-meter Blanco Telescope. Far behind the dome are thousands of individual stars and diffuse light from three galaxies: the Small Magellanic Cloud (upper left), the Large Magellanic Cloud (lower left), and our Milky Way Galaxy (right).  Also visible just to Blanco's right is the famous superposition of four bright stars known as the Southern Cross. A single 20 second exposure, this digital image was recorded with a sensitive detector intended for astronomical imaging. The observatory structures are lit solely by starlight.
Comet Hale-Bopp Over Val Parola Pass (2004-03-14)

Comet Hale-Bopp became much brighter than any surrounding stars. It was seen even over bright city lights. Out away from city lights, however, it put on quite a spectacular show. Here Comet Hale-Bopp was photographed above Val Parola Pass in the Dolomite mountains surrounding Cortina d'Ampezzo, Italy. Comet Hale-Bopp's blue ion tail was created when fast moving particles from the solar wind struck expelled ions from the comet's nucleus. The white dust tail is composed of larger particles of dust and ice expelled by the nucleus that orbit behind the comet. Observations showed that  Comet Hale-Bopp's nucleus spins about once every 12 hours.

Image credit: A. Dimai, (Col Druscie Obs.),
AAC
The Orion Nebula from CFHT (2004-03-15)

Few astronomical sights excite the imagination like the nearby stellar nursery known as the Orion Nebula.  The Nebula's glowing gas surrounds hot young stars at the edge of an immense interstellar molecular cloud only 1500 light-years away.  The Great Nebula in Orion can be found with the unaided eye just below and to the left of the easily identifiable  belt of three stars in the popular constellation Orion. The above image from the 3.6-meter Canada-France-Hawaii Telescope atop a dormant volcano in Hawaii brings out Orion's detail in spectacular fashion.  Buried in the complex nebulosity are the bright stars of the Trapezium in Orion's heart, the sweeping lanes of dark dust that cross the center, the pervasive red glowing hydrogen gas, and the  blue tinted dust that reflects the light of newborn stars.  The whole Orion Nebula cloud complex, which includes the Horsehead Nebula, will slowly disperse over the next 100,000 years.

Image credit: Canada-France-Hawaii Telescope
Sedna of the Outer Solar System (2004-03-16)

What is the most distant known object in our Solar System?  A new answer to this centuries-old question was announced yesterday by NASA with the discovery of a dark red object dubbed Sedna.  Although over twice the distance to Pluto, Sedna is near its closest approach to the Sun.  Sedna's highly elliptical orbit will further displace it by 10 times, making it a candidate for the long-hypothesized Oort cloud of icy objects thought to extend to the Solar System's edge.  Sedna is estimated to be about three-quarters the size of Pluto and therefore the largest Solar System object found since Pluto in 1930.  Whether Sedna is ever designated a planet is at the discretion of the International Astronomical Union.  The above drawing depicts how Sedna might look facing the distant Sun.  The unexpectedly red color, the unusual orbit, and the origin of Sedna will surely be topic of much future research.
Redshift 10: Evidence for a New Farthest Galaxy (2004-03-17)

What's the farthest galaxy known?  The answer keeps changing as astronomers compete to find galaxies that top the list.  The new claimed record holder is now the faint smudge indicated in the above images by an 8.2-meter Very Large Telescope (VLT) operating in Chile.  Detected light left this galaxy 13.2 billion of years ago, well before the Earth formed, when the universe was younger than 3 percent of its present age.  Astronomers have estimated a redshift of 10 for this galaxy, the first double-digit claim for any galaxy.  Young galaxies are of much interest to astronomers because many unanswered questions exist on when and how galaxies formed in the early universe.  The distant redshift, if confirmed, would also give valuable information about galaxy surroundings at the end of the universe's dark age.  Although this galaxy's distance exceeds that of even the farthest known quasar, it is still in front of the pervasive glowing gas that is now seen as the cosmic microwave background radiation.
Spirit Pan from Bonneville Crater's Edge (2004-03-18)

Scroll right and follow this breathtaking view of the martian surface from the southern edge of a small crater dubbed Bonneville. NASA's Spirit rover recorded the sharp 180-degree panorama on sols 68 and 69 of its stay on the Red Planet, following the completion of a 300+ meter journey from its landing site within Mars' expansive Gusev Crater region. Bonneville crater itself is about 200 meters across. Rocks scattered about the area are potentially "ejecta" from Bonneville, debris blasted from below the martian surface by the impact which created the crater. Researchers are eager to confirm this scenario since such material could be a guide to the geological history of the area. So what's that shiny patch on the left, just beyond the crater's far rim? It's the Spirit lander's heat shield.