juliensimon/nasa-apod huggingface dataset:
Hyakutake: Comet Atmosphere (1998-04-10)

The atmosphere of a comet comes and goes. Approaching the sun, it swells as material from the icy cometary nucleus is warmed and evaporated by increasing sunlight. Immense but tenuous and fleeting, the inner atmosphere or inner "coma" of comet Hyakutake is seen in this false color picture. Oriented with the sunward direction toward the upper right, the picture is a composite of Hubble Space Telescope images recorded on April 3 and 4, 1996. It is about 14,000 km across (comparable to Earth's diameter) and is a combination of images showing dust reflected light (red) and ultraviolet light scattered from Hydrogen atoms (blue). Hyakutake's Hydrogen atoms were produced by the breakup of water (H20) molecules evaporating from its nucleus. The Hydrogen data, combined with other observations, indicate that this comet's nucleus, itself only a few km across, was producing about 7 to 8 tons of water per second.
NGC 604: Giant Stellar Nursery (1998-04-11)

Scattered within this cavernous nebula, cataloged as NGC 604, are over 200 newly formed hot, massive, stars. At 1,500 light-years across, this expansive cloud of interstellar gas and dust is effectively a giant stellar nursery located some three million light-years distant in the spiral galaxy, M33. The newborn stars irradiate the gas with energetic ultraviolet light stripping electrons from atoms and producing a characteristic nebular glow. The details of the nebula's structure hold clues to the mysteries of star formation and galaxy evolution.
Stars from Eagle's EGGs (1998-04-12)

Newborn stars are forming in the Eagle Nebula.  This image, taken with the Hubble Space Telescope in 1995, shows evaporating gaseous globules (EGGs) emerging from pillars of molecular hydrogen gas and dust.  The giant pillars are light years in length and are so dense that interior gas contracts gravitationally to form stars.  At each pillars' end, the intense radiation of bright young stars causes low density material to boil away, leaving stellar nurseries of dense EGGs exposed. The Eagle Nebula, associated with the open star cluster M16, lies about 7000 light years away.
The Sun Changes (1998-04-13)

Our Sun changes every day. This recent picture was taken in a very specific red color called Hydrogen-Alpha.  Dark spots that might appear on the image are usually sunspots, dark magnetic depressions that are slightly cooler than the rest of the Sun's surface.  Bright spots that might appear are usually plages, active regions that are slightly hotter than the rest of the Sun's surface.  Over the next few years the average number of sunspots and plages will increase until "Solar Maximum" occurs in 2001.  The Sun usually goes through a maximum and minimum every 11 years. From 1645 to 1715, however, almost no sunspots at all were recorded, for reasons unknown. (An updated picture can be found here.)
Starlight Reflections (1998-04-14)

Wisps of dust fill the space between the stars. This dust is usually invisible, subtly acting to dim the light of more distant stars.  Sometimes this dust is thick and prominent as dark patches on otherwise bright emission nebulae.  Other times this dust may show itself by reflecting the light of bright, nearby stars. Because bright stars tend to be blue, and because dust reflects blue light more easily than red, the resulting reflection nebula usually appears blue.  Pictured above is the reflection nebula Sharpless 2-1 in the constellation of Scorpius.

Image credit: G. Greaney
NGC 1818: Pick A Star (1998-04-15)

This is NGC 1818, a youthful, glittering cluster of 20,000 stars residing in the Large Magellanic Cloud, 164,000 light-years away. Pick a star. Any star. Astronomers might pick the unassuming bluish-white one (circled) which appears to be a hot newly formed white dwarf star. What makes it so interesting? The standard astronomical wisdom suggests that stars over 5 times as massive as the sun rapidly exhaust their nuclear fuel and end their lives in a spectacular supernova explosion. With less than this critical mass they evolve into red giants, pass through a relatively peaceful planetary nebula phase, and calmly fade away as white dwarf stars like this one. Except that as a member of the NGC 1818 cluster, this new white dwarf would have evolved from a red giant star over 7.6 times as massive as the sun - which should have exploded! Its discovery will likely force astronomers to revise the limiting mass estimate for supernovae upward.
Mars: Cydonia Close-Up (1998-04-16)

The Mars Global Surveyor spacecraft has returned another close-up of the Cydonia region on Mars. Orbiting over clear Martian skies at a range of about 200 miles, the Mars Orbiter Camera looked down on features known as the "City" on Mars and produced a high resolution image covering a swath around 1.5 by 15 miles at a pixel size of about 8.2 feet. This cropped portion of the processed image shows an area approximately 1.5 miles wide. Heavily weathered hills and pocked surfaces suggest the erosion of layers of the ancient Martian crust.
Mars: Looking For Viking (1998-04-17)

On July 20, 1976, the Viking 1 lander touched down on the Martian Chryse Planitia. Its exact landing site is somewhere in the white rectangle above. Unfortunately, this wide angle Mars Global Surveyor image taken on April 12 reveals a substantial dust storm in the area with light colored plumes apparently blowing toward the upper right of the picture. Attempts to find the first spacecraft to land on Mars in the corresponding high resolution narrow field images have not been successful due in part to the increased atmospheric haze. The region shown here is about 100 miles across.
Star Wars in NGC 664 (1998-04-18)

Long ago in a galaxy far, far away, locked in their final desperate struggle against the force of gravity ... two stars exploded! stellar explosions - Supernovae - are among the most powerful events in the Universe, estimated to release an equivalent energy of up to 1 million trillion trillion (1 followed by 30 zeros) megatons of TNT. After the explosion, an expanding supernova envelope is observed to brighten over a a period of days to a maximum light output which rivals that of an entire galaxy before fading from view over the following months. Triggered by the collapsing core of a massive star or the nuclear demise of a white dwarf supernovae occur in average spiral galaxies only about once every 25-100 years. But a recent observation of NGC 664, a spiral galaxy about 300 million light years distant, captured a rare and colorful performance - two supernovae from the same galaxy. In this monitoring exposure the two supernovae, one reddish yellow and one blue, form a close pair just below the image center (to the right of the galaxy nucleus). The color difference is due to temperature - blue is hotter.
Betelgeuse (1998-04-19)

Here is the first direct picture of the surface of a star other than our Sun. Taken by the Hubble Space Telescope in 1995, the atmosphere of Betelgeuse reveals some unexpected features, including a large bright hotspot visible below the center.  Betelgeuse (sounds like "beetle juice") is a red supergiant star about 600 light years distant, easily recognizable from its brightness and reddish color in the constellation of Orion. While Betelgeuse is cooler than the Sun, it is more massive and over 1000 times larger.  If placed at the center of our Solar System, it would extend past the orbit of Jupiter. Betelgeuse is nearing the end of its life and will become a supernova in a perhaps a few tens of millions of years.
Name This Satellite (1998-04-20)

Can you name this satellite? In December, NASA's third Great Observatory is planned for launch. The two NASA Great Observatories currently in orbit are the Hubble Space Telescope and the Compton Gamma-Ray Observatory, both now named for famous scientists. But after whom should the Advanced X-ray Astrophysics Facility (AXAF) be named? If your submitted suggestion conforms with contest rules and is chosen, you will have named the most powerful X-ray satellite ever built, and may even win a prize.  AXAF is the size of a bus, has strange mirrors polished to atomic smoothness, and will produce X-ray images five times clearer of objects twice as faint as any previous X-ray satellite.  This should allow AXAF the ability to see X-rays emitted near small black holes, from distant active galaxies, and inside huge clusters of galaxies. Astronomers now hope for an uneventful launch, routine operations, and spectacular discoveries.
Water in Orion (1998-04-21)

Is Orion all wet?  Recent observations have confirmed that water molecules now exist in the famous Orion Nebula, and are still forming.  The Orion Nebula (M42, shown above) is known to be composed mostly of hydrogen gas, with all other atoms and molecules being comparatively rare. The nebula is so vast, though, that even the measured minuscule production rate creates enough water to fill Earth's oceans 60 times over every day, speculate discoverers led by M. Harwit (Cornell).  The water that composes comets, the oceans of Earth, and even humans may have been created in a cloud like the Orion Nebula.

Image credit: Anglo-Australian Telescope
Board
HR 4796A: A Recipe for Planets (1998-04-22)

Two hundred and twenty light years from Earth, planets are forming.  Recent observations of the binary star system HR 4796 have shown that one of the stars is surrounded by a dusty gaseous disk.  This disk is of the right size, age, and density for dust pellets to accrete surrounding matter.  A hole in the disk's center indicates that increasingly larger condensates are colliding and sticking together, coalescing into moons and planets. Pictured above is a false-color image of the system, with the bright star HR 4796A indicated by a cross. The disk measures about five times the size of our Solar System, and is seen nearly edge-on. HR 4796 is in the southern constellation Centaurus.
Three Dusty Stars (1998-04-23)

These separate radio images reveal three dusty debris disks surrounding three bright, young, nearby stars - evidence for solar systems in formation. From left to right are the stars Fomalhaut, Beta Pictoris, and Vega, their positions indicated by star symbols. The false color maps show the intensity of submillimeter radio emission from the surrounding dust. Next to each dust "disk", a vertical bar illustrates the present size of our own solar system. These observations are likely examples of what our solar system would have looked like to distant radio astronomers when it was only a few hundred million years old! Astronomers speculate that bright blobs of emission near Vega and Beta Pictoris may represent dust clouds around developing giant planets. The radio images were made using detectors cooled to near absolute zero and the James Clerk Maxwell Telescope at Mauna Kea Observatory in Hawaii.
Infrared Saturn (1998-04-24)

This delightfully detailed false color image of Saturn has been earmarked to celebrate the 8th anniversary of the orbiting Hubble Space Telescope. The picture is a combination of three images taken in January of this year with the Hubble's new NICMOS instrument and shows the lovely ringed planet in reflected infrared light. Different colors indicated varying heights and compositions of cloud layers generally thought to consist of ammonia ice crystals. The eye-catching rings cast a shadow on Saturn's upper hemisphere, while the bright stripe seen within the left portion of the shadow is infrared sunlight streaming through the large gap in the rings known as the Cassini Division. Two of Saturn's many moons have also put in an appearance, Tethys just beyond the planet's disk at the upper right, and Dione at the lower left.
Supernova Remnant and Neutron Star (1998-04-25)

A massive star ends life as a supernova, blasting its outer layers back to interstellar space. The spectacular death explosion is initiated by the collapse of what has become an impossibly dense stellar core. However, this core is not necessarily destroyed. Instead, it may be transformed into an exotic object with the density of an atomic nucleus but more total mass than the sun - a neutron star. A neutron star is hard to detect directly because it is small (roughly 10 miles in diameter) and therefore dim, but newly formed in this violent crucible it is intensely hot, glowing in X-rays. These X-ray images from the orbiting ROSAT observatory may offer a premier view of such a recently formed neutron stars' X-ray glow. Pictured is the supernova remnant Puppis A, one of the brightest sources in the X-ray sky, with shocked gas clouds still expanding and radiating X-rays. In the inset close-up view, a faint pinpoint source of X-rays is visible which is most likely the young neutron star, kicked out by the asymmetric explosion and moving away from the site of the original supernova at about 600 miles per second.
NGC 2440: Cocoon of a New White Dwarf (1998-04-26)

Like a butterfly, a white dwarf star begins its life by casting off a cocoon that enclosed its former self. In this analogy, however, the Sun would be a caterpillar and the ejected shell of gas would become the prettiest of all! The above cocoon, the planetary nebula designated NGC 2440, contains one of the hottest white dwarf stars known. The white dwarf can be seen as the bright dot near the photo's center. Our Sun will eventually become a "white dwarf butterfly", but not for another 5 billion years. The above false color image and was post-processed by F. Hamilton.
IC 4406: A Seemingly Square Nebula (1998-04-27)

How can a round star make a square nebula? This conundrum came to light with the discovery of planetary nebulae like IC 4406.  IC 4406 is most probably cylindrical, with its square appearance the result of our vantage point in viewing the cylinder.  Hot gas is known to be flowing out the ends of the cylinder, while filaments of dark dust and molecular gas lace the bounding walls. The star primarily responsible for this interstellar sculpture can be found in the nebula's center. In a few million years, the only thing left visible in IC 4406 will be a fading white dwarf star.
A Double Conjunction Eclipse (1998-04-28)

The crescent Moon, Venus, and Jupiter all appeared together in the early morning hours of April 23rd.  Some locations on Earth were able to witness a rare double conjunction eclipse, where the Moon occulted both Jupiter and Venus at the same time. The next double conjunction eclipse will involve Mercury and Mars and will occur on February 13, 2056.

Image credit: S. Barnes, Sky Optics
Tornadoes on the Sun (1998-04-29)

Giant spinning clouds of gas, similar to Earth's tornadoes, have been found on the Sun. Solar tornadoes, however, can be larger than the entire Earth, and sustain wind gusts over 1000 times stronger than their Earth counterparts. The SOHO spacecraft has found that solar tornadoes start low in the Sun's atmosphere and spiral outwards, gathering speed as they enter the Solar System.  Earthlings have more to fear from Earth's own weather phenomena, though, because the high speed particles that result from solar tornadoes are easily stopped by the Earth's thick atmosphere.  Earthlings may have much to learn from solar tornadoes, including details of how the solar wind and corona are powered, and how to better predict future solar particle storms that could damage sensitive satellites.
Mars: Big Crater in Stereo (1998-04-30)

Get out your red/blue glasses and check out this stereo picture of "Big Crater" on Mars! (Pieces of red and blue or green clear plastic will do. Your right eye should look through the red piece.) The stereo perspective was created by combining images from the Mars Global Surveyor spacecraft taken on two different orbits, each with a slightly different viewing angle. At just under a mile in diameter, Big Crater is not all that big but it is an important landmark in the vicinity of the Mars Pathfinder landing site on an ancient flood plain in Ares Vallis. Identifying corresponding smaller scale features in Pathfinder and Surveyor images will help to precisely locate the lander. Meanwhile, the line of sight between the Earth and Mars is approaching the Sun. During this period, known as solar conjunction, communicating with Mars Global Surveyor will be difficult.
Venus: Just Passing By (1998-05-01)

Venus, the second closest planet to the Sun, is a popular way-point for spacecraft headed for the gas giant planets in the outer reaches of the solar system. Why visit Venus first? Using a " gravity assist " maneuver, spacecraft can swing by planets and gain energy during their brief encounter saving fuel for use at the end of their long interplanetary voyage. This colorized image of Venus was recorded by the Jupiter-bound Galileo spacecraft shortly after its gravity assist flyby of Venus in February of 1990. Galileo's glimpse of the veiled planet shows structure in swirling sulfuric acid clouds. The bright area is sunlight glinting off the upper cloud deck. The Saturn-bound Cassini spacecraft just completed its own flyby of Venus on April 26. Launched in October of 1997, Cassini should reach Saturn in July 2004.
The Frothy Milky Way (1998-05-02)

Astronomers have discovered that looking at dust along the plane of our Milky Way Galaxy is a bit like looking into a frothy glass of beer. The dust between stars in our galaxy appears to be arranged like a foam with bubbles and voids -- churned by shocks and winds generated as stars cycle through their lives. This processed infrared image, based on data from NASA's IRAS satellite, maps the radiation from the edges of galactic dust clouds and reveals the complex distribution. The image covers an area of about 40x60 degrees centered on the galactic plane near the Cygnus region. It shows bright bubble-shaped and arc-like dust clouds around the supernova remnants and starbirth regions embedded in the galactic disk.
Standing on the Moon (1998-05-03)

Humans once walked on the Moon. Pictured above is the second person to stand on the lunar surface: Edwin "Buzz" Aldrin. During this Apollo 11 mission, Neil Armstrong (the first person to walk on the moon) and Buzz Aldrin landed on the Moon while Michael Collins circled in the Command Module above. The lunar team erected a plaque on the surface that reads: HERE MEN FROM THE PLANET EARTH FIRST SET FOOT UPON THE MOON JULY 1969 A.D. WE CAME IN PEACE FOR ALL MANKIND. The Apollo missions demonstrated that it is possible to land humans on the Moon and return them safely.
M57: The Ring Nebula (1998-05-04)

It looked like a ring on the sky. Hundreds of years ago astronomers noticed a nebula with a most unusual shape.  Now known as M57 or NGC 6720, the gas cloud became popularly known as the Ring Nebula. It is now know to be a planetary nebula, a gas cloud emitted at the end of a Sun-like star's existence.  As one of the brightest planetary nebula on the sky, the Ring Nebula can be seen with a small telescope in the constellation of Lyra. The Ring Nebula lies about 4000 light years away, and is roughly 500 times the diameter of our Solar System. In this recent picture by the Hubble Space Telescope, dust filaments and globules are visible far from the central star. This helps indicate that the Ring Nebula is not spherical, but cylindrical. Perhaps the Ring Nebula would appear differently if viewed sideways.
Aurora at Midnight (1998-05-05)

What's happening behind those trees?  Aurora. This picture was taken at midnight near Fairbanks, Alaska, and captures familiar trees, common clouds, and a glowing sky markedly different than a sunset. Particularly strange is the green auroral ring caused by ionized oxygen high in the Earth's atmosphere.  The small water droplets composing clouds reflect and absorb aurora light, giving clouds a reddish tinge.  The above picture was taken on September 20th of last year.  In the next few years the Sun will reach the most active part of its 11 year cycle, meaning more puffs of high energy solar particles will be released, and more spectacular auroras will occur when these particles strike the Earth.

Image credit: J. Curtis  
(U. Alaska Fairbanks)
Beijing Ancient Observatory (1998-05-06)

Did observatories exist before telescopes? One example that still stands today is the Beijing Ancient Observatory in China. Starting in the 1400s astronomers erected large instruments here to enable them to measure star and planet positions with increasing accuracy.  Pre-telescopic observatories throughout the world date back to before recorded history, providing measurements that helped to determine when to plant crops, how to navigate ships, and when religious ceremonies should occur. The above hand-painted lantern slide was originally taken in 1895.
A Powerful Gamma-ray Burst (1998-05-07)

Gamma-ray bursts are thought to be the most powerful explosions in the Universe, yet the cause of these high-energy flashes remains a mystery. Blindingly bright for space-based gamma-ray detectors the burst sources are so faint at visible wavelengths that large telescopes and sensitive cameras are required to search for them. The faint optical flash from a relatively intense gamma-ray burst detected on December 14th of last year seems to have originated in the galaxy indicated in this Hubble Space Telescope image - taken months after the burst had faded from view. Astronomers have recently announced that this galaxy's spectrum, recorded using the large Keck telescope atop Hawaii's Mauna Kea, indicates that it lies at a distance of about 12 billion light-years. The energy required to produce the observed flash of gamma-rays from this distance would be staggering! Some estimates suggest that in a few seconds the burster released the equivalent energy of several hundred supernovae (exploding stars). The eruption of such a large amount of energy in such a short time is so extreme that even exotic theoretical models of the bursters are being challenged. Could the bursts be caused by the cataclysmic merger of neutron stars with black holes ... or something as yet unknown?
A Gamma-Ray Burst Supernova? (1998-05-08)

Did a gamma-ray burst precede this supernova? This intriguing suggestion came to light yesterday with the discovery of an evolving supernova that is potentially coincident with the position of gamma-ray burst GRB 980425, which occurred just two weeks ago.  If true, this would tie together the two most violent phenomena known in the universe. The supernova, indicated by the arrow, appears to be somewhat unusual, for one reason because of its extremely bright radio emission. The host galaxy has a redshift of 0.0085, placing it at the relatively close distance of about 125 million light years away.  Today it remains undetermined whether the two events are related - perhaps the evolution of the supernova over the next few weeks will provide some clues.
The Water Vapor Channel (1998-05-09)

What alien planet's bizarre landscape lurks below these fiery-looking clouds? It's only Planet Earth, of course ... as seen on the Water Vapor Channel. Hourly, images like this one (an infrared image shown in false color) are brought to you by the orbiting Geostationary Operational Environmental Satellites' (GOES) multi-channel imagers. These instruments can produce images at the infrared wavelength of 6.7 microns or about 10 times the wavelength of visible light, recording radiation emitted by water vapor in the upper troposphere. Bright regions correspond to high concentrations of water vapor while dark spots are relatively dry areas. Atmospheric water vapor is invisible to the eye and produced by evaporation from the oceans. Convected upward in the tropical zones it affects the climate by contributing substantially to the greenhouse effect.
Skylab Over Earth (1998-05-10)

Skylab was an orbiting laboratory launched by a Saturn V rocket in May 1973.  Skylab was visited three times by NASA astronauts who sometimes stayed as long as two and a half months. Many scientific tests were performed on Skylab, including astronomical observations in ultraviolet and X-ray light. Some of these observations yielded valuable information about Comet Kohoutek, our Sun and about the mysterious X-ray background - radiation that comes from all over the sky. Skylab fell back to earth on 11 July 1979.