juliensimon/nasa-apod huggingface dataset:
The Virgo Cluster: Hot Plasma and Dark Matter (1996-04-19)

This ROSAT image of the Virgo cluster of galaxies reveals a hot X-ray emitting plasma or gas with a temperature of 10-100 million degrees pervading the cluster. False colors have been used to represent the intensity of X-ray emission. The large area of X-ray emission, just below and left of center, is about 1 million light-years across. The giant elliptical galaxy M87, the biggest member of the cluster, is centered in that area while other cluster members are scattered around it. By adding up the amount of X-ray emitting gas astronomers have found that its total mass is up to 5 times the total mass of the cluster galaxies themselves - yet all this matter still does not produce nearly enough gravity to keep the cluster from flying apart! Where is the unseen mass? Because galaxy clusters are the largest structures in the Universe, this mysterious Dark Matter must dominate the cosmos but its nature is still an open question.   Information: The Scale of the Universe Debate in April 1996
Apollo 17 Lunarscape: A Magnificent Desolation (1996-04-20)

Buzz Aldrin, Apollo 11 Lunar Module pilot and the second human to walk on the Moon described the lunar landscape as "a magnificent desolation". Dramatic pictures from the Apollo missions to the moon's surface testify to this apt turn of phrase. Near the Apollo 17 landing site, Family Mountain (center background) and the edge of South Massif (left) frame the lunarscape in this photo of astronaut Harrison Schmitt working alongside the lunar roving vehicle. Schmitt and fellow astronaut Eugene Cernan were the last to walk on this magnificent desolation.   Information: The Scale of the Universe Debate in April 1996
A Supernova in the Whirlpool (1996-04-21)

In 1994, a new star in a distant galaxy was seen by amateur astronomers, who alerted the world to their discovery of a supernova. Near the nucleus of spiral galaxy M51, popularly known as the Whirlpool, this supernova (1994I) is identified as the bright spot indicated by the arrow in the lower left of this Hubble Space Telescope image. Supernovae are violent death explosions of stars that eject radioactive debri clouds. They are often discovered by amateur observers dedicated to systematic searches of the sky and are of intense interest to astronomers who hope to learn what kind of stars generate these explosions and what chemical elements are produced and mixed into space. Distances to these these intrinsically bright events can also be determined, providing crucial yardsticks for measuring the Scale of the Universe.
At the Edge of the Helix (1996-04-22)

While exploring the inner edge of the Helix Nebula with the Hubble Space Telescope's Wide Field Planetary Camera 2, astronomers were able to produce this striking image - rich in details of an exotic environment. This planetary nebula, created near the final phase of a sun-like star's life, is composed of tenuous shells of gas ejected by the hot central star. The atoms of gas, stripped of electrons by ultraviolet radiation from the central star, radiate light at characteristic energies allowing specific chemical elements to be identified. In this image, emission from nitrogen is represented as red, hydrogen emission as green, and oxygen as blue. The inner edge of the nebula (the direction to the central star) is toward the bottom right. Clearly visible close to the inner edge are finger shaped "cometary knots".
Comet Hyakutake on a Starry Night (1996-04-23)

It was a starry night in April (April 9th, 1996, 9:32 pm CDT to be exact) near Lone Jack, Missouri when Comet Hyakutake graced this astronomically rich field. Making an appearance as the brilliant evening star, Venus is overexposed at the far left. Just below Venus and slightly to the right, the Pleiades star cluster (M45) glistens. On the right hand side of the image, the comet itself shows a bright blue tail extending upwards past the nearby star cluster in the constellation Perseus (top right, the Alpha Persei Group). Hyakutake, receding from the Earth and appoaching the Sun, will sink into the western horizon at sunset in late April, disappearing from Northern Hemisphere skies.  Latest Comet Hyakutake images: APOD Hyakutake Archive, JPL, Fayetteville Observer-Times, NASA's Night of the Comet, ICSTARS, Jerry Lodriguss, ScienceWeb, Crni Vrh Obs.,

Image credit: Vic 
Winter, Courtesy
ICSTARS
Giant Cluster Bends, Breaks Galaxy Images (1996-04-24)

What are those strange blue objects? Many are images of a single, unusual, beaded, blue, ring-like galaxy which just happens to line-up behind a giant cluster of galaxies. Cluster galaxies here appear yellow and -- together with the cluster's dark matter -- act as a gravitational lens. A gravitational lens can create several images of background galaxies, analogous to the many points of light one would see while looking through a wine glass at a distant street light. The distinctive shape of this background galaxy -- which is probably just forming -- has allowed astronomers to deduce that it has separate images at 4, 8, 9 and 10 o'clock, from the center of the cluster. Possibly even the blue smudge just left of center is yet another image! This spectacular photo from HST was taken in October 1994. The first cluster lens was found unexpectedly by Roger Lynds (NOAO) and Vahe Petrosian (Stanford) in 1986 while testing a new type of imaging device.  Lensed arcs around this cluster, CL0024+1654, were first discovered from the ground by David Koo (UCO Lick) in 1988.   Congratulations: Hubble Space Telescope is six years old today!
In the Center of the Whirlpool (1996-04-25)

In the center of M51, a spiral galaxy 23 million light-years away, astronomers have identified a dense region of young stars. The graceful swirling arcs of this galaxy's spiral arms have inspired its popular name, "The Whirlpool Galaxy". This Hubble Space Telescope image of its nucleus reveals the light from millions of stars, perhaps one tenth the age of the Sun, packed into the bright central region. This area is a mere 80 light-years across -- the stars are so crowded here that the view from a hypothetical planet orbiting one of these distant suns would be of a continuously bright sky! The dark "Y" shape visible within this region is an indication that lanes of dust are present, partially blocking the intense starlight.
A Giant Globular Cluster in M31 (1996-04-26)

G1, pictured above, is the brightest known globular cluster in the whole Local Group of galaxies. Also called Mayall II, it orbits the center of the largest nearby galaxy: M31.  G1 contains over 300,000 stars and is almost as old as the entire universe.  In fact, observations of this globular cluster show stars as old as the oldest of the roughly 250 known globular clusters in our own Milky Way Galaxy.  This image was taken with the Hubble Space Telescope in July of 1994.  It shows, for the first time, the same fine detail in a distant globular cluster as can be discerned from a ground-based telescope of a globular cluster in our own Galaxy.
Apollo 14: Rickshaw Tracks Across the Moon (1996-04-27)

Brilliant sunlight glints off tracks gently winding across the the Moon's Frau Maro highlands. The tracks were made by the Apollo 14 crew's two-wheeled Modularized Equipment Transporter (MET), fondly known as the "rickshaw". The MET was designed as an aid to surface exploration. Serving as a workbench with a place for the lunar handtools, it also carried cameras, sample containers, spare film and a Lunar Surface Penetrometer. The path outlined in the lunar soil leads away from Alan Shepard and Edgar Mitchell's Lunar landing Module, tracing the path of their first venture across the lunar landscape.
The Sun Sets on Comet Hyakutake (1996-04-28)

Comet Hyakutake is seen here just as the Sun sets on April 22. As April draws to a close, Comet Hyakutake will be visible only just after sunset and will be hard to discern against the brightly lit sky. Unfortunately, Comet Hyakutake did not brighten as much as hoped during its journey to the inner Solar System, and is now not supposed to get as bright as it did when it passed the Earth in late March. Nevertheless, Comet Hyakutake is still a bright comet and spectacular sight.  Latest Comet Hyakutake images: APOD Hyakutake Archive, JPL, Fayetteville Observer-Times, NASA's Night of the Comet, ICSTARS, Jerry Lodriguss, ScienceWeb, Crni Vrh Obs.,
Saturn's Rings Seen Sideways (1996-04-29)

Saturn's rings are actually very thin.  This picture from the Hubble Space Telescope was taken on August 6, 1995 when the rings lined up sideways as seen from Earth.  Saturn's largest moon Titan is seen on the left, and Titan's shadow can be seen on Saturn's cloud tops!  Titan itself looks a brownish color because of its thick atmosphere. Four other moon's of Saturn can be seen just above the ring plane, which are, from left to right: Mimas, Tethys, Janus, and Enceladus. If you look carefully, you will note that the dark band across the planet is actually the shadow of the rings, and is slightly displaced from the real rings - which are best seen away from the planet.  Saturn's rings are not solid - they are composed of ice chunks which range in size from a grain of sand to a house.
Uranus' Ring System (1996-04-30)

The rings of Uranus are thin, narrow, and dark compared to other planetary ring systems. Brightened artificially by computer, the ring particles reflect as little light as charcoal, although they are really made of ice chucks darkened by rock. This false-color, infrared picture from the Hubble Space Telescope taken in July 1995 shows the rings in conjunction to the planet. The infrared light allows one to see detail in different layers of Uranus' atmosphere, which has been digitally enhanced with false color. Three other planets in our Solar System are known to have rings: Jupiter, Saturn, and Neptune.  Four of Uranus' moons are visible outside the ring plane. The rings of Uranus were discovered from ground-based observations in 1977.
Comet Hyakutake and a Cactus (1996-05-01)

Comet Hyakutake is shown photographed the night of March 27 in Arizona, USA, with a cactus in the foreground.  Polaris, the north star, is the bright star seen just to the upper right of the comet's head. Today Comet Hyakutake reaches its closest approach to the Sun. Comet Hyakutake is now at its intrinsic brightest, but because of its distance from the Earth, it will appear less bright to us than it did during its closest approach to the Earth in late March. In fact, due to the comet's angular proximity to the Sun, it will difficult to see at all from the Earth! Comet Hyakutake will reach less than one quarter of the Earth-Sun distance - inside the orbit of Mercury. Comet Hyakutake will not venture near the Sun again for another about 15,000 years.   Latest Comet Hyakutake images: APOD Hyakutake Archive, JPL, Fayetteville Observer-Times, NASA's Night of the Comet, ICSTARS, Jerry Lodriguss, ScienceWeb, Crni Vrh Obs.,
The Tails of Comet Hyakutake (1996-05-02)

What makes comet tails so colorful?  This photograph of Comet Hyakutake was taken the night of April 18th and highlights different components of the tail. The gold and red tail features are dust, made predominately of little bits of rock and carbon. The dust tail shines by reflecting sunlight. Extending past the dust tail is the comet's ion tail, shown here glowing in blue. The ion tail is composed mostly of ions of water, carbon monoxide, and cyanogen. The ion tail glows by emitting light when elections re-combine with electrically charged ions to make uncharged molecules. The photograph was taken just north of Kansas City, Missouri, USA.   Latest Comet Hyakutake images: APOD Hyakutake Archive, JPL, Fayetteville Observer-Times, NASA's Night of the Comet, ICSTARS, Jerry Lodriguss, ScienceWeb, Crni Vrh Obs.,

Image credit: Vic 
Winter, Courtesy
ICSTARS
The Milky Way Near the Southern Cross (1996-05-03)

This breathtaking patch of sky would be above you were you to stand at the south pole of the Earth. Just above and to the right of the photograph's center are the four stars that mark the boundaries of the famous Southern Cross.  At the top of this constellation, also known as "The Crux," is the orange star Gamma Crucis. The band of stars, dust, and gas crossing the middle of the photograph is part our Milky Way Galaxy.  In the very center of the photograph is the dark Coal Sack Nebula, and the bright nebula on the far right is the Eta Carina Nebula. The southern cross is so famous a constellation that it is depicted on the Australian National Flag.

Image credit: Greg
Bock, Courtesy 
Southern
Astronomical Society
Astro 1 In Orbit (1996-05-04)

In December of 1990, the Space Shuttle Columbia carried an array of astronomical telescopes high above the Earth's obscuring atmosphere to observe the Universe at ultraviolet and x-ray wavelengths. The telescopes, known by the acronyms UIT, HUT, WUPPE, and BBXRT, are seen here in Columbia's payload bay against a spectacular view of the constellation Orion. The ultraviolet telescopes were mounted on a common structure - HUT is visible in this view along with a star tracker (the silver cone at the left). The mission studied solar system, galactic, and extra-galactic sources.
Planet Near a Galaxy Core (1996-05-05)

What would the night sky look like if you lived on a planet near the center of a galaxy? Now imagine that this galaxy houses a black hole billions of times more massive than a star. From this spectacular vantage point, the sky might look like the above illustration. This drawing is based on recent observations of the center of NGC 4261, made by the Hubble Space Telescope.  Results indicate that a disk of dust 800-light years wide surrounds the black hole. The hypothetical planet depicted above lies within this disk. The black hole itself heats gas to white-hot temperatures, generating light that is reddened when reflected off the dust. Jets shoot off from the poles of the black hole, perpendicular to the disk. However, friction with the dust and gas would cause planets near the black hole to spiral in and disappear forever.  NASA has recently announced a new initiative to search for Earth-like planets in our Galaxy.
Southern Lights and Shuttle Glow (1996-05-06)

A background of distant stars, sinuous and spiky bands of Southern Lights (Aurora Australis), and the faint glow of charged plasma (ionized atomic gas) surrounding the Space Shuttle Discovery's engines give this photo from the STS-39 mission an eerie, otherworldly look. This image reflects Discovery's April 1991 mission well - its payload bay (PLB) was filled with instruments designed to study celestial objects, aurora and atmospheric phenomena, and the low Earth orbit environment around the PLB itself. The aurora seen here are at a height of about 50-80 miles and caused by charged particles in the solar wind, channeled through the van Allen Radiation Belts which excite atoms of oxygen in the upper atmosphere.
The Clouds of Neptune (1996-05-07)

These Hubble Space Telescope (HST) images reveal glimpses of the dynamic atmosphere of Neptune, the Solar System's most distant gas giant planet. The first close-up of Neptune's clouds was provided by NASA's Voyager 2 spacecraft during its August 1989 flyby, giving a tantalizingly brief look. From its vantage point in Earth orbit, HST can patiently watch in detail as the planet's massive weather systems change over time. These three pictures, taken during October and November 1994 when Neptune was only 2.8 billion miles distant, show the planet's characteristic aqua-blue colored atmosphere highlighted by pink, high altitude clouds and streaked with dark bands. The aqua-blue color is caused by gaseous methane which absorbs red light. Here, the high altitude clouds were intentionally given a pink tint to indicate that they were imaged in near infrared light - in true color images they would appear white.
Neptune's Great Dark Spot: Gone But Not Forgotten (1996-05-08)

When NASA's Voyager 2 spacecraft flew by distant Neptune in August of 1989, astronomers were shocked. Since Neptune receives only 3 percent the sunlight Jupiter does, they expected to find a dormant, dark, frigid planet. Instead, the Voyager images revealed evidence of a dynamic and turbulent world. One of the most spectacular discoveries was of the Great Dark Spot, shown here in close-up.  Surprisingly, it was comparable in size and at the same relative southern latitude as Jupiter's Great Red Spot, appearing to be a similar rotating storm system. Winds near the spot were measured up to 1500 miles per hour, the strongest recorded on any planet. The Voyager data also revealed that the Great Dark Spot varied significantly in size during the brief flyby.  When the Hubble Space Telescope viewed the planet in 1994, the spot had vanished -- only to be replaced by another dark spot in the planet's northern hemisphere!
Supernova Remnant: Cooking Elements In The LMC (1996-05-09)

Massive stars cook elements in their cores through nuclear fusion. Starting with the light elements of hydrogen and helium, their central temperatures and pressures produce progressively heavier elements, carbon, oxygen, nitrogen, etc. up through iron. At the end of their lives they explode in a spectacular supernova, scattering these elements into space, contributing material to the formation of other stars and star systems. In fact, the elements making up life on Earth were baked in such a stellar oven! This Hubble Space Telescope image of a supernova remnant known as N132D in the Large Magellanic Cloud (LMC) allows astronomers to explore the details of this nuclear processing and mixing. It reveals luminous clouds of cooked supernova debris energized by shocks -- singly ionized sulfur appears red, doubly ionized oxygen, green, and singly ionized oxygen, blue. The region shown above is about 50 lightyears across.
Henize 70:  A SuperBubble In The LMC (1996-05-10)

Massive stars (tens of times the mass of the Sun) profoundly affect their galactic environment. Churning and mixing the clouds of gas and dust between the stars, they leave their mark in the compositions and locations of future generations of stars and star systems. Dramatic evidence of this is beautifully illustrated in our neighboring galaxy, the Large Magellanic Cloud (LMC), by the lovely ring shaped nebula, Henize 70 (also known as N70 and DEM301).  It is actually a luminous "superbubble" of interstellar gas about 300 lightyears in diameter, blown by winds from hot, massive stars and supernova explosions, its interior filled with tenuous hot expanding gas. These superbubbles offer astronomers a chance to explore this crucial connection between the lifecycles of stars and the evolution of galaxies.

Image credit: Anglo-Australian Telescope
Board
Sunlight Through Saturn's Rings (1996-05-11)

Normally, Earth based astronomers view Saturn's spectacular ring system fully illuminated by reflected sunlight. However, this November 1995 Hubble Space Telescope composite image was made to take advantage of an unusual perspective, with the Sun actually illuminating the rings from below. The three bright ring features are visible because the rings themselves are not solid. Composed of many separate chunks of rocky, icy material, the rings allow the scattered sunlight to pass through them -- offering a dramatic demonstration that they are not continuous, uninterrupted bands of material.
Tracking Saturn's Moons (1996-05-12)

These five pairs of Hubble Space Telescope images track some of Saturn's moons as they orbit the ringed planet. A pair of images was taken every 97 minutes on November 21, 1995 with the Wide Field Planetary Camera-2, the normally bright ring system appearing nearly edge-on. In the top pair, the large bright moon Dione hangs above center while the smaller moons Pandora, Prometheus, and Mimas (top right image) appear near the planet's disk close to the outer ring. By the second and third pair of images, moons Rhea and Epimetheus have joined the dance. During the Saturn ring plane crossings, the reduction in light from the edge-on rings provided an opportunity for astronomer's to explore Saturn's complex moon system and search for elusive undiscovered satellites.
Hubble's Constant and The Expanding Universe (I) (1996-05-13)

Our Universe is expanding. Distant galaxies appear to recede from us at ever-increasing speeds. What is the rate of expansion? How long has it been expanding? What will be its ultimate fate? Two groups of astronomers are searching vigorously for answers to these fundamental questions using the Hubble Space Telescope (HST). The teams have recently announced conflicting measurements of the Hubble constant, a number which represents the expansion rate of the Universe. Astronomer Wendy Freedman and her collaborators have used pulsating stars called Cepheids to measure the distance to galaxies like the Fornax cluster barred spiral galaxy NGC1365 shown above. The ground based photo (left) shows an inset locating the HST image (right) which Freedman and team have used to identify some 50 Cepheids. Their distance and velocity measurements determine Hubble's constant to be about 80 kilometers per second per megaparsec which means that galaxies one megaparsec (3 million lightyears) distant appear to recede from us at a speed of 80 kilometers per second. Conflicting results indicating a substantially slower expansion rate (smaller Hubble constant) are being reported by astronomer Allan Sandage and collaborators. The value of Hubble's constant was recently the subject of a popular public debate titled "The Scale of the Universe 1996: The Value of Hubble's Constant".
Hubble's Constant and The Expanding Universe (II) (1996-05-14)

In this century, the discovery that the Universe is expanding has produced a revolution in human thought about the Cosmos. American astronomer Edwin Hubble played a major role in this profound discovery, coining the "Hubble constant". This single number describes the rate of the cosmic expansion, relating the apparent recession velocities of external galaxies to their distance. Two groups of astronomers trying to measure this fundamental constant using the Hubble Space Telescope (HST) are continuing to report conflicting results. One group, led by astronomer Allan Sandage, measures distances to galaxies using pulsating Cepheid variable stars and supernovae observed in galaxies like the Virgo Cluster spiral galaxy, NGC4639, shown above. This galaxy is the most distant one to which Cepheid-based determinations have been made and was also the site of a well-studied 1990 supernova. Their results favor a relatively small Hubble constant (slow expansion rate) of about 55 kilometers per second per megaparsec which means that galaxies one megaparsec (3 million lightyears) distant appear to recede from us at a speed of 55 kilometers per second. A substantially faster expansion rate (larger Hubble constant) is being reported by astronomer Wendy Freedman and collaborators, also based on HST data. The value of Hubble's constant was recently the subject of a popular public debate titled "The Scale of the Universe 1996: The Value of Hubble's Constant".
The Milky Way Near the Northern Cross (1996-05-15)

This beautiful image of the sky near the bright star Deneb (just above center) reveals the stars, nebulae, and dark clouds along the plane of our Milky Way Galaxy as seen from the northern hemisphere (near Columbia Missouri, USA). Just below Deneb lies the suggestively shaped North America emission nebula. Deneb is the brightest star in the constellation Cygnus, located in the tail of this celestial swan. Cygnus contains the asterism known as the Northern Cross and marks one side of the "Great Rift" in the Milky Way, a series of dark obscuring dust clouds which stretches on through the constellation Sagittarius. Deneb defines the top of the Northern Cross while the body of the cross extends past the upper right corner of the picture. Cygnus also harbors the most famous candidate for a black hole in our galaxy, Cygnus X-1.

Image credit: Andy Steere