juliensimon/nasa-apod huggingface dataset:
Hale-Bopp Polarized 
Credit and Copyright: (1997-04-25)

Light polarization is familiar to many outdoor enthusiasts who use polarizing sunglasses to cut the glare of reflected light. These two views of comet Hale-Bopp also demonstrate the effect of polarization. At left is an "ordinary intensity image" of Hale-Bopp's coma taken April 14, while on the right a similar image made with polarizing filters represents the intensity of polarized light. The arcs visible in the polarized view probably correspond to concentrations of ejected cometary dust that produce polarization by reflecting sunlight. Any sort of reflection - from clumps of comet dust or the surface of your favorite lake or ski slope - can polarize light by causing the light waves to vibrate in a plane defined by the reflecting surface.
The Perseus Cluster of Galaxies 
Credit: (1997-04-26)

Here is one of the largest objects that anyone will ever see on the sky. Each of the fuzzy blobs in the above picture is a galaxy, together making up the Perseus Cluster, one of the closest clusters of galaxies. We view the cluster through the foreground of faint stars in our own Milky Way Galaxy. It takes light roughly 300 million years to get here from there, so we only see this cluster as it existed during the age of the dinosaurs. Also known as Abell 426, the center of Perseus cluster is a prodigious source of X-ray radiation, and so helps astronomers study how clusters formed and how gas and dark matter interact. The Perseus Cluster of Galaxies is part of the Pisces-Perseus supercluster of galaxies, which spans over 15 degrees and contains over 1000 galaxies.

Image credit: STScI,
AAO, UK-PPARC, ROE
Sputnik: Traveling Companion 

Courtesy: (1997-04-27)

Sputnik means "traveling companion". Despite the innocuous sounding name, the launch of the Earth's first "artificial moon", Sputnik 1, by the Soviets on October 4, 1957 shocked the free world, setting in motion events which resulted in the creation of NASA and the race to the Moon. Sputnik 1 was a 184 pound, 22 inch diameter sphere with four whip antennas connected to battery powered transmitters. The transmitters broadcast a continuous "beeping" signal to an astounded earthbound audience for 23 days. A short month later, on November 3, the Soviets followed this success by launching a dog into orbit aboard Sputnik 2.
Io's Sodium Cloud
Credit: (1997-04-28)

Io has its own cloud.  Jupiter's most active moon is visible on the left of the above false-color photograph, with its left side illuminated by sunlight.  But what is happening on Io's right side?  Here a plume of gas from the active volcano Prometheus also reflects sunlight.  A second active volcano, Pele, is also visible: marked by the red spot just below Io's center. Surrounding the moon is a yellow haze originating from gas ejected by Io's volcanos.  Sodium atoms in the gas cloud are particulary efficient at reflecting yellow light.  Several points of light are background stars.
Hale Bopp and Orion
Credit and Copyright: (1997-04-29)

Comet Hale-Bopp is still brighter than most constellations.  In fact, Comet Hale-Bopp may now hold the record for staying bright the longest. Last week the comet was photographed above in the same field as the constellation Orion, visible in the photograph's center, as well as with Sirius, the brightest star in the night, visible on the far left. Just below Comet Hale-Bopp on the right is the volcanic caldera known as White Mountain.. As the comet heads south, it has become visible to most of the world, now including many observers in the Earth's Southern Hemisphere.
Milky Way Molecule Map
Credit: (1997-04-30)

Where are the Milky Way's gas clouds and where are they going?  Stars form in gas clouds, and the motion of gas clouds tell us about the size and rotation speed of our own Milky Way Galaxy. But gas clouds are hard to detect - they are composed mostly of nearly invisible molecular hydrogen and helium. Fortunately, at least small amounts of heavier gases co-exist, one of them being carbon monoxide (CO), which is relatively easy to detect at radio wavelengths. Therefore, over the past decade, a team of astronomers have carefully mapped out the molecular sky to unprecedented clarity - to about four times previous resolution and about eight times previous sensitivity. The resulting map is shown above, rescaled and in false color, with dark blue being relatively low emission.  The band of our Milky Way Galaxy spans the middle. The data have not only helped our understanding of the Galaxy, but highlight a few mysteries too. For example: what causes the rapid speed of the gas near the Galactic Center?
A Galactic Cloud of Antimatter
Credit: (1997-05-01)

The center of our Milky Way Galaxy is full of surprises. Its latest spectacular is a mysterious cloud glowing in gamma rays produced by annihilating antimatter particles! Star Trek fans are all too familiar with the consequences of mixing matter (electrons) and antimatter (positrons) - the particles catastrophically annihilate converting their masses to energy according to Einstein's famous E=mc2. Positron/electron annihilation energy is emitted as gamma rays with photon energies of 511,000 electron volts. Searching for these high energy photons, the OSSE instrument onboard NASA's orbiting Compton Gamma Ray Observatory has recently produced this map of the Galactic Center (GC) region. As anticipated, it shows annihilation gamma rays as a bright spot at the GC with fainter horizontal emission from the galactic plane. Astoundingly, it also reveals a large and unexpected cloud of annihilation radiation, probably about 4,000 light years across, extending nearly 3,500 light years above the GC. What could have created this cloud? Associated with no previously known object, it seems to imply that a fountain of antimatter positrons streams from the GC. Present guesses about the source of the positrons include the violent and exotic environments surrounding starbirth, neutron star collisions, and black holes at the GC. Are there other such clouds in our Galaxy?
X-Rays From IC 443
Credit: (1997-05-02)

The life-cycles of stars help drive the ecology of our Galaxy, churning, processing, and redistributing matter. Massive stars reach a spectacular evolutionary endpoint - supernovae explosions which blast off their outer layers, violently merging stellar material with the gas and dust of the Milky Way. The supernova remnant IC 443 is typical of the aftermath. Seen in this false color X-ray image are the shocked, expanding shells of gas from a star which exploded thousands of years ago. Known to be interacting with galactic molecular clouds, the expanding supernova remnant was also recently discovered to have regions of intense higher energy X-ray emission (coded blue in this map) near the molecular cloud boundaries. This X-ray emission may indicate that electrons are being accelerated within the remnant, gaining in energy as they surf back and forth across the expanding shock wave. If so, IC 443 could also be one source of our Galaxy's puzzling high energy cosmic-rays.
Giant Cluster Bends, Breaks Galaxy Images
Credit: (1997-05-03)

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.
The Last Moon Shot

Credit: (1997-05-04)

In 1865 Jules Verne predicted the invention of a space capsule that could carry people. In his science fiction story "From the Earth to the Moon", he outlined his vision of a cannon in Florida so powerful that it could shoot a "Projectile-Vehicle" carrying three adventurers to the Moon. Over 100 years later, NASA, guided by  Wernher Von Braun's vision, produced the  Saturn V rocket. From a spaceport in Florida, this rocket turned Verne's fiction into fact, launching 9 Apollo Lunar missions and allowing 12 astronauts to walk on the Moon. Pictured is the last moon shot, Apollo 17, awaiting a night launch in December of 1972. Spotlights play on the rocket and launch pad while the full Moon looms in the background.  Humans have not walked on the lunar surface since. Should we return to the Moon?
Sunset with Hale-Bopp at Keck
Credit: (1997-05-05)

A famous star cluster and observatory highlight this picture of Comet Hale-Bopp. Taken last week from the observatory summit of Hawaii's Mauna Kea Volcano, the dome of the new 10-meter Keck II telescope appears silhouetted on the lower left.  Comet Hale-Bopp is visible on the upper right, and the Pleiades star cluster is visible below the comet. Normally sunset and clouds are to be avoided when making astronomical observations, but Comet Hale-Bopp is not a normal astronomical object. In fact, were it cloudless, Professor Keel would be inside NASA's nearby IRTF dome preparing to observe something else.  Comet Hale-Bopp continues to look impressive, although it is fading and moving towards the south.
NGC4039: Starbirth and Galaxy Death
Credit: (1997-05-06)

Do star clusters form when galaxies collide? Quite possibly, according to Hubble Space Telescope  observations of the "Antennae", two galaxies thought to be in the early stages of a collision. As NGC 4038 and NGC 4039 slowly merge, the combined gravity of each pulls the other apart, huge gas clouds collide, and new bright stars and dark dust are dispersed.  Many blue knots of stars appear to be newly formed globular clusters.  Red star knots are particularly interesting, as they might be globular clusters that have not yet expelled early dust from their system. The above picture is centered around the smaller of the two interacting galaxies: NGC 4039. The color contrast in the above three-color mosaic was chosen to highlight extended features.

Image credit: AURA
Ultraviolet Venus
Credit: (1997-05-07)

The forecast for Venus is cloudy, cloudy, cloudy. Although similar to the Earth in size and mass, Venus' slightly closer orbit to the Sun create for it a much thicker atmosphere and a much hotter surface. The thick atmosphere was photographed above in ultraviolet light in 1979 by the Pioneer Venus Orbiter. Whether or not Venus has a moon was the center of a great controversy in the 1700s and 1800s. Today we know Venus has no natural satellites.  Venus's extremely uncomfortable climate was likely caused by a runaway greenhouse effect.  Could Earth ever undergo runaway greenhouse heating like Venus?
Detailing Hale-Bopp
Credit and Copyright: (1997-05-08)

This enhanced composite image detailing structure in the coma and dust tail of Hale-Bopp was recorded May 5 - one day before the comet's passage from north to south across the plane of Earth's orbit. As the comet descends into murky twilight for northern hemisphere observers it will become increasingly easy to view from the south. Along with Southern Hemisphere observers, astronomers and a fleet of spacecraft of the International Solar-Terrestrial Physics program have been anxiously awaiting this north/south crossing.  The comet's interaction with the changing equatorial solar wind and magnetic field during this crossing is expected to produce distortions and disconnections of Hale-Bopp's ion tail. Whisker-like structures, probably part of the ion tail, are visible above extending from the lower left of the bright coma.
Apollo 12: Self-Portrait
Credit: (1997-05-09)

Is it art? In November of 1969, Apollo 12 astronaut-photographer Charles "Pete" Conrad recorded this masterpiece while documenting colleague Alan Bean's lunar soil collection activities on the Oceanus Procellarum. The image is dramatic and stark. Bean is faceless - the harsh environment of the Moon's Ocean of Storms is echoed in his helmet's perfectly composed reflection of Conrad and the lunar horizon. Works of photojournalists orginally intent on recording the human condition on planet Earth, such as Lewis W. Hine and Margaret Bourke-White are widely regarded as photographic art. Similarly many documentary astronomy and space images can be appreciated for their artistic and esthetic appeal.
Apollo 15's Home on the Moon (1997-05-10)

Could you ever call this place home? The lunar module shown above, named "Falcon," served as home for Apollo 15 astronauts David Scott and James Irwin during their stay on the Moon in July and August 1971. Meanwhile, astronaut Alfred Worden circled in the command module overhead. Harsh sunlight on the grey lunar surface lends the image an eerie quality, while the Lunar Apennine Mountains frame the background with Mount Hadley Delta visible on the right. Visible in the foreground are tracks from the first Lunar Roving Vehicle, an electric car which enabled the astronauts to explore extended areas on the lunar surface.  Apollo 15 confirmed that most lunar surface features were created by impacts. Rocks returned by the Apollo 15 crew included green glasses whose formation mechanism is still unclear.
M42: A Mosaic of Orion's Great Nebula (1997-05-11)

The Great Nebula in Orion, an immense, nearby starbirth region, is probably the most famous of all astronomical nebulae.  Here, 15 pictures from the Hubble Space Telescope have been mosaicked to cover the inner 2.5 light years of the nebula and illustrate its diverse nature. In addition to housing a bright open cluster of stars known as the Trapezium, the Orion Nebula contains many stellar nurseries. These nurseries contain hydrogen gas, hot young stars, proplyds, and stellar jets spewing material at high speeds. Most of the filamentary structures visible in this image are actually shock waves - fronts where fast moving material encounters slow moving gas. Shocks are particularly apparent near the bright stars in the lower left of the picture. The Orion Nebula is about 1500 light years distant, located in the same spiral arm of our Galaxy as the Sun.
Lightning on Jupiter (1997-05-12)

Does lightning occur only on Earth? Spacecraft in our Solar System have detected radio signals consistent with lightning on other planets, including Venus, Jupiter, Saturn, Uranus, and Neptune.  In the above photograph, optical flashes from Jupiter were photographed recently by the Galileo orbiter. Each of the circled dots indicates lightning.  The numbers label lines of  latitude.  The size of the largest spot is about 500 kilometers across and might be high clouds illuminated by several bright lightning strokes.
Optical Transient Near GRB970508 Shows Distant Redshift (1997-05-13)

The GRB distance scale controversy may have just ended with a flash. Gamma Ray Bursts (GRBs) are powerful explosions occurring in seemingly random positions on the sky.  They are so featureless and so poorly resolved, however, that their distances could not be determined.  Last Thursday, May 8th, the orbiting Beppo-Sax Satellite detected and precisely located a GRB (GRB970508), quickly relaying its position to astronomers. Within hours, many of the world's most powerful telescopes were re-pointed in the direction of the new GRB.  There they found a faint but variable optical source: a potential counterpart to the gamma-ray burst that was getting brighter.  Most importantly, continued scrutiny showed that this optical transient had absorption lines with a redshift of about 0.8 - the distance scale of galaxies and quasars. If this source and GRB970508 are related, the GRB itself must be many billions of light-years away and the 30-year controversy on the distance scale to GRBs will draw dramatically to a close. GRBs could move from the realm of astronomical mystery to useful beacons of the early universe.  Above is a "negative" image of the GRB970508 field, located near the north celestial pole, taken hours after the initial flash of gamma rays. The faint optical transient source is inside the box. North is up and the image is 2.5 arcminutes across (about a tenth the size of the full moon).
Hale-Bopp's Fickle Ion Tail (1997-05-14)

What's happening to Comet Hale-Bopp's blue ion tail?  The comet's ion tail is fluctuating more rapidly as it passes a region of changing solar wind.  As the comet passes from north to south, it crosses the plane of the Sun's equator, where the solar magnetic field changes direction. Ions from the solar wind, which cause Comet Hale-Bopp's ion tail, act unpredictably here. Therefore, Comet Hale-Bopp's ion tale may show unusual structure or even a disconnection - where the tail appears to break off and then reestablish itself later. The above picture, taken April 30th, indeed shows unusual structure in the blue ion tail.

Image credit: A. Dimai, D. Ghirardo and R. Volcan
 (Col Druscie Obs.,
ACC)
Hale-Bopp: Climbing Into Southern Skies (1997-05-15)

Fighting the glow of the setting sun and the city lights of Cape Town, South Africa, comet Hale-Bopp is just visible near the center of this panoramic view - photographed on May 3rd. In the foreground is the Strand beach front, about 50 km East of Cape Town, while the Cape Peninsula mountain ranges can be seen at the left along the horizon. The bright star visible above and to the left of the comet is the red giant Aldebaran in the constellation Taurus. As Hale-Bopp continues its outbound journey during the month of May it will climb higher into evening southern skies. Still a bright comet it is now providing an enjoyable and much anticipated showing for Southern Hemisphere observers.

Image credit: Jurgen Blignaut
Signed, "A Black Hole" (1997-05-16)

This artistic image is actually the signature of a supermassive black hole in the center of distant galaxy M84 - based on data recently recorded by Hubble's new Space Telescope Imaging Spectrograph (STIS). Very near black holes the force of gravity is so strong that even light can not escape ... but the presence of a black hole can also be revealed by watching matter fall into it. In fact, material spiraling into a black hole would find its speed increasing at a drastic rate. These extreme velocity increases provide a "signature" of the black hole's presence. STIS relies on the Doppler effect to measure gas velocity rapidly increasing to nearly 240 miles per second within 26 light years of the center of M84, a galaxy in the Virgo Cluster about 50 million light years away. The STIS data show that radiation from approaching gas, shifted to blue wavelengths left of the centerline, is suddenly redshifted to the right of center indicating a rapidly rotating disk of material near the galactic nucleus. The resulting sharp S-shape is effectively the signature of a black hole estimated to contain at least 300 million solar masses. Do all galaxies have central black holes?
7,000 Stars And The Milky Way (1997-05-17)

This panorama view of the sky is really a drawing. It was made in the 1940s under the supervision of astronomer Knut Lundmark at the Lund Observatory in Sweden. To create the picture, draftsmen used a mathematical distortion to map the entire sky onto an oval shaped image with the plane of our Milky Way Galaxy along the center and the north galactic pole at the top. 7,000 individual stars are shown as white dots, size indicating brightness. The "Milky Way" clouds, actually the combined light of dim, unresolved stars in the densely populated galactic plane, are accurately painted on, interrupted by dramatic dark dust lanes. The overall effect is photographic in quality and represents the visible sky. Can you identify any familiar landmarks or constellations? For starters, Orion is at the right edge of the picture, just below the galactic plane and the Large and Small Magellanic Clouds are visible as fuzzy patches in the lower right quadrant.

Image credit: Lund Observatory,
The First Explorer (1997-05-18)

The first US spacecraft was Explorer 1. The cylindrical 30 pound satellite was launched (above) as the fourth stage of a Jupiter-C rocket (a modified US Army Redstone ballistic missile) and achieved orbit on January 31, 1958. Explorer I carried instrumentation to measure internal and external temperatures, micrometeorite impacts, and an experiment designed by James A. Van Allen to measure the density of electrons and ions in space. The measurements made by Van Allen's experiment led to an unexpected and startling discovery -- an earth-encircling belt of high energy electrons and ions trapped in the magnetosphere now known as the Van Allen Belt. Explorer I ceased transmitting on February 28 of that year but remained in orbit until March of 1970.
Barred Spiral Galaxy NGC 1365 (1997-05-19)

Many spiral galaxies have bars across their centers. Even our own Milky Way Galaxy is thought to have a bar, but perhaps not so prominent as the one in NGC 1365, shown above.  The persistence and motion of the bar imply relatively massive spiral arms. The placements of bright young blue stars and dark dust lanes also indicate a strong rotating density wave of star formation. NGC 1365 is a member of the Fornax Cluster of Galaxies. Because NGC 1365 is relatively nearby, simultaneous measurements of its speed and distance are possible, which help astronomers estimate how fast our universe is expanding.

Image credit: D. Malin (AAO),
AATB
Shells in the Egg Nebula (1997-05-20)

The Egg Nebula is taking a beating. Like a baby chick pecking its way out of an egg, the star in the center of the Egg Nebula is casting away shells of gas and dust as it slowly transforms itself into a white dwarf star. The above picture was taken by the newly installed Near Infrared Camera and Multi-Object Spectrometer (NICMOS) now on board the Hubble Space Telescope. A thick torus of dust now surrounds the star through which the shell gas is escaping.  Newly expelled gas shells escape in beams as can be seen in the original HST image and in the recently released image shown above. This infrared image is coded in false color to highlight two different types of emission. The red light represents hot hydrogen gas heated by the collisions of expanding shells. The blue light represents light from the central star scattered by the dust in the nebula.  It takes light about 3000 years to reach us from the Egg Nebula, which is hundreds of times the size of our Solar System.
GRB970508 Delivers Predicted Radio Emission (1997-05-21)

New evidence bolsters once controversial claims that Gamma-Ray Bursts (GRBs) are the most powerful explosions ever found by humanity.  Two weeks ago, an average GRB became instantly historic when prompt, coincident X-ray and optical emissions were identified.  The glow in visible light was particularly interesting because it showed color gaps indicative of absorbing gas at the very distant redshift of 0.8. If GRBs do occur this far into our universe, then radio emission was predicted to peak about a week after the optical emission. Now, late reports do indicate that radio emission has been detected and does peak a week later than the optical. This adds credibility to the claim that these bursts of gamma-radiation indeed occur far from our familiar home Galaxy. Shown above are two photos highlighting the changing optical emission from GRB970508 when it was increasing in brightness.  Now light from this enigmatic explosion is fading.