juliensimon/nasa-apod huggingface dataset:
The Center of NGC 6251 is Glowing (1997-09-12)

Something is lighting up the center of galaxy NGC 6251. Leading speculation holds that it is a large black hole not shrouded by gas and dust typically found near the center of a galaxy.  Observations taken with the Hubble Space Telescope and released earlier this week indicate a new perspective on the strange beasts that rule the centers of galaxies: a bright central object that is illuminating a surrounding material disk, shown in blue.  The lack of reflection from the upper part of the disk indicates that this disk is warped in shape. Although not visible in the above composite image, a huge plasma jet streams out from the central object, perpendicular to the warped disk.
Kepler Discovers How Planets Move (1997-09-13)

Johannes Kepler used simple mathematics to describe how planets move. Kepler was an assistant to the most accurate astronomical observer of the time, Tycho Brahe. Kepler was able to use Brahe's data to show that planets move in ellipses around the Sun (Kepler's First Law), that planets move proportionally faster in their orbits when they are nearer the Sun (Kepler's Second Law), and that more distant planets take proportionally longer to orbit the Sun (Kepler's Third Law). Kepler lived from 1571 to 1630, during the time of discovery of the telescope. Kepler was one of the few vocal supporters of Galileo's discoveries and the Copernican system of planets orbiting the Sun instead of the Earth.
MyCn18: An Hourglass Nebula (1997-09-14)

The sands of time are running out for the central star of this hourglass-shaped planetary nebula. With its nuclear fuel exhausted, this brief, spectacular, closing phase of a Sun-like star's life occurs as its outer layers are ejected - its core becoming a cooling, fading white dwarf. Astronomers have recently used the Hubble Space Telescope (HST) to make a series of images of planetary nebulae, including the one above. Here, delicate rings of colorful glowing gas (nitrogen-red, hydrogen-green, and oxygen-blue) outline the tenuous walls of the "hourglass". The unprecedented sharpness of the HST images has revealed surprising details of the nebula ejection process and may help resolve the outstanding mystery of the variety of complex shapes and symmetries of planetary nebulae.
Olympus Mons on Mars: The Largest Volcano (1997-09-15)

The largest volcano in the Solar System is on Mars. Olympus Mons rises 24 kilometers high and measures 550 km across. By comparison, Earth's largest volcano, Mauna Loa in Hawaii, rises 9 km high and measures 120 km across.  Such large volcanoes can exist on Mars because of the low gravity and lack of surface tectonic motion.  Olympus Mons is a shield volcano, built by fluid lava. Over the next three years, Mars Global Surveyor, which arrived at Mars last week, will photograph the planet at such high resolution that objects only 100 meters across will be visible.  The above image was taken by Mariner 9, which orbited and photographed Mars during 1971 and 1972.
Moon Over Mongolia (1997-09-16)

Fighting clouds and the glow of city lights, a young Moon shines over the western horizon of Mongolia's capital Ulaan-Baatar. The thin sunlit crescent is 1.5 days old and strongly over exposed in this image taken on March 10. The night side of the moon is also visible due to Earthshine - sunlight reflected from the Earth to the Moon. Just below the Moon, bright Saturn shines through the clouds. Early morning risers can see the Moon near Saturn this week. In fact, from North America the Moon can be seen to pass in front of the ringed planet on Thursday morning, September 18th.

Image credit: Olivier Staiger
GRB Fireball Persists (1997-09-17)

It's still there. The optical counterpart to the instantly famous gamma-ray burst (GRB) that occurred last February 28th has faded - but not completely. The astronomical community had waited patiently for months for the Earth to proceed far enough along in its orbit so that the Sun's glare no longer ruined inspection of the location of this historic flash. The above picture of GRB 970228, taken September 5th and released just yesterday, shows an extended structure in the center that hasn't changed.  However, the arrow indicates a point on the upper right that is now five times dimmer than in preceding months. This fading point fits a model where the GRB originated in a fireball across the universe.  The constant brightness of the extended feature indicates that it is not reflected emission from the GRB and might be the host galaxy. Even so, the great GRB mystery is not yet over: for one thing, how come other GRBs don't appear to occur in galactic hosts?
Erupting Sun (1997-09-18)

On August 27th twisting magnetic fields propelled this huge eruptive prominence a hundred thousand miles above the Sun's surface. The seething plasma of ionized gases is at a temperature of about 150,000 degrees Farenheit and spans over 200,000 miles (about 27 Earths). The Extreme ultraviolet Imaging Telescope (EIT) onboard the space-based SOHO observatory recorded this exquisitely detailed image in the light of ionized Helium atoms from its vantage point in a Halo orbit. This is the largest solar prominence observed by SOHO instruments since they began exploring solar phenomena in early 1996.
Globular Cluster 47 Tucanae (1997-09-19)

Stars come in bunches. Of the over 200 globular star clusters that orbit the center of our Milky Way Galaxy, 47 Tucanae is the second brightest globular cluster (behind Omega Centauri).  Known to some affectionately as 47 Tuc or NGC 104, it is only visible from the Southern Hemisphere.  Light takes about 20,000 years to reach us from 47 Tuc which can be seen near the SMC in the constellation of Tucana.  Red Giant stars are particularly easy to see in the above photograph.   The dynamics of stars near the center of 47 Tuc are not well understood, particularly why there are so few binary systems there.
The Clouds of Jupiter (1997-09-20)

What makes the colors in Jupiter's clouds? With a mean temperature of 120 degrees Kelvin (-153 degrees Celsius) and a composition dominated by hydrogen (about 90%), and helium (about 10%) with a smattering of hydrogen compounds like methane and ammonia, the blue, orange, and brown cloud bands and the salmon colored "red" spot are hard to explain. Trouble is -- at the cool cloud temperatures Jupiter's atmospheric constituents should be colorless! Some suggest that more colorful hydrogen compounds well up from warmer regions in the atmosphere, tinting the cloud tops. Alternatively, compounds of trace elements like sulfur may color the clouds. Jupiter's colors do indicate the clouds' altitudes, blue is lowest through red as highest. The dark colored bands are called belts and the light colored ones zones. In addition to the belts and zones, the Voyager missions revealed the presence of intricate vortices visible, for example, in this 1979 image from the Voyager I flyby. Centuries of visual observations of Jupiter have revealed that the colors of its clouds are ever changing.
Looking Down on Saturn (1997-09-21)

This picture of Saturn could not have been taken from Earth. No Earth based picture could possibly view the night side of Saturn and the corresponding shadow cast across Saturn's rings. Since Earth is much closer to the Sun than Saturn, only the day side of the planet is visible from the Earth. In fact, this photo was taken by the Voyager 1 spacecraft as it flew by Saturn in November 1980. The next spacecraft to approach Saturn will be Cassini which is currently scheduled to be launched later this year and reach Saturn in 2004.
Antares and Rho Ophiuchi (1997-09-22)

Why is the sky near Antares and Rho Ophiuchi so colorful?  The colors result from a mixture of objects and processes.  Fine dust illuminated from the front by starlight produces blue reflection nebulae. Gaseous clouds whose atoms are excited by ultraviolet starlight produce reddish emission nebulae. Backlit dust clouds block starlight and so appear dark.   Antares, a red supergiant and one of the brighter stars in the night sky, lights up the yellow-red clouds on the upper left.  Rho Ophiuchi lies at the center of the blue nebula on the right.  The distant globular cluster M4 is visible just below Antares, and to the left of the red cloud engulfing Sigma Scorpii.  These star clouds are even more colorful than humans can see, emitting light across the electromagnetic spectrum.

Image credit: D. Malin 
(AAO),
AATB, 
ROE,
UKS Telescope
A Martian Autumn Begins (1997-09-23)

Today is the first day of Autumn in the Northern Hemisphere of Planet Earth. The Autumnal Equinox occured yesterday at 7:56 pm EDT as the Sun crossed the celestial equator from North to South. Mars has seasons too and for the same reason that Earth does -- like Earth, Mars' axis of rotation is tilted with respect to the plane of its orbit around the sun. Eleven days ago Autumn also came to Mars' Northern Hemisphere and the Hubble Space Telescope recorded this image of the red planet to look for seasonal changes in the Martian weather. Clouds appear to cover the north polar regions while a dust storm rages in the south. The subject of weather on Mars is important to the just arrived Mars Global Surveyor spacecraft as it uses the innovative technique of aerobraking to establish a suitable mapping orbit around Mars.
Moon Occults Saturn (1997-09-24)

Many stargazers in the U. S. were able to watch a lovely lunar occultation early last Thursday morning as a bright Moon passed in front of Saturn. Using a 1.2 meter reflector, astronomer Kris Stanek had an excellent view of this dream-like event from the Whipple Observatory atop Arizona's Mount Hopkins. This animated gif image was constructed by Wes Colley from 4 frames taken by Stanek at 35 second intervals as the ringed planet emerged from behind the Moon's dark limb. While lunar occultations of fairly bright stars and planets are not extremely rare events, their exact timing depends critically on the observer's location.

Image credit: K. Stanek
(CfA),

W. Colley
(Princeton)
T Pyxidis: Recurrent Nova (1997-09-25)

What happens when a thermonuclear blast occurs on the surface of a white dwarf star? Over the years astronomers have watched (at a safe distance ...) as, 6,000 light years from Earth in the southern constellation Pyxis, a binary star known as T Pyxidis repeatedly produces these fearsome explosions. This Hubble Space Telescope image of nova T Pyx captures what appear to be blobs rather than the expected shells of material expanding from this interacting star system. Like other binary star systems which produce nova outbursts, T Pyx is composed of a dense white dwarf and a close companion star. An outburst occurs when the temperature and density of the sea of matter dumped from the companion onto the surface of the white dwarf reach the nuclear flash point for hydrogen. While material is violently blown off, the white dwarf itself is not disrupted and soon begins to accumulate more matter from its companion, repeating the cataclysm a few years later.
A Lonely Neutron Star (1997-09-26)

How massive can a star get without imploding into a black hole?  These limits are being tested by the discovery of a lone neutron star in space. Observations by the Hubble Space Telescope released Wednesday, have been combined with previous observations by the X-ray ROSAT observatory and ultraviolet EUVE observatory for the isolated star at the location of the arrow. Astronomers are able to directly infer the star's size from measurements of its unblended brightness, temperature, and an upper limit on the distance. Assuming that the object is a neutron star of typical mass, some previous theories of neutron star structure would have predicted an implosion that would have created a black hole. That this neutron star even exists therefore allows a window to the extreme conditions that exist in the interiors of neutron stars.
The Ecliptic Plane (1997-09-27)

The Plane of the Ecliptic is illustrated in this Clementine star tracker camera image which reveals (from right to left) the Moon lit by Earthshine, the Sun's corona rising over the Moon's dark limb, and the planets Saturn, Mars, and Mercury. The ecliptic plane is defined as the imaginary plane containing the Earth's orbit around the Sun. In the course of a year, the Sun's apparent path through the sky lies in this plane. The planetary bodies of our solar system all tend to lie near this plane, since they were formed from the Sun's spinning, flattened, proto-planetary disk. The snapshot above nicely captures a momentary line-up looking out along this fundamental plane of our solar system.
A Wolf Rayet Star Bubble (1997-09-28)

What's a Wolf-Rayet star, and how did it create that spherical bubble and sweeping arc? A Wolf-Rayet star is a star that originated with a mass over 40 times that of our Sun. An extremely hot, luminous star, it has since expelled shells of material through its strong stellar wind which could account for the bubble shaped nebula that surrounds it. But astronomers are unsure how the central Wolf-Rayet created both the bubble and the arc seen above, and even whether it acted alone in doing so. Together, this bubble and the arc are known as NGC 2359.

Image credit: Anglo-Australian Telescope
Board
Half Dome Rock on Mars (1997-09-30)

Who ordered the Martian pizza? As Mars Pathfinder and its mobile robot Sojourner continue to explore Mars, new and interesting rocks are being discovered, even though the primary mission has been successfully completed. The rock pictured above has been dubbed "Half Dome" and was visited by Sojourner in late August.  The shape and fine-scale textures of Half Dome are still being studied.  As part of the extended mission, Sojourner has been sent on its longest journey yet - a 50 meter stroll around the Sagan Memorial Station.
Maria Mitchell Inspires a Generation (1997-10-01)

"Do not look at stars as bright spots only - try to take in the vastness of the universe." Today is the 150th anniversary of the day Maria Mitchell swept the sky with her telescope and discovered the comet of 1847 (comet Mitchell 1847VI). Honored and recognized internationally for her discovery, Mitchell, who lived from 1818 to 1889, became one of the most famous American scientists of her day.  Vassar College appointed Mitchell the first woman Professor of Astronomy and she remained the only woman ever elected to the American Academy of Arts and Sciences until 1943.  Mitchell mentored a generation of scientists, and is fondly remembered for her ability to motivate.  "We especially need imagination in science," Maria Mitchell said. "Question everything."

Image credit: Helen Wright
Colliding Supernova Remnants (1997-10-02)

When a massive star exhausts its nuclear fuel it explodes. This stellar detonation, a supernova, propels vast amounts of starstuff outwards, initially at millions of miles per hour. For another 100,000 years or so the expanding supernova remnant gradually slows as it sweeps up material and ultimately merges with the gas and dust of interstellar space. Short lived by cosmic standards, these stellar debris clouds are relatively rare and valuable objects for astronomers exploring the life cycles of stars.  Yet this double bubble-shaped nebula 160,000 light-years away in the Large Magellanic Cloud may represent something rarer still - the collision of two supernova remnants. This image in the light of excited Hydrogen atoms along with images at X-Ray, radio and other optical wavelengths, suggests that the bubbles are indeed two separate regions of hot gas surrounded by cooler dense shells begining to interact as they expand and make contact.
Comet Halley and the Milky Way (1997-10-03)

Comet Halley was photographed superposed in front of the disk of our Milky Way Galaxy in 1986 by the Kuiper Airborne Observatory.  Comet Halley is the bright white streak near this photograph's center.  Comet Halley is the most famous comet in history, and returns to the inner Solar System every 76 years.  Stars visible in our Milky Way Galaxy typically lie millions of times farther in the distance and orbit the Galactic center every 250 million years.  Billions of comets are thought to orbit our Sun but most do not get close enough for us to see.  Similarly, billions of stars orbit our Milky Way's center but do not get close enough for us to see.
In the Center of 30 Doradus (1997-10-04)

In the center of 30 Doradus lies a huge cluster of the largest, hottest, most massive stars known. The center of this cluster, known as R136, is boxed in the upper right portion of the above picture. The gas and dust filling the rest of the picture is predominantly ionized hydrogen from the emission nebula 30 Doradus. R136 is composed of thousands of hot blue stars, some about 50 times more massive than our Sun. 30 Doradus and R136 lie in the LMC - a satellite galaxy to our own Milky Way Galaxy. Although the ages of stars in R136 cause it to be best described as an open cluster, R136's density will likely make it a low mass globular cluster in a few billion years.
Worlds of a Distant Sun: 47 Ursae Majoris b (1997-10-05)

In the last few years, observational astronomy has given humanity evidence of the existence of worlds beyond the solar system. Solar-type stars are now inferred to harbor planets of approximately Jupiter mass - some residing in temperature zones which could conceivably support liquid water and therefore life! Above is a hypothetical scene near one such planet whose sun, 47 Ursae Majoris (47 UMa), is very similar to our own. In our sky, 47 UMa appears as a faint, inconspicuous star below the cup of the "the Big Dipper". (Our own sun would be equally inconspicuous when viewed from 47 UMa ...) Astronomers G. Marcy and P. Butler announced the discovery of a planet associated with this star in 1996 and reported it to have a mass of about 2.4 Jupiters or more with an orbital period of 3 years. This artist's extrasolar vision pictures the detected planet, 47 UMa b, as a gas giant surrounded by a ring of material - analogous to our own gas giant Saturn. In the foreground lies a hypothetical moon of 47 UMa b.  Could such a moon support life? 47 UMa is only 44 light years distant, fairly close by astronomical standards - yet there is evidence for planetary systems which are closer still. NASA plans to explore nearby planetary systems using spaceborne observatories.

Image credit: John Whatmough,
Extrasolar Visions
Surveyor At Mars (1997-10-06)

Mars Global Surveyor achieved Martian orbit on September 11 and began aerobraking into its final mapping orbit, a process that will take until March next year. Anticipating the labors ahead, Mars Orbital Camera operators have begun acquiring test images. This dramatic detail of a recent image shows a 10 mile wide swath of a highland valley, part of the Nirgal Vallis system. The original image was recorded from an altitude of 250 miles at a resolution of about 30 feet per pixel and has been rotated to represent the camera's perspective view. Were these valleys formed by flowing water or did collapse and erosion caused by ground water produced the channel? What other processes were important? Time will tell. From its planned mapping orbit, with four times better resolution, Mars Global Surveyor's images should provide answers to these and other questions about Mars.
Europe at Night (1997-10-07)

This is what Europe looks like at night! Can you find your favorite European city? Although not all of Europe is shown, city lights might make this task possible.  Continental outlines have been artificially drawn in yellow. The above picture is actually a composite of over 200 images made by satellites orbiting the Earth. Scans were made by the USAF Defense Meteorological Satellite Program (DMSP) Operational Linescan System. The DMSP satellites continue to help in the understanding and prediction of weather phenomena as well as provide key information about population patterns, city light levels, and even rural forest fires.

Image credit: NOAA/
NGDC, 
DMSP 
Digital Archive
The Brightest Star Yet Known (1997-10-08)

Star light, star bright, a new brightest star has been discovered in the night.  This new brightest star is so far away and so obscured by dust, however, that it took the Hubble Space Telescope to confirm it.  In 1990 a star named the Pistol Star was known to lie at the center of the Pistol Nebula.  In 1995 it was suggested that the Pistol Star was so massive it was throwing off the mass that actually created the Pistol Nebula. Now, observations from the Hubble Space Telescope released today confirm the spectral relation between the star and the nebula.  Dramatic implications include that the star emits 10 million times more light than our Sun, and is about 100 times more massive.  Astronomers are currently unsure how a star this massive could have formed and how it will act in the future.
Hale-Bopp and the North American Nebula (1997-10-09)

Comet Hale-Bopp's recent encounter with the inner Solar System allowed many breath-taking pictures. Above, Comet Hale-Bopp was photographed on March 8th in the constellation of Cygnus. Visible on the right in red is the North American Nebula, a bright emission nebula observable from a dark location with binoculars. The North American Nebula is about 1500 light years away, much farther than the comet, which was about 8 light minutes away.  Several bright blue stars from the open cluster M39 are visible just above the comet's blue ion tail.

Image credit: J. C. Casado
Mars Pathfinder Super Pan (1997-10-10)

Spectacular details of rover tracks, wind-driven soil, and textured rocks on the Martian surface fill this color mosaic. The view is north-northeast from the Sagan Memorial Station at the Pathfinder landing site on Mars. These images are just part of the "Super Panorama" - a detailed color and stereo imaging data set being compiled by Pathfinder's IMP camera. The data set will be used to derive detailed topographic maps of the landing site and to further explore the mineralogy of the martian rocks and soil. The forward rover deployment ramp and the rock named Barnacle Bill, appear in the foreground at the left while the larger Yogi rock is partly visible at the upper right. Criss-crossing tracks were made by the cruising Sojourner robot rover's spiked wheels. With three wheels on each side, the two foot long rover makes tracks about 1.5 feet apart.
Floating Free in Space (1997-10-11)

NASA astronauts can float free in space without any connection to a spaceship. Here astronaut Bruce McCandless maneuvers outside the Space Shuttle Challenger by firing nitrogen gas thrusters on his manned maneuvering unit (MMU).  This picture was taken in 1984 and records this first untethered spacewalk. The MMU was developed because astronauts found tethers restrictive.
Impact! 65 Million Years Ago (1997-10-12)

What killed the dinosaurs? Their sudden disappearance 65 million years ago, along with about 70 percent of all species then living on Earth, is known as the K-T event (Cretaceous-Tertiary Mass Extinction event). Geologists and paleontologists often entertain the idea of a large asteroid or comet impacting the Earth as the culprit. Besides the firestorms, tidal waves, earthquakes, and hurricane winds such an impact would generate, the debris thrown into the atmosphere would have a serious global environmental impact -- creating extended periods of darkness, low temperatures, and acid rains. In 1990, dramatic support for this theory came from cosmochemist Alan Hildebrand's revelation of a 65 million year old, 112 mile wide ring structure still detectable under layers of sediment in the Yucatan Peninsula region of Mexico. The outlines of the structure, called the Chicxulub crater (named for a local village), are visible in the above representation of gravity and magnetic field data from the region. In addition to having the right age, the crater is consistent with the impact of an asteroid of sufficient size (6 to 12 miles wide) to cause the global disruptions. Regardless of the true cause of the K-T event, it is fortunate that such impacts are presently believed to happen only about once every 100 million years!