juliensimon/nasa-apod huggingface dataset:
M81: A Bulging Spiral Galaxy (1996-07-13)

Few stars are still forming in the old giant spiral galaxy M81. The blue regions in this picture - representing ultraviolet light - highlight regions of bright young stars and star formation and appear rare than in M74 and M33. The red regions - representing the visible light - show a large population of older, less massive stars.  M81 is therefore classified as spiral galaxy type "Sab" on the Hubble Sequence of Galaxies. One distinguishing feature of these types of galaxies is the relatively large central bulge surrounding the center of the galaxy. A massive density wave circulates around the center of spiral galaxies. It is not well understood why the bulge of M81 glows as bright as it does in ultraviolet light. Speculation includes that this may be due to hot evolved stars such as those found in the ancient globular cluster Omega Centauri.
M81 in True Color (1996-07-14)

Here's what a spiral galaxy REALLY looks like.  Yesterday, M81 was shown in two colors only, but here we see M81 at its most colorful. In the above picture, note how blue the spiral arms are - this indicates the presence of hot young stars and on-going star formation.  Also note the yellow hue of the nucleus, possibly designating a population of older stars many billions of years old.  M81 is actually a dominant member of a group of galaxies which includes M82 and several other galaxies.  Unlike our Local Group of galaxies, large galaxies in the M81 group are actually colliding. It is possible that M81's interaction with M82 create the density waves which generate M81's spiral structure.
Keck: The Largest Optical Telescope (1996-07-15)

In buildings eight stories tall rest mirrors ten meters across that are slowly allowing humanity to map the universe. Alone, each is the world's largest optical telescope: Keck. Together, the twin Keck telescopes have the resolving power of a single telescope 90-meter in diameter, able to discern sources just milliarcseconds apart. Since opening in 1992, the real power of Keck I (left) has been in its enormous light-gathering ability - allowing astronomers to study faint and distant objects in our Galaxy and the universe. Keck II, completed earlier this year, and its twin are located on the dormant volcano Mauna Kea, Hawaii, USA. In the distance is Maui's volcano Haleakala. One reason Keck was built was because of the difficultly for astronomers to get funding for a smaller telescope.
A Portrait of Saturn from Titan (1996-07-16)

The above artistic portrait of Saturn depicts how it might look from Titan, Saturn's largest moon. In the foreground sits ESA's Huygens probe, which will be released by NASA's Cassini spacecraft. The Cassini mission to Saturn in currently planned for launch in late 1997. Cassini will reach Saturn in 2004 and will release the Huygens probe later that year. Titan is one of only two moons in the Solar System to have an atmosphere, It has been suggested Titan might have gasoline-like lakes, and may even harbor life.
Looking Down on Saturn (1996-07-17)

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 be launched next year and reach Saturn in 2004.
Nebulosity in Sagittarius (1996-07-18)

What causes the colors in this beautiful nebulosity in Sagittarius?  Dubbed NGC 6589 and NGC 6590, the colors of this nebulosity, are caused by gas and dust. The blue color of the nebula nearest the bright stars is caused by reflection off interstellar dust. The dust emits little visible light of its own - in the absence of a nearby star the dust would appear dark, blocking light from background stars. The red color of the nebula farthest from the bright stars is caused by glowing hydrogen gas. Energetic light from the central stars ionizes hydrogen gas - which glows red when recombining with a local electron. In the absence of a nearby star, the gas would neither glow on its own nor absorb much background starlight.

Image credit: Anglo-Australian Telescope
Board
Galileo's First Color Image of Io (1996-07-19)

Above is the first color image of Jupiter's volcanic moon Io released by the Galileo Project. (Io sounds like "eye-oh".) The image was made on June 25 when the Galileo spacecraft approached within 1.4 million miles. It reveals features as small as 14 miles across - comparable to the resolution of the best 1979 vintage Voyager images. The Voyager flybys discovered active volcanos on Io's mottled surface and this image indicates that dramatic changes have occurred since, notably in the region of the Masubi volcano located in Io's southern hemisphere. This region, apparently covered with new deposits of sulfur and sulfur dioxide frost deposited by volcanic eruption, is seen as the pronounced white area at the bottom of the picture. While scientists continue to analyze this image and other recent Galileo data the robot spacecraft will continue to explore Jupiter's moons. Its next scheduled close encounter is set for September 6th with the moon Ganymede. Higher resolution images of Io are also expected during the ongoing mission.
20 Years Ago: Vikings on Mars (1996-07-20)

On July 20, 1976, NASA's Viking 1 lander become the first spacecraft to land on Mars, followed weeks later by its twin robot explorer, the Viking 2 lander. Operating on the Martian surface into the early 1980s, the Vikings took thousands of pictures, conducted sophisticated chemical searches for life and studied the Martian weather and geology. In the dramatically detailed image above, a field of rocks and boulders is viewed from the Viking 1 landing sight on Chryse Planitia (the Plain of Chryse). Viking 1's dusty foot pad is just visible at the lower right. The image was created by combining high resolution black and white images with lower resolution color images of the same area. NASA is planning to continue its extremely productive and well chronicled exploration of the mysterious Red Planet with the Mars Global Surveyor and the Mars Pathfinder missions. What's Mars like today?
The Eagle Soars (1996-07-21)

On July 21, 1969 Apollo 11 astronauts Neil A. Armstrong and Edwin E. "Buzz" Aldrin Jr. lifted off the lunar surface in the ascent stage of their lunar module dubbed "The Eagle" -- after becoming the first to walk on the moon. Seen here the Eagle soars towards a rendezvous with the orbiting Command Module piloted by Michael Collins. The smooth, dark mare area on the surface below is Mare Smithii located just below the equator on the extreme eastern edge of the lunar nearside. The Earth is visible hanging above the moon's western horizon.
Utopia on Mars (1996-07-22)

The Viking 2 spacecraft was launched on the Road to Utopia in September of 1975 (30 years after Bing, Dotty, and Bob). In August of 1976, after making the second successful Martian landing, Viking 2's lander began recording data used to produce this exquisitely detailed image of the Martian surface in the area of Utopia Planitia (the Plain of Utopia). Visible at the lower right are the protective shroud that covered the lander's soil collector head, ejected after the descent, along with one of the lander's dust covered footpads. Seen near the center are shallow trenches dug by the sampler arm. Mars looks red because its surface is covered with reddish iron oxide dust (rust). This dust, suspended in the thin carbon dioxide atmosphere, also filters the sunlight causing surface views to take on a reddish tinge. The Vikings made the first successful landings on Mars 20 years ago. What does Mars look like today?
Hale-Bopp, Jupiter, and the Milky Way (1996-07-23)

Shining brightly, the mighty Jupiter rules this gorgeous Kodacolor photo of the Milky Way near Sagittarius. Astronomer Bill Keel took the picture earlier this month (July 7) while standing near the summit of Hawaii's Mauna Kea contemplating the sky in the direction of the center of the Galaxy (right of picture center). In addition to the gas giant planet, which is well placed for evening viewing, the image contains an impressive sampler of celestial goodies. Many famous emission nebulae are visible as reddish patches - M16, the Eagle nebula, is just above and right of center, with the Horseshoe nebula, M17, just below it and farther to the right. Also, look for the Lagoon Nebula, M8, as the brightest red patch at the right of the picture with the Trifid Nebula, M20, just above it and to the left. The milky glow of distant unresolved stars in the plane of our Galaxy (thus the term Milky Way) runs through the image cut by dark, absorbing, interstellar dust clouds. The much anticipated comet Hale-Bopp is also clearly visible. Where's the comet? Click on the picture to view the comet's location flanked by superposed vertical lines. The comet was discovered while still beyond the orbit of Jupiter a year ago today independently by Alan Hale and Thomas Bopp. Astronomers monitoring Hale-Bopp's activity report that having now brightened to almost 6th magnitude it is still on track for becoming an extremely bright naked-eye comet in early 1997.

Image credit: W. Keel, UA
COMPTEL Explores The Radioactive Sky (1996-07-24)

Diffuse gas clouds laced with radioactive aluminum atoms (Al26) line the plane of our Milky Way Galaxy! How do we see them? Relying on the Compton Effect, the COMPTEL instrument onboard NASA's immense orbiting Compton Gamma Ray Observatory can "see" the 1.8 million electron Volt gamma rays emitted by the radioactive decay. COMPTEL's first ever survey image of the entire sky in the light of gamma rays produced by this exotic radioactivity is shown above. The Galactic plane is horizontal, passing through the Galactic center in the middle of the picture, as indicated by the superposed coordinate grid. The radioactive Al26 clouds are seen to lie in clumps near the plane, with some slightly above and below it. The brightest feature looks like a mysterious inverted "V", just to the left of center. Where do they come from? Al26 decays to magnesium (Mg26) with a half-life of about a million years, a very short time compared to the age of the Galaxy -- so the clouds must have been produced relatively "recently". COMPTEL astronomers are exploring several origins for the radioactive clouds including nuclear processing (nucleosynthesis) by aging massive stars and supernova explosions. Because they are generally thought to be associated with short lived massive stars, the radioactive clouds are expected to be located near sites of recent star formation.  (Note added in press: Don't worry - the aluminum atoms in the foil in your kitchen are Al27 and are not radioactive!)
Hale-Bopp on Schedule (1996-07-25)

Late March and early April of 1996 marked a banner season for viewing the spectacular naked-eye comet Hyakutake. The spring of 1997 could well offer a similar cometary wonder, Comet Hale-Bopp. Discovered last year while approaching the inner solar sytem Hale-Bopp has been eagerly watched for signs that it will indeed brighten spectacularly. So far, things look good! This recent image of the comet was made on May 14 at the European Southern Observatory when the comet about 340 million miles from Earth. It is shown here using false colors to indicate relative brightness. Stars are visible through the expanding coma. Recent reports are that its level of activity, the rate of dust and gas production from the solar heating of the icy nucleus is as expected. An analysis of its orbit indicates that this comet will approach to within nearly 120 million miles of Earth on March 22, 1997. The orbital calculations also reveal that Hale-Bopp has visited the inner solar system before ... about 4200 years ago.
The Cygnus Loop (1996-07-26)

15,000 years ago a star in the constellation of Cygnus exploded -- the shockwave from this supernova explosion is still expanding into interstellar space! The collision of this fast moving wall of gas with a stationary cloud has heated it causing it to glow in visible as well as high energy radiation, producing the nebula known as the Cygnus Loop (NGC 6960/95). The nebula is located about 2500 light years away. The colors used here indicate emission from different kinds of atoms excited by the shock; oxygen-blue, sulfur-red, and hydrogen-green. This picture was taken with the Wide Field and Planetary Camera 2 on board the  Hubble Space Telescope.
Driving to the Sun (1996-07-27)

How long would it take to drive to the Sun? Brittany, age 7, and D.J., age 12, ponder this question over dinner one evening. James, also age 7, suggests taking a really fast racing car while Christopher, age 4, eagerly agrees. Jerry, a really old guy who is used to estimating driving time on family trips based on distance divided by speed, offers to do the numbers. "Let's see ... the Sun is 93 million miles away. So, if we drove 93 miles per hour the trip would only take us 1 million hours." How long is 1 million hours? One year is 365 days times 24 hours per day, or 8,760 hours. One hundred years would be 876,000 hours, still a little short of the 1 million hour drive time -- so the Sun is really quite far away. Christopher is not impressed, but as he grows older he will be. You've got to be impressed by something that's 93 million miles away and still hurts your eyes when you look at it!
Huck Finn's New Sky View (1996-07-28)

"We had the sky up there, all speckled with stars, and we used to lay on our backs and look up at them, and discuss about whether they was made or only just happened."  (from Mark Twain's The Adventures of Huckleberry Finn)  Explanation: Even the faintest stars Huck and Jim could see as their raft drifted down the Mississippi were in our own Galaxy. The faintest objects astronomers can see today are the distant galaxies -- entire systems of stars comparable to our own Milky Way, which fill the Universe. Despite the advances, the sense of wonder so simply expressed in Huck's musing is still the same.
A Dust Jet From Hale-Bopp (1996-07-29)

Approaching the inner Solar System, comet Hale-Bopp's icy nucleus is heated by sunlight, accelerating its production of dust and gas. Shrouded in the resulting cloud, known as the coma, the cometary nucleus remains hidden from direct view. However, astronomers using a 2.2 meter telescope at the European Southern Observatory in May, were able to detect an enormous jet of dust extending northward (up) from the nuclear region as seen in this false color image. Dust jets may arise from vents on the surface of the nucleus. Early estimates of the size of Hale-Bopp's nucleus have indicated that it could be as large as 40 kilometers (24 miles) in diameter -- several times larger than comet Halley's. Hale-Bopp's copious dust production bodes well for it becoming a bright naked-eye comet in the spring of 1997.
Tonight: A Blue Moon (1996-07-30)

How often does a Full Moon occur twice in a single month? Exactly once in a Blue Moon. In fact, the modern usage of the term "Blue Moon" refers to the second Full Moon in a single month. Tonight's Blue Moon (Universal Time) will be the first since September 1993.  A Blue Moon typically occurs every few years. The reason for the rarity of the Blue Moon is that the 29.53 days between full moons is just slightly shorter than the number of days in the average month.  Don't, however, expect the moon to look blue tonight!  The term "Blue Moon" is thought to derive from common language expressions used hundreds of years ago. It is possible for the Moon to appear tinged by a blue hue, sometimes caused by fine dirt circulating in the Earth's atmosphere, possibly from a volcanic explosion.  The above picture is of our Moon taken was taken in a dark blue morning sky. The bright crescent is the only part directly exposed to sunlight - the rest of the Moon glows from sunlight reflected from the Earth. In this dramatic photo, however, the planet Jupiter is also visible along with its four largest moons.

Image credit: Vic Winter,
ICSTARS
A Violet Moon (1996-07-31)

Checking out the Galileo spacecraft's cameras during its December 1992 flyby of Earth's Moon, controllers took this dramatically illuminated picture through a violet filter. The view looks down on the Moon's north polar region with the Sun shining from the left at a low angle and the direction toward the moon's North pole toward the lower right. Across the image upper left stretches the smooth volcanic plain of the Mare Imbrium. Pythagoras crater, 65 miles wide, is near the center of the image -- mostly in shadow, its central peak just catches the sunlight. Yesterday, the Moon made its closest approach to Earth and was full for the second time in July (as reckoned by UT dates). The closest point in the Moon's orbit is referred to as Lunar Perigee, a mere 221,797 miles at 8 hours UT. The second full moon in a month is known as a "Blue Moon".
The Hydra Cluster of Galaxies (1996-08-01)

You are flying through space and come to ... the Hydra Cluster of Galaxies. Listed as Abell 1060, the Hydra Cluster contains well over 100 bright galaxies.  Clusters of galaxies are the largest gravitationally-bound objects in the universe. All of the bright extended images in the above picture are galaxies in the Hydra Cluster with the exception of unrelated  diffraction crosses centered on bright stars. Several proximate clusters and galaxy groups might together create an even larger entity - a supercluster - but these clumps of matter are not (yet) falling toward each other. In fact, the Hydra cluster is thought to be part of the Hydra-Centaurus Supercluster of galaxies. Similarly, our own Milky Way Galaxy is part of the Local Group of Galaxies which is part of the Virgo Supercluster of Galaxies.

Image credit: STScI, 
AAO, UK-PPARC, ROE
Galileo, Cassini, and the Great Red Spot (1996-08-02)

Imagine a hurricane that lasted for 300 years! Jupiter's Great Red Spot indeed seems to be a giant hurricane-like storm system rotating with the Jovian clouds. Observed in 1655 by Italian-French astronomer Jean-Dominique Cassini it is seen here over 300 years later - still going strong - in a mosaic of recent Galileo spacecraft images. The Great Red Spot is a cold, high pressure area 2-3 times wider than planet Earth. Its outer edge rotates in a counter clockwise direction about once every six days. Jupiter's own rapid rotation period is a brief 10 hours. The Solar System's largest gas giant planet, it is presently well placed for evening viewing. (APOD thanks to Alan Radecki for assembling a preliminary mosaic from the Galileo imagery!)
Jupiter's Colorful Clouds (1996-08-03)

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, astronomers have been hard pressed to explain the blue, orange and brown cloud bands and the salmon colored "red" spot. 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. The 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.
NGC 3393: A Super Spiral? (1996-08-04)

A bird? A plane? No, but pictured here is something physically much larger, flying much higher, and moving much faster than either of these. It is, in fact, a Seyfert type 2 spiral galaxy. The "S" is actually a lane of stars, gas and dust circling the core. Designated NGC 3393, the bright core makes this galaxy a Seyfert and the infrared glow of central dust help distinguish it as "type 2." Seyfert galaxies have extremely energetic nuclei similar to more powerful quasars.  Seyferts are thought to have black holes in their centers. Most of the lines and small spots in this image are due to cosmic rays striking the imager and are unrelated to structure in the galaxy.
Erupting Volcanoes on Io (1996-08-05)

Jupiter's moon Io has active volcanoes. The Voyager spacecraft caught several erupting when they passed the energetic moon in 1979. In the above picture, several of Io's volcanoes are visible and one is seen actually erupting. Debris from this explosive event can be seen on the upper left of the photo, just beyond Io's edge.  Io's volcanism is thought to be caused by the large tidal distortions raised by Jupiter, Europa, and Ganymede. These tides stretch Io, cause internal friction, and thus heat the interior. The hot interior then expands and forces its way out through volcanoes. Currently, the spacecraft Galileo is orbiting Jupiter and photographing Jupiter's Galilean moons.
Europa: Oceans of Life? (1996-08-06)

Is there life beneath Europa's frozen surface? Some believe the oceans found there of carbon-enriched water are the best chance for life, outside the Earth, in our Solar System. Europa, the fourth largest moon of Jupiter, was recently discovered to have a thin oxygen atmosphere by scientists using the Hubble Space Telescope. Although Earth's atmospheric abundance of oxygen is indicative of life, astronomers speculate that Europa's oxygen arises purely from physical processes. But what an interesting coincidence! The above picture was taken by a Voyager spacecraft in 1979, but the spacecraft Galileo is currently circling Jupiter and has been photographing Europa. The first of these pictures will be released two days from today. Will they show the unexpected?  Late News: NASA to Release New Europa and Jupiter Pictures Thursday  More Late News: New Evidence Suggests Microscopic Life Existed on Mars
Early Microscopic Life on Mars? (1996-08-07)

Today a team of NASA and Stanford scientists announced the discovery of strong circumstantial evidence that microscopic life once existed on Mars. Dr. David McKay, Dr. Everett Gibson, and Kathie Thomas-Keprta of Lockheed-Martin, all from (NASA /JSC), and Dr. Richard Zare (Stanford) have led a team that has found chemical evidence for past life on Mars - including what they interpret as possible microscopic fossil remains (tube-like structures pictured above) - in a meteorite thought to have originated on Mars. A small fraction of the many meteorites that fall to Earth from space have composition similar to the Martian surface. Many scientists believe that these meteorites are indeed Martian rocks that have been catapulted into space during a catastrophic event on Mars, such as an asteroid impact.  The escaped rocks would then circle the inner Solar System, some of them falling to Earth.  The meteorite containing the evidence landed on Earth 13,000 years ago, but may indicate a life-form that existed on Mars billions of years ago. The team's findings will be published in the August 16 issue of Science Magazine. Even skeptical scientists look forward to future research confirming or refuting these exciting claims.