juliensimon/nasa-apod huggingface dataset:
The Local Interstellar Cloud (2000-04-11)

The stars are not alone.  In the disk of our Milky Way Galaxy about 10 percent of visible matter is in the form of gas, called the interstellar medium (ISM).  The ISM is not uniform, and shows patchiness even near our Sun.  It can be quite difficult to detect the local ISM because it is so tenuous and emits so little light.  This mostly hydrogen gas, however, absorbs some very specific colors that can be detected in the light of the nearest stars.  A working map of the local ISM within 10 light-years based on recent observations is shown above.  These observations show that our Sun is moving through a Local Interstellar Cloud as this cloud flows outwards from the Scorpius-Centaurus Association star forming region.  Our Sun may exit the Local Interstellar Cloud during the next 10,000 years.  Much remains unknown about the local ISM, including details of its distribution, its origin, and how it affects the Sun and the Earth.

Image credit: Linda Huff 
(American Scientist), 
Priscilla Frisch
(U. Chicago)
The Local Bubble and the Galactic Neighborhood (2000-04-12)

What surrounds the Sun in this neck of the Milky Way Galaxy?  Our current best guess is depicted in the above map of the surrounding 1500 light years constructed from various observations and deductions.  Currently, the Sun is passing through a Local Interstellar Cloud (LIC), shown in violet, which is flowing away from the Scorpius-Centaurus Association of young stars.  The LIC resides in a low-density hole in the interstellar medium (ISM) called the Local Bubble, shown in black.  Nearby, high-density molecular clouds including the Aquila Rift surround star forming regions, each shown in orange.  The Gum Nebula, shown in green, is a region of hot ionized hydrogen gas.  Inside the Gum Nebula is the Vela Supernova Remnant, shown in pink, which is expanding to create fragmented shells of material like the LIC.  Future observations should help astronomers discern more about the local Galactic Neighborhood and how it might have affected Earth's past climate.

Image credit: Linda Huff 
(American Scientist), 
Priscilla Frisch
(U. Chicago)
Exploring Comet Tails (2000-04-13)

Comets are known for their tails. In the spring of 1997 and 1996 Comet Hale-Bopp (above) and Comet Hyakutake gave us stunning examples as they passed near the Sun. These extremely active comets were bright, naked-eye spectacles offering researchers an opportunity to telescopically explore the composition of primordial chunks of our solar system by studying their long and beautiful tails. But it has only recently been discovered that surprising readings from experiments on-board the interplanetary Ulysses probe which lasted for several hours on May 1, 1996, indicate the probe passed through comet Hyakutake's tail! Ulysses experiments were intended to study the Sun and solar wind and the spacecraft-comet encounter was totally unanticipated. Relative positions of Ulysses and Hyakutake on that date indicate that this comet's ion tail stretched an impressive 360 million miles or about four times the Earth-Sun distance. This makes Hyakutake's tail the longest ever recorded and suggests that comet tails are much longer than previously believed.

Image credit: Joe Orman
Supernova Remnant E0102-72 from Radio to X-Ray (2000-04-14)

Not all stars form a big Q after they explode.  The shape of supernova remnant E0102-72, however, is giving astronomers a clue about how tremendous explosions disperse elements and interact with surrounded gas.  The above image is a composite of three different photographs in three different types of light.  Radio waves, shown in red, trace high-energy electrons spiraling around magnetic field lines in the shock wave expanding out from the detonated star.  Optical light, shown in green, traces clumps of relatively cool gas that includes oxygen.  X-rays, shown in blue, show relatively hot gas that has been heated to millions of degrees.  This gas has been heated by an inward moving shock wave that has rebounded from a collision with existing or slower moving gas.  This big Q currently measures 40 light-years across and was found in our neighboring SMC galaxy.  Perhaps we would know even more if we could buy a vowel.
Surveyor Hops (2000-04-15)

This panorama of the cratered lunar surface was constructed from images returned by the US Surveyor 6 lander. Surveyor 6 was not the first spacecraft to accomplish a soft landing on the Moon ... but it was the first to land and then lift off again! After the spacecraft touched down near the center of the Moon's nearside in November of 1967, NASA controllers commanded it to hop. Briefly firing its rocket engine and lifting itself some 4 meters above the surface, the Surveyor moved about 2.5 meters to one side before setting down again. The hopping success of Surveyor 6 essentially marked the completion of the Surveyor series main mission - to determine if the lunar terrain was safe for the planned Apollo landings.
IC 4406: A Seemingly Square Nebula (2000-04-16)

How can a round star make a square nebula?  This conundrum came to light with the discovery of planetary nebulae like IC 4406.  IC 4406 is most probably cylindrical, with its square appearance the result of our vantage point in viewing the cylinder.  Hot gas is known to be flowing out the ends of the cylinder, while filaments of dark dust and molecular gas lace the bounding walls.  The star primarily responsible for this interstellar sculpture can be found in the planetary nebula's center.  In a few million years, the only thing left visible in IC 4406 will be a fading white dwarf star.
Flying Over Asteroid Eros' West End (2000-04-17)

The robot spacecraft NEAR Shoemaker continues to orbit asteroid Eros.  This condensed 40-minute long time-lapse sequence taken last month shows what it looks like to pass within 200 kilometers of Eros' west end.  The north pole of the rotating mountain is toward the bottom of the picture.  This month NEAR-Shoemaker closes to within 100 kilometers, and by the end of this month will orbit only 50 kilometers from the center of this 33-kilometer long asteroid.  One reason for moving in so close is to determine if 433 Eros has a magnetic field.  NEAR Shoemaker, launched in 1996, is run by a computer similar to a PC released 15 years ago (12 MHz, 256K).
Europa: Ice Line (2000-04-18)

This bright white swath cutting across the surface of icy Jovian moon Europa is known as Agenor Linea. In all about 1000 kilometers long and 5 kilometers wide, only a section is pictured here as part of a combined color and black and white image based on data from the Galileo spacecraft. Most linear features on Europa are dark in color but Agenor Linea is uniquely bright for unknown reasons. Also unknown is the origin of the reddish material along the sides. While these and other details of Europa's surface formations remain mysterious, the general results of Galileo's exploration of Europa have supported the idea that an ocean of liquid water lies beneath the cracked and frozen crust. An extraterrestrial liquid ocean holds out the tantalizing possibility of life.
Redshift 5.8: A New Farthest Quasar (2000-04-19)

The distance record for a quasar has been broken yet again.  At the present time, no other object in the universe has been found to be more distant than the above speck.  The recently discovered quasar has been clocked at redshift 5.82.  The exact relation between redshift and distance remains presently unknown, although surely higher redshifts do mean greater distance.  The above quasar is likely billions of light-years away and so is seen when the universe was younger than one billion years old, less than a tenth of its present age.  Like all quasars, this object is probably a large black hole in the center of a distant galaxy. Don't close the record book yet, though.  The redshifts to several other SDSS-discovered quasars are currently being measured, some of which might have redshifts greater than six.
Blue Marble 2000 (2000-04-20)

This newly released digital portrait of our planet is reminiscent of the Apollo-era pictures of the "big blue marble" Earth from space. To create it, researchers at Goddard Space Flight Center's Laboratory for Atmospheres combined data from a Geostationary Operational Environmental Satellite (GOES), the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), and the Polar Orbiting Environmental Satellites (POES) with a USGS elevation model of Earth's topography. Stunningly detailed, the planet's western hemisphere is cast so that heavy vegetation is green and sparse vegetation is yellow, while the heights of mountains and depths of valleys have been exaggerated by 50 times to make vertical relief visible.  Hurricane Linda is the dramatic storm off North America's west coast. And what about the Moon? The lunar image was reconstructed from GOES data and artistically rescaled for this visualization.
M82: Starburst In X-rays (2000-04-21)

Star formation occurs at a faster pace in M82 -- a galaxy with about 10 times the rate of massive star birth (and death) compared to our Milky Way. Winds from massive stars and blasts from supernova explosions have created the expanding cloud of million degree gas filling the above Chandra X-ray Observatory image of this remarkable starburst galaxy. The false color image even resolves bright spots which are likely shocked supernova remnants and X-ray bright binary stars. Also observed as a radio galaxy and a bright celestial infrared source, M82's aspect in optical pictures has led to its popular moniker, the Cigar Galaxy. M82's burst of star formation was likely triggered a mere 100 million years ago in the latest of a series of bouts with another large galaxy, M81.
Journey to the Center of the Galaxy (2000-04-22)

In Jules Verne's science fiction classic A Journey to the Center of the Earth, Professor Hardwigg and his fellow explorers encounter many strange and exciting wonders. What wonders lie at the center of our Galaxy? Astronomers now know of some of the bizarre objects which exist there, like vast dust clouds, bright young stars, swirling rings of gas, and even a large black hole. Much of the Galactic Center is shielded from our view in visible light by the intervening dust and gas, but it can be explored using other forms of electromagnetic radiation. This haunting image of the Galactic Center region in infrared light was made in 1996 by a telescope onboard the Midcourse Space Experiment. The center itself appears as a bright spot near the middle of this roughly 1 degree field of view, and the north galactic pole is towards the top. The picture is in false color - starlight appears blue while dust is greenish grey, tending to red in the cooler areas.
Giant Cluster Bends, Breaks Images (2000-04-23)

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 the Hubble Space Telescope was taken in October 1994.
Reflection Nebula M78 (2000-04-24)

An eerie blue glow and ominous columns of dark dust highlight M78, a bright reflection nebula in the constellation of Orion.  The dust not only absorbs light, but also reflects the light of several bright blue stars that formed recently in the nebula.  The same type of scattering that colors the daytime sky further enhances the blue color.  M78 is about five light-years across and visible through a small telescope.  M78 appears above only as it was 1600 years ago, however, because that is how long it takes light to go from there to here.   M78 belongs to the larger Orion Molecular Cloud Complex that contains the Great Nebula in Orion and the Horsehead Nebula.
Layers of the Martian South Polar Cap (2000-04-25)

The South Pole of Mars is stranger than was previously thought.  Pictured above are unexpectedly complex layers photographed recently by the Mars Global Surveyor spacecraft currently orbiting Mars.  The layers probably include carbon dioxide ice, water ice, rock and dust.  The intricate structures might indicate erosion patterns that hold clues to the history of the Martian climate over the past 100 million years.  The above image covers a region five kilometers across, resolving details as small as 25 meters across.
Filaments In The Cygnus Loop (2000-04-26)

Subtle and delicate in appearance, these are filaments of shocked interstellar gas -- part of the expanding blast wave from a violent stellar explosion. Recorded in November 1997 with the Wide Field and Planetary Camera 2 onboard the Hubble Space Telescope, the picture is a closeup of a supernova remnant known as the Cygnus Loop. The nearly edge-on view shows a small portion of the immense shock front moving toward the top of the frame at about 170 kilometers per second while glowing in light emitted by atoms of excited Hydrogen gas. Not just another pretty picture, this particular image has provided some dramatic scientific results. In 1999, researchers used it to substantially revise downward widely accepted estimates of distance and age for this classic supernova remnant. Now determined to lie only 1,440 light-years away, the Cygnus Loop is thought to have been expanding for 5 - 10 thousand years.
Calderas And Cliffs Near Io's South Pole (2000-04-27)

Braving intense radiation belts, the Galileo spacecraft once again flew past the surface of Jupiter's moon Io (sounds like EYE-oh) on February 22. Combining high resolution black and white images from that flyby with color data recorded last summer has resulted in this dramatic view of a region near the volcanic moon's south pole. An active and alien landscape, the bright white areas are likely due to sulfur dioxide frost and seem to be concentrated near ridges and cliffs. The three ominous black spots, each about 6-12 miles across, are volcanic craters or calderas covered with recent dark lava. A sinuous channel connects the lower left caldera with a yellowish lava flow. Io is small, but its continuous activity is driven by the drastic tides induced by Jupiter and the other Jovian moons. It is estimated that the resulting volcanism completely resurfaces Io every million years.
Leonid Glowworm (2000-04-28)

Recent Leonid meteor showers have been rich in bright fireball meteors which leave lingering trails stretching across the night sky. These trails, or persistent trains, are mysteriously self-luminescent and do not shine by reflected light. Visible for many minutes, they are blown by winds at altitudes up to 100 kilometers and can take on progressively twisted worm-like shapes. Recorded in November, during the 1998 Leonid meteor shower, this picture shows a persistent train dubbed "Glowworm" by astronomers at Kirtland Air Force Base's Starfire Optical Range (SOR). What makes the Glowworm glow? To find out, SOR astronomers engaged in a unique experiment, tracking and probing both 1998 and 1999 Leonid meteor trains with pulsed laser lidar (light detection and ranging) systems and other instruments. A copper vapor laser produced the intense streak seen shooting from the lower left of the image. While the cause of the Glowworm's glow remains enigmatic for now, the SOR results will help unravel the mystery.
3D View Of Jupiter's Clouds (2000-04-29)

very day is a cloudy day on Jupiter, the Solar System's reigning gas giant. This 3-dimensional visualization presents a simplified model view from between Jovian cloud decks based on imaging and spectral data recorded by the Galileo spacecraft. The separation between the cloud layers and the height variations have been exaggerated. The upper cloud layer is haze a few tens of miles thick. Heights in the lower cloud layers have been color coded; light bluish clouds are high and thin, reddish clouds are low, and white clouds are high and thick. Streaks in the lower layer suggestively lead to a dark blue area, a relatively clear, dry region similar to the site where Galileo's atmospheric probe made the first entry into a gas giant planet's atmosphere on December 7th, 1995.
The Small Cloud of Magellan (SMC) (2000-04-30)

Almost unknown to casual observers in the northern hemisphere, the southern sky contains two diffuse wonders known as the Magellanic Clouds.  The Magellanic Clouds are small irregular galaxies orbiting our own larger Milky Way spiral galaxy.  The Small Magellanic Cloud (SMC), pictured here, is about 250,000 light years away and contains a preponderance of young, hot, blue stars indicating it has undergone a recent period of star formation.  There is evidence that the SMC is not gravitationally bound to the LMC.

Image credit: D. Malin 
(AAO),
AATB, 
ROE,
UKS Telescope
The North America Nebula (2000-05-01)

Here's a familiar shape in an unfamiliar location!  This emission nebula is famous partly because it resembles Earth's continent of North America.  To the right of the North America Nebula, cataloged as NGC 7000, is a less luminous Pelican Nebula.  The two emission nebula measure about 50 light-years across, are located about 1500 light-years away, and are separated by a dark absorption cloud.  The nebulae can be seen with binoculars from a dark location.  Look for a small nebular patch north-east of bright star Deneb in the constellation of Cygnus.  It is still unknown which star or stars ionize the red-glowing hydrogen gas.

Image credit: Jason Ware
An Iridium Flash Sunset (2000-05-02)

Did you see that flash?  Lasting only about 15 seconds, it's possible that nobody you ask can confirm it, but what you might have seen is sunlight reflecting off an orbiting Iridium satellite.  Satellites of all types have been providing streaks and glints visible only since the launch of Sputnik I in 1957.  Of these, flares from any of the 66 Iridium satellites can be particularly bright, sometimes even approaching the brightness of the Moon.  If the Iridium satellites are programmed to re-enter the Earth's atmosphere, they might provide even brighter flares as they burn up.  Pictured above, the streak from an Iridium satellite punctuates a picturesque sunset in San Sebastian, Spain.  Then again, that sky-flash you saw? If it lasted only a second or two, it might have been a meteor.

Image credit: Jon Teus
(Science Society Aranzadi, Spain)
BOOMERANG Images The Early Universe (2000-05-03)

Drifting through the stratosphere above Antarctica in late 1998, the balloon-borne BOOMERANG telescope peered into the cosmos at millimeter wavelengths. The blotchy structures it detected are seen above in the sharpest yet picture of the universe at an early age, perhaps a mere 300,000 years old. The false-color image shows subtle fluctuations in the temperature of the hot plasma which filled the universe before expansion cooled the material, producing the familiar stars and galaxies. Dramatically, the size of the fluctuations in the BOOMERANG image has convinced many cosmologists that the universe contains just exactly enough matter and energy to be flat -- a powerful prediction of the popular theory of Inflation which describes the earliest moments of the Big Bang. A flat universe will expand forever and two light rays which start out parallel will never spread or intersect. But all is not solved (!) as the BOOMERANG results don't show clearly details confirming the suspected amounts of dark matter and the cosmological constant.
Planets Above The Clouds (2000-05-04)

Clouds scatter the faint orange rays of the setting sun in the foreground of this breathtaking photograph from the summit of Mauna Kea, Hawaii. Taken on April 7th, this skyscape features a dramatic lunar and planetary alignment. An overexposed crescent moon dominates the celestial scene, but the bright "star" just below and to its right is Saturn while further below Saturn is a close pairing of brilliant Jupiter and a fainter, yellowish Mars. Red giant star Aldebaran is almost directly above the moon near the top of the image and the bright blue stars of the Pleiades cluster are visible about midway up and to the right of the moon-Aldebaran line. The good news is that planetary alignments like this one do not portend disasters, are relatively common, and can clearly make inspirational viewing for casual stargazers and astronomers alike. The bad news is that the world is not going to end because of the highly publicized planetary alignment occurring tomorrow, May 5th -- so you probably will have to go to work!

Image credit: B. Magrath
Planets In The Sun (2000-05-05)

Today, all five naked-eye planets (Mercury, Venus, Mars, Jupiter, Saturn) plus the Moon and the Sun will at least approximately line-up. As viewed from planet Earth, they will be clustered within about 26 degrees, the closest alignment for all these celestial bodies since February 1962, when there was a solar eclipse! Such planetary alignments are not dangerous, except of course that the Sun might hurt your eyes when you look at it. So it might be easier to appreciate today's solar system spectacle if you use a space-based coronagraph ... like the LASCO instrument onboard the SOHO observatory. In this recent LASCO image, an occulting disk supported by a structure seen projecting from the lower left blocks out the overwhelming sunlight. It shows three of the planets along with the Sun's location and bright solar wind regions against a background of stars, but Mars and Venus are unfortunately outside LASCO's roughly 15 degree field of view. The horizontal bars through the planets are digital image artifacts. And what about the Moon? The SOHO spacecraft is positioned well beyond lunar orbit where its view of the Sun is never interrupted by the Moon.
The Heart Of Orion (2000-05-06)

Newborn stars lie at at the heart of the the Orion Nebula, hidden from view by the dust and gas of the giant Orion Molecular Cloud number 1 (OMC-1). Sensitive to invisible infrared wavelengths, Hubble's NICMOS camera can explore the interior of OMC-1 detecting the infrared radiation from infant star clusters and the interstellar dust and atoms energized by their intense starlight. In this false color picture, stars and the glowing dust clouds which also scatter the starlight appear yellowish orange while emission from hydrogen gas is blue. The dramatic image reveals a wealth of details, including many filaments and arcs of gas and dust -- evidence of violent motions stirred-up by the emerging stars. The bright object near the center is the massive young star "BN" (named for its discoverers Becklin and Neugebauer). The pattern of speckles and ripples surrounding BN and other bright stars are image artifacts.
A Green Flash from the Sun (2000-05-07)

Many think it is just a myth.  Others think it is true but its cause isn't known.  Adventurers pride themselves on having seen it.  It's a green flash from the Sun.  The truth is the green flash does exist and its cause is well understood.  Just as the setting Sun disappears completely from view, a last glimmer appears startlingly green.  The effect is typically visible only from locations with a low, distant horizon, and lasts just a few seconds. A green flash is also visible for a rising Sun, but takes better timing to spot.  A slight variant of this was caught in the above photograph, where much of the Sun was still visible, but the very top appeared momentarily green. The Sun itself does not turn partly green, the effect is caused by layers of the Earth's atmosphere acting like a prism.

Image credit: Juan Carlos Casado