juliensimon/nasa-apod huggingface dataset:
Leonids Above Torre de la Guaita (2004-11-14)

The 1999 Leonids Meteor Shower came to an impressive crescendo.  Observers in Europe observed a sharp peak in the number of meteors visible around 0210 UTC during the early morning hours of November 18.  Meteor counts then exceeded 1000 per hour - the minimum needed to define a true meteor storm.  At other times and from other locations around the world, observers typically reported respectable rates of between 30 and 100 meteors per hour.  The above photograph is a 20-minute exposure ending just before the main Leonids peak began.  Visible are at least five Leonids meteors streaking high above the Torre de la Guaita, an observation tower used during the 12th century in Girona, Spain.  This year's Leonids should peak twice on November 19th, but is predicted to be less impressive than in 1999.

Image credit: Juan Carlos Casado
Burns Cliff on Mars (2004-11-15)

The majestic walls of Endurance Crater contain layers of clues about the ancient past of Mars.  In fact, the deeper the layer, the older the clue.  The particular crater wall imaged above was dubbed Burns Cliff and was in front of the robot rover Opportunity last week.  Close inspection of different layers has found slightly different compositions as well as interesting trends in relative compositions.  For example, deeper layers contain similarly decreasing amounts of both magnesium and sulfur, indicating a common reason for their decline -- possibly dissolution in water.  Today, more practically, Burns Cliff blocks one exit direction for Opportunity to leave Endurance Crater.  When combined with slippery sand elsewhere on the crater floor, controllers have decided to program Opportunity to back out of the crater the way it came in - after a few more days exploration.
Sagittarius Dwarf Irregular Galaxy (2004-11-16)

How old is this galaxy?  The nearby Local Group galaxy dubbed the Sagittarius Dwarf Irregular Galaxy (SagDIG) is not only very small but also has relatively few elements more massive than helium.  Now the lack of heavy elements might mean that SagDIG is very young, so that component stars had little time to create and disperse massive elements.  Conversely, SagDIG's diminutive size could indicate that it formed in the early universe, being a surviving building block of modern large galaxies.  The above detailed image from the Hubble Space Telescope has now resolved enough stars to solve this mystery: SagDIG is ancient.  Although SagDIG does have some groups of young stars, many stars are very old, and the galaxy as a whole helps astronomers to understand how the universe evolved, and show that at least one metal-poor galaxy is almost as old as the universe.  Pictured above, SagDIG spans about 1,500 light years and lies about 3.5 million light years away toward the constellation of Sagittarius.
Aurora Over Wisconsin (2004-11-17)

The auroral displays of the past week are being reported as some of the most beautiful in memory.  In particular, impressive auroral bands fanned out over much of eastern North America after sunset on November 8.  The multicolored aurora pictured above was caught reflecting in one of the many small lakes in central Wisconsin near that time.  Continued solar activity might create more aurora visible over the next few nights as the Leonids meteor shower peaks.   News flash: NASA air breathing jet sets new air speed record.

Image credit: Chris VenHaus
A Sharper View of a Tilted Planet (2004-11-18)

These sharp views of tilted gas giant Uranus show dramatic details of the planet's atmosphere and ring system. The remarkable ground-based images were made using a near-infrared camera and the Keck Adaptive Optics system to reduce the blurring effects of Earth's atmosphere. Recorded in July, the pictures show two sides of Uranus (careful how you pronounce that ...). In both, high, white cloud features are seen mostly in the northern (right hand) hemisphere, with medium level cloud bands in green and lower level clouds in blue. The artificial color scheme lends a deep reddish tint to the otherwise faint rings. Because of the severe tilt of its rotational axis, seasons on Uranus are extreme and last nearly 21 Earth years on the distant planet. Uranus is now slowly approaching its southern autumnal equinox - the beginning of fall in the southern hemisphere - in 2007.
Phobos: Doomed Moon of Mars (2004-11-19)

This moon is doomed.  Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These martian moons may well be captured asteroids originating in the main asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. The largest moon, Phobos, is indeed seen to be a cratered, asteroid-like object in this stunning new color image from the Mars Express spacecraft, recorded at a resolution of about seven meters per pixel. But Phobos orbits so close to Mars - about 5,800 kilometers above the surface compared to 400,000 kilometers for our Moon - that gravitational tidal forces are dragging it down. In 100 million years or so it will likely crash into the surface or be shattered by stress caused by the relentless tidal forces, the debris forming a ring around Mars.
Stereo Phobos (2004-11-20)

Get out your red/blue glasses and float next to Phobos, grooved moon of Mars! Also featured in yesterday's episode, the image data from the Mars Express High Resolution Stereo Camera was recorded at a distance of about 200 kilometers. This tantalizing stereo anaglyph view shows the Mars-facing side of the asteroid-like moon's cratered and grooved surface. Up to hundreds of meters wide, the mysterious grooves may be fractures related to the impact which created 10 kilometer wide Stickney crater, the large crater at the left.
Spiral Galaxies in Collision (2004-11-21)

Billions of years from now, only one of these two galaxies will remain.  Until then, spiral galaxies NGC 2207 and IC 2163 will slowly pull each other apart, creating tides of matter, sheets of shocked gas, lanes of dark dust, bursts of star formation, and streams of cast-away stars.  Astronomers predict that NGC 2207, the larger galaxy on the left, will eventually incorporate IC 2163, the smaller galaxy on the right.  In the most recent encounter that peaked 40 million years ago, the smaller galaxy is swinging around counter-clockwise, and is now slightly behind the larger galaxy.  The space between stars is so vast that when galaxies collide, the stars in them usually do not collide.
Swift Launches (2004-11-22)

Where do gamma ray bursts occur?  To help find out, NASA launched the Swift satellite on Saturday, as pictured above.  What Swift is designed to do better than any previous satellite is to quickly locate these enigmatic explosions in both sky position and distance.  Once a gamma ray burst (GRB) is located, Swift itself will rotate to face it head-on and determine its distance directly.  Swift locations will be immediately beamed down to Earth-based telescopes operating around the world and across the electromagnetic spectrum that are just waiting for a place on the sky to point.  Subsequently collected GRB and afterglow information will help astronomers not only determine the nature of the initial explosions, but also the nature of the expanding shockwaves.  Some Swift-located GRBs could turn out to be the most distant transient events ever detected, holding unique clues to the nature and rate of the very first wave of star formation in the early universe.
Leonid Meteors Streak (2004-11-23)

The 2004 Leonids meteor shower had its ups and downs.  Although average rates were significantly less than many previous years, as expected, at least two unexpected "mini-outbursts" of several bright meteors over a few minutes were reported.  Pictured above, a bright Leonid meteor was imaged by a Night Sky Live camera over Kitt Peak National Observatory early in the morning of November 19 during one mini-outburst.  The meteor appears as the streak just to the left of the green "Ursa Major" label.  Moving your cursor over the image will show the image taken about 25 minutes later where two bright Leonid meteors from a second mini-outburst were recorded, visible on the lower right just to the left of the green words "Canis" and "Minor".  The stars appear to shift between the two images because of the rotation of the Earth.  The Night Sky Live frames show fisheye images, capturing the entire sky as a person would see it looking straight up, including peripheral vision.  In mid-December, the Geminids meteor shower will give sky enthusiasts another good chance to see live meteors.
A Radar View of Titan (2004-11-24)

Where are Titan's craters?  Throughout our Solar System's five billion-year history, dangerous rocks and chunks of ice have continually slammed into planets and moons - usually creating numerous long lasting impact craters.  When the robot spacecraft Cassini swooped past Saturn's moon Titan last month, however, radar images showed few craters.  One such image, spanning 75 kilometers across, is shown above.  The imaged structures are not yet understood, but may involve some sort of flows.  Titan is already known to be an unusual moon, sporting a thick atmosphere, large size, small amounts of organic compounds.  Craters are surely created on all surfaces in the Solar System, but might be destroyed later, as on Earth and Jupiter's moon Io.  How craters are destroyed on Titan remains a topic of speculation, but might become better understood by consideration of data returned by future flybys of Cassini and by the probe Huygens that will descend toward Titan's surface in December.
What the Hubble Saw (2004-11-25)

In this striking 41 inch by 38 inch quilt, astronomy enthusiast Judy Ross has interpreted some of the Hubble Space Telescope's best galactic and extragalactic vistas. Featured in past APODs, clockwise from the lower right are; the Red Rectangle Nebula, NGC 2392, the Sleeping Beauty Galaxy, V838 Monocerotis - the Milky Way's most mysterious star, and supernova remnant N49 - the cosmic debris from an exploded star. Of course, quilts have been used historically to represent astronomical concepts. And while inspired by the images of the cosmos that she incorporates into her quilts, Ross reports that she is still a little daunted by the intricacies of the Cat's Eye Nebula revealed by the Hubble's sharp vision.

Image credit: Judy W. Ross, 
Point Roberts, WA
Magnetars In The Sky (2004-11-26)

Indicated on this infrared image of the galactic center region are positions of candidate magnetars -- believed to be the strongest magnets in the galaxy. Classified by observers as Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs), these cosmic powerhouses are likely city-sized, spinning, highly-magnetized neutron stars. How strong is a magnetar's magnetic field?  The Earth's magnetic field which deflects compass needles is measured to be about 1 Gauss, while the strongest fields sustainable in earthbound laboratories are about 100,000 Gauss. A magnetar's monster magnetic field is estimated to be as high as 1,000,000,000,000,000 Gauss. A magnet this strong, located at about half the distance to the Moon would easily erase your credit cards and suck pens out of your pocket. In 1998, from a distance of about 20,000 light-years, one magnetar, SGR 1900+14 generated a powerful flash of gamma-rays detected by many spacecraft. That blast of high-energy radiation is now known to have had a measurable effect on Earth's ionosphere. At the surface of the magnetar, its powerful magnetic field is thought to buckle and shift the neutron star crust generating the intense high-energy flares
NGC 2683: Spiral Edge-On (2004-11-27)

This gorgeous island universe, cataloged as NGC 2683, lies a mere 16 million light-years distant in the northern constellation Lynx. A spiral galaxy comparable to our own Milky Way, NGC 2683 is seen nearly edge-on in this cosmic vista, with more distant galaxies scattered in the background. Blended light from a large population of old yellowish stars forms the remarkably bright galactic core. Starlight silhouettes the dust lanes along winding spiral arms, dotted with the telltale pink glow of ionized hydrogen gas from this galaxy's star forming regions.

Image credit: Adam Block
Doomed Star Eta Carinae (2004-11-28)

Carinae may be about to explode.  But no one knows when - it may be next year, it may be one million years from now.  Eta Carinae's mass - about 100 times greater than our Sun - makes it an excellent candidate for a full blown supernova. Historical records do show that about 150 years ago Eta Carinae underwent an unusual outburst that made it one of the brightest stars in the southern sky.  Eta Carinae, in the Keyhole Nebula, is the only star currently thought to emit natural LASER light.  This image, taken in 1996, resulted from sophisticated image-processing procedures designed to bring out new details in the unusual nebula that surrounds this rogue star.  Now clearly visible are two distinct lobes, a hot central region, and strange radial streaks.  The lobes are filled with lanes of gas and dust which absorb the blue and ultraviolet light emitted near the center.  The streaks remain unexplained. Will these clues tell us how the nebula was formed?  Will they better indicate when Eta Carinae will explode?
Saturn's Moon Tethys from Cassini (2004-11-29)

Tethys is one of the larger and closer moons of Saturn.  The Cassini spacecraft now orbiting Saturn passed near the frozen moon at the end of October, capturing the most detailed images since the Voyager spacecrafts in the early 1980s.  Tethys is composed almost completely of water ice and shows a large impact crater that nearly circles the moon. Because this crater did not disrupt the moon, Tethys is hypothesized to be at least partly liquid in its past.  Two smaller moons, Telesto and Calypso, orbit Saturn just ahead of and behind Tethys.  Giovanni Cassini discovered Tethys in 1684.   The Cassini spacecraft is scheduled for a close fly-by of Tethys in September 2005.
Lake Effect Snow on Earth (2004-11-30)

What are those strange clouds stretching out from these lakes?  The clouds are caused by cold air moving over a warm water and result in bands of lake-effect snow.  The rising bands of moistened, warmed air that drop lake-effect snow alternate with clear bands of falling cold air.  During a winter, such bands can create hundreds of centimeters of snow more than upwind areas only a hundred kilometers away.  During this lake-effect snowfall of 2000 December 5, practically all of the state of Michigan, USA got covered.  A cold northwesterly wind over Great Lakes Superior and Michigan created the unusual clouds.  The above image was taken with NASA's SeaWiFS satellite.
Saturn's Moon Dione from Cassini (2004-12-01)

What causes the bright streaks on Dione?  Recent and likely future images of this unusual moon by the robot Cassini spacecraft now orbiting Saturn might help us find out.  The above image was taken at the end of October from a distance of about one million kilometers. The bright streaks run across some of Dione's many craters, indicating that the process that created them occurred later than the impacts that created those craters.  Dione is made of mostly water ice but its relatively high density indicates that it contains much rock inside.  Giovanni Cassini discovered Dione in 1684.  The Cassini spacecraft is scheduled to photograph Dione at higher resolution in mid-December.  Currently, the highest resolution images of Dione remain those taken by the passing Voyager spacecraft in 1980.
Mimas, Rings, and Shadows (2004-12-02)

Caught in sunlight, icy moon Mimas shines above a broad shadow across gas giant Saturn. In this remarkable image from the Cassini spacecraft, tiny Mimas is at the upper right. The broad shadow across the giant planet is cast by Saturn's dense B ring with intriguing threadlike shadows from Saturn's inner C ring arrayed below. While the B and C rings are otherwise not visible here, the very narrow outer F ring lies toward the bottom of the image as well as a section of the partly transparent A ring and its 300 kilometer wide Encke gap crisscrossing the ring shadows. Sunlight streaming through the much larger Cassini gap that separates the A and B rings is responsible for the bright band seen above Mimas. The Cassini gap itself is just off the bottom of this cropped view. Orbiting well beyond Saturn's F ring, Mimas is a mere 400 kilometers in diameter.
I Zwicky 18: Young Galaxy (2004-12-03)

The Milky Way is an ordinary 12 billion year old spiral galaxy, and even our middle-aged Sun is pushing 4.5 billion years. But all the stars in dwarf galaxy I Zwicky 18 are much younger. In fact, based on Hubble Space Telescope image data, that galaxy's first stars formed only about 500 million years ago, making it the youngest known galaxy.  In this view, the bright knots are the two major star forming regions of I Zwicky 18, embedded in expanding filaments of glowing interstellar gas. Scattered, much older background galaxies are seen as small red blobs, and a companion galaxy lies just beyond the upper right corner of the cropped picture. Astronomers believe that diminutive I Zwicky 18 resembles the earliest galaxies formed, but also want to understand how such a young galaxy can be only 45 million light-years away - surrounded by mature galaxies in an aging Universe. The tiny galaxy itself is a mere 3,000 light-years across.
Reflecting Merope (2004-12-04)

In the well known Pleiades star cluster, a star's light is slowly destroying this wandering cloud of gas and dust.  Above, the star Merope lies just off the upper left edge of this picture from the Hubble Space Telescope.  The cloud, known as IC 349, and the star have been in existence for millions of years.  In the past 100,000 years, however, part of the cloud has by chance moved so close to the star - only 3,500 times the Earth-Sun distance - that the starlight itself is having a very dramatic effect. Pressure of the star's light significantly repels the dust in the reflection nebula with smaller dust particles being repelled more strongly.  Eventually parts of the dust cloud have become stratified and point toward Merope, with the closest particles being the most massive and so the least affected by the radiation pressure.  A longer-term result will be the general destruction of the dust by the energetic starlight.
Kemble's Cascade (2004-12-05)

A picturesque chain of unrelated stars is visible with strong binoculars towards the constellation of Camelopardalis.  Known as Kemble's Cascade, the asterism contains about 20  stars nearly in a row stretching over five times the width of a full moon.  Made popular by astronomy enthusiast Lucian Kemble (1922-1999), these stars appear as a string only from our direction in the Milky Way Galaxy.  The above photograph of Kemble's Cascade was made with a small telescope in New Mexico, USA.  The bright object near the bottom left is the relatively compact open cluster of stars known as NGC 1502.

Image credit: Walter MacDonald
Filaments Across the Sun (2004-12-06)

Two unusually long filaments crossed part of the Sun last week.  The filaments are actually relatively cool and dark prominences of solar plasma held up by the Sun's magnetic field but seen against the face of the Sun.  Filaments typically last a few weeks before falling back.  Pictured above, the two filaments are visible on the Sun's right side.  It would take twenty Earths, set end-to-end, to match the length of one of the filaments.  Also visible are bright hot regions called plages and a carpet of hundreds of granules that provide the Sun's texture.   The above image was taken early last week through a small telescope in a very specific color of light emitted primarily by hydrogen.

Image credit: Greg Piepol (sungazer.net)
A Strange Streak Imaged in Australia (2004-12-07)

Meteor experts don't think it's a meteor.  Atmospheric scientists don't think it's lightning.  The photographer insists that the streak and flash on the above image has not been created digitally.  So what is it? Nobody is sure.  APOD's editors do not claim to know - one purpose of posting this image is to mine the eclectic brain trust of APOD's readers to help see if some unusual phenomenon was caught serendipitously.  The strange features were captured on a series of images intended to monitor cloud changes in the background.  Images taken just before and after the above frame show no streak or flash.  The light pole near the flash has been inspected and does not show any damage, although the light inside was not working.  If you think you know what is going on, instead of sending us email please participate in an online discussion.  If a convincing argument or consensus is reached, the answer will be posted on APOD at a later date.   Discussion consensus: Australian strange streak is plausibly just a flying insect. News: The answer to Lewin's Challenge APOD can be found here.

Image credit: Wayne Pryde
In the Center of the Heart Nebula (2004-12-08)

What powers the Heart Nebula?  The large emission nebula dubbed IC 1805 looks, in whole, like a human heart.  The nebula glows brightly in red light emitted by its most prominent element: hydrogen.  The red glow and the larger shape are all created by a small group of stars near the nebula's center.  A close up spanning about 30 light years contains many of these stars is shown above in a recent image taken by the Canada France Hawaii Telescope.  This open cluster of stars contains a few bright stars nearly 50 times the mass of our Sun, many dim stars only a fraction of the mass of our Sun, and an absent microquasar that was expelled millions of years ago.  The Heart Nebula is located about 7,500 light years away toward the constellation of Cassiopeia.    News: The answer to Lewin's Challenge APOD can be found here.

Image credit: Canada-France-Hawaii Telescope
Jupiter and the Moon's Shadowed Horizon (2004-12-09)

rly Tuesday morning, December 7th, June Croft thought the southeastern sky above Atmore, Alabama, USA was beautiful. Watching the Moon rise through gossamer clouds, she noted, " ... the crescent Moon looked like it was held in the sky by a star just off its shadowed horizon." What was that star?  Bright Jupiter of course, and some watched as the Moon actually occulted or passed in front of the Solar System's reigning gas giant planet. For astronomer Jimmy Westlake in Colorado, Jupiter was already hidden at moonrise that morning, but later he was able to record this lovely image, not unlike the view that inspired Croft. Seen through gossamer clouds, Jupiter along with large Jovian satellites Ganymede and Callisto (bottom to top) has emerged from behind the crescent Moon's shadowed horizon.   News: The answer to Lewin's Challenge APOD can be found here.

Image credit: Jimmy
Westlake
(Colorado
Mountain College)
Debris Disks Surround Distant Suns (2004-12-10)

In this dramatic artist's vision, debris along the outer reaches of a planet forming disk orbits in the glare of a distant sun. But inset are actual images of such disks around two nearby stars - AU Microscopii (top left; edge-on) and HD107146 (right: face-on) - as seen by the Hubble Space Telescope. Combined with infrared images from the Spitzer Space Telescope that show debris disks around known planet bearing stars, the data provide the first direct link between extrasolar disks and planets, suggesting a scenario where evolving planets scatter debris produced by collisions into giant disks. In time, the dusty disks may dwindle and become like our own Solar System's comet reservoir, the Kuiper Belt.   News: The answer to Lewin's Challenge APOD can be found here.