juliensimon/nasa-apod huggingface dataset:
Expansive Comet Holmes (2007-11-21)

The spherical coma of Comet Holmes has swollen to a diameter of over 1.4 million kilometers, making the tenuous, dusty cloud even bigger than the Sun. Scattering sunlight, all that dust and gas came from the comet's remarkably active nucleus, whose diameter before the late October outburst was estimated to be a mere 3.4 kilometers. In this sharp image, recorded on November 14 with the Canada-France-Hawaii Telescope, stars are easily visible right through the outer coma, while the nucleus is buried inside the condensed, bright region. The bright region of the coma seems offset from the center, consistent with the idea that a large fragment drifted away from the nucleus and disintegrated, producing the comet's spectacular outburst. Of course, more recent images of Holmes also show the bright star Mirfak (Alpha Persei) shining through as the comet sweeps slowly through the constellation Perseus.

Image credit: Jean-Charles
Cuillandre
Pleiades and Stardust (2007-11-22)

Hurtling through a cosmic dust cloud a mere 400 light-years away, the lovely Pleiades or Seven Sisters star cluster is well-known for its striking blue reflection nebulae. This remarkable wide-field (3 degree) image of the region shows the famous star cluster at the right, while highlighting lesser known dusty reflection nebulae nearby, across an area that would span over 20 light-years. In this case, the sister stars and cosmic dust clouds are not related, they just happen to be passing through the same region of space. But astronomers using infrared detectors have recently found a dusty disk that really does belong to one young Pleiades star -- HD 23514. Surrounding HD 23514, the disk is estimated to be comparable in size to the terrestrial planet zone in our own solar system and likely represents the debris from the process of rocky planet formation.

Image credit: Tony Hallas
The Medusa Nebula (2007-11-23)

Braided, serpentine filaments of glowing gas suggest this nebula's popular name, The Medusa Nebula. Also known as Abell 21, this Medusa is an old planetary nebula some 1,500 light-years away in the constellation Gemini. Like its mythological namesake, the nebula is associated with a dramatic transformation. The planetary nebula phase represents a final stage in the evolution of low mass stars like the sun, as they transform themselves from red giants to hot white dwarf stars and in the process shrug off their outer layers. Ultraviolet radiation from the hot star powers the nebular glow. The Medusa's transforming hot central star is visible in the detailed color image as the small blue star within the upper half of the overall bright crescent shape. Fainter filaments clearly extend above and to the left of the bright crescent region. The Medusa Nebula is estimated to be over 4 light-years across.

Image credit: Don Goldman
Galaxies in Pegasus (2007-11-24)

This wide, sharp telescopic view reveals galaxies scattered beyond the stars near the northern boundary of the high-flying constellation Pegasus. Prominent at the upper right is NGC 7331. A mere 50 million light-years away, the large spiral is one of the brighter galaxies not included in Charles Messier's famous 18th century catalog. The disturbed looking group of galaxies at the lower left is well-known as Stephan's Quintet. About 300 million light-years distant, the quintet dramatically illustrates a multiple galaxy collision, its powerful, ongoing interactions posed for a brief cosmic snapshot. On the sky, the quintet and NGC 7331 are separated by about half a degree.

Image credit: Dietmar Hager
An Iridescent Cloud Over Colorado (2007-11-25)

Why would a cloud appear to be different colors? A relatively rare phenomenon known as iridescent clouds can show unusual colors vividly or a whole spectrum of colors simultaneously. These clouds are formed of small water droplets of nearly uniform size. When the Sun is in the right position and mostly hidden by thick clouds, these thinner clouds significantly diffract sunlight in a nearly coherent manner, with different colors being deflected by different amounts. Therefore, different colors will come to the observer from slightly different directions. Many clouds start with uniform regions that could show iridescence but quickly become too thick, too mixed, or too far from the Sun to exhibit striking colors. This iridescent cloud was photographed above Boulder, Colorado last week.

Image credit: August Allen
Moon Over Pigeon Point Lighthouse (2007-11-26)

This spectacular sky is mostly human-made. Once a year, the Light Station at Pigeon Point near San Francisco, California, USA is lit as it was over 100 years ago. During this time, light generated by five kerosene lamps pours through 24 rotating Fresnel lenses, warning approaching ships to stay away. Early last week, light emanating from the Pigeon Point Lighthouse was particularly picturesque because of a thin fog, also blurring the distant Moon. During the latter 1970s, the lighthouse was guarded by an 800 pound pig named Lester. In modern times, the light house is still active but has been supplied with a more efficient flashing aerobeacon.

Image credit: Tyler Westcott
Space Station Over the Ionian Sea (2007-11-27)

Last August, the Space Shuttle Endeavour crew captured this shot of the International Space Station (ISS) against the backdrop of Planet Earth. During that trip to the ISS, the space shuttle crew re-supplied the station, repaired the station, and even built more of the station. Its primary mission complete, the crew took the premier spaceship on a tour around the premier space station. Pictured during this inspection tour, the ISS is visible in front the Ionian Sea. The boot of Italy is visible on the left, while the western coastlines of Greece and Albania stretch across the top. The dorsal fin of the upside-down shuttle orbiter pokes into the very top of the image. The Space Shuttle Discovery subsequently visited the ISS in October while the next shuttle mission to the ISS is scheduled for next week.
Comet Holmes from the Hubble Space Telescope (2007-11-28)

Why did Comet Holmes brighten? The unexpectedly bright Comet 17P/Holmes continues to grace northern skies as a naked-eye addition to the constellation Perseus. Any northern sky enthusiast with a dark sky, a bright curiosity, and a recent sky map should still be able to locate the comet in a few minutes. What is seen, however, is primarily the sun-light reflecting dust coma. It surrounds an iceberg nucleus too small and too faint to discern. Clues to the nearly million-fold brightness increase are therefore being sought in dramatic images of the enigmatic comet's central regions taken earlier this month by the Hubble Space Telescope. One such Hubble image, shown above, indicates a still unresolved dense central dust cloud near the nucleus, surrounded by a more complex, anisotropic coma. The Hubble images do not show any obvious fragmentation of the nucleus, however, as was seen last year in Comet Schwassman-Wachmann 3. Observers around and above the world will continue to study this unusual addition to the night sky.
Stardust in Perseus (2007-11-29)

This cosmic expanse of dust, gas, and stars covers some 4 degrees on the sky in the heroic constellation Perseus. Centered in the gorgeous skyscape is the dusty blue reflection nebula NGC 1333, about 1,000 light-years away. At that estimated distance, the field of view is nearly 70 light-years across. Other reflection nebulae are scattered around, along with remarkable dark dust nebulae and the faint reddish glow of hydrogen gas. These dust and gas clouds lie near the edge of a large molecular cloud. Themselves telltale signs of star-forming regions, they tend to hide the newly formed stars and young stellar objects or protostars from prying optical telescopes. Collapsing due to self-gravity, the protostars form around dense cores embedded in the molecular cloud.

Image credit: Jeff Lunglhofer
Aristarchus Plateau (2007-11-30)

Anchored in the vast lava flows of the Moon's Oceanus Procellarum lies the Aristarchus Plateau. The bright impact crater at the corner of the plateau is Aristarchus, a young crater 42 kilometers wide and 3 kilometers deep. Only slightly smaller, lava flooded Herodotus crater is above and to the left. A valley or rille feature likely carved by rapidly flowing lava or a collapsed lava tunnel, Vallis Schroteri begins just to the right of Herodotus and winds across the plateau for about 160 kilometers, eventually turning toward the top of the picture. Aristarchus Plateau itself is like a rectangular island about 200 kilometers across, raised up to 2 kilometers or so above the smooth surface of the lunar Ocean of Storms. Recorded from a backyard observatory in Buffalo, New York, the contrast of light-colored ejecta around Aristarchus with surrounding dark, smooth, lava flooded surfaces suggests more familiar snowy scenes of planet Earth.

Image credit: Alan Friedman
M74: The Perfect Spiral (2007-12-01)

If not perfect, then this spiral galaxy is at least one of the most photogenic. An island universe of about 100 billion stars, 32 million light-years away toward the constellation Pisces, M74 presents a gorgeous face-on view. Classified as an Sc galaxy, the grand design of M74's graceful spiral arms are traced by bright blue star clusters and dark cosmic dust lanes. Constructed from image data recorded in 2003 and 2005, this sharp composite is from the Hubble Space Telescope's Advanced Camera for Surveys. Spanning about 30,000 light-years across the face of M74, it includes exposures recording emission from hydrogen atoms, highlighting the reddish glow of the galaxy's large star-forming regions.
Gibbous Europa (2007-12-02)

Although the phase of this moon might appear familiar, the moon itself might not. In fact, this gibbous phase shows part of Jupiter's moon Europa. The robot spacecraft Galileo captured this image mosaic during its mission orbiting Jupiter from 1995 - 2003. Visible are plains of bright ice, cracks that run to the horizon, and dark patches that likely contain both ice and dirt. Raised terrain is particularly apparent near the terminator, where it casts shadows. Europa is nearly the same size as Earth's Moon, but much smoother, showing few highlands or large impact craters. Evidence and images from the Galileo spacecraft, indicated that liquid oceans might exist below the icy surface. To test speculation that these seas hold life, ESA has started preliminary development of the Jovian Europa Orbiter, a spacecraft proposed to orbit Europa. If the surface ice is thin enough, a future mission might drop hydrobots to burrow into the oceans and search for life.
A Complete Solar Cycle from SOHO (2007-12-03)

very eleven years, our Sun goes through a solar cycle. A complete solar cycle has now been imaged by the sun-orbiting SOHO spacecraft, celebrating the 12th anniversary of its launch yesterday. A solar cycle is caused by the changing magnetic field of the Sun, and varies from solar maximum, when sunspot, coronal mass ejection, and flare phenomena are most frequent, to solar minimum, when such activity is relatively infrequent. Solar minimums occurred in 1996 and 2007, while the last solar maximum occurred in 2001. This picture is composed of a SOHO image of the Sun in extreme ultraviolet light for each year of the last solar cycle, with images picked to illustrate the relative activity of the Sun.
Movie: Analemma Over New Jersey (2007-12-04)

An analemma is that figure-8 curve that you get when you mark the position of the Sun at the same time each day throughout planet Earth's year. Above, 26 separate exposures were recorded to illustrate the regular solar motion -- a difficult project performed mostly during the calendar year 2006. The images were taken at 8 am in the morning in northern New Jersey, USA, and digitally combined with a single foreground image later. The individual images have since been combined into a movie. Solstices correspond to the top and bottom of the figure-8, indicating the northern and southernmost excursions of the Sun in the sky. The tilt of planet Earth's axis and the variation in speed as it moves around its orbit combine to produce the graceful analemma curve.

Image credit: Tom Matheson
(Guidescope.net)
Comet Holmes Over Hungary (2007-12-05)

Comet Holmes refuses to fade.  The unusual comet that surprisingly brightened nearly a million-fold in late October continues to remain visible to the unaided eye from dark locations.  Night to night, Comet 17P/Holmes is slowly gliding through the constellation Perseus, remaining visible to northern observers during much of the night right from sunset.  Pictured above, Comet Holmes was captured from Hungary last week.  The remarkable snowball continues to retain a huge coma, but now shows very little of a tail.   To the far right is the open cluster of stars NGC 1245. How much longer Comet Holmes will remain visible to the unaided eye is unknown.

Image credit: Tamas Ladanyi
Mars in View (2007-12-06)

Very good telescopic views of Mars can be expected in the coming weeks as the Red Planet nears opposition on December 24th. Of course, opposition means opposite the Sun in planet Earth's sky - an arrangement that occurs every 26 months for Mars. Because of Mars' more elliptical orbit, the actual date of closest approach to Earth will be December 18, when Mars will be within about 88 million kilometers of our fair planet. Situated in the constellation Gemini and rising after evening twilight, the bright, ruddy disk of Mars will reach nearly 16 arcseconds in diameter (about 1/100th the diameter of the Full Moon). In this already exceptional image taken on November 18, north is down and surface markings around the sprawling, dark, albedo feature Syrtis Major are remarkably clear. The image was recorded with a video camera and filters on a 1 meter telescope at Pic Du Midi, a mountain top observatory in the French Pyrenees. NASA launched the Phoenix lander to Mars in August, scheduled to arrive in May 2008.

Image credit: Jean-Luc Dauvergne
Double Cluster in Perseus (2007-12-07)

Skygazers recently following Comet Holmes have probably also chanced across this lovely starfield, not far from the comet on the sky in the constellation Perseus. Some 7,000 light-years away, this pair of open or galactic star clusters is an easy binocular target and is visible to the unaided eye from dark sky areas. In fact, it was cataloged in 130 BC by Greek astronomer Hipparchus. Now known as h and chi Persei, or NGC 869(left) and NGC 884, the clusters themselves are separated by only a few hundred light-years and contain stars much younger and hotter than the Sun.  In addition to being physically close together, the clusters' ages based on their individual stars are similar - evidence that both clusters were likely a product of the same star-forming region.

Image credit: Volker Wendel
Star Trails at Dawn (2007-12-08)

Just fix your camera to a tripod and you too can make an image of graceful trails traced by the stars as planet Earth rotates on its axis. Made on September 14 from Montlaux, France, this wide-angle view nicely shows the stars near the celestial equator tracing nearly straight lines in projection, while stars north and south of the equator, respectively, appear to circle the north and south celestial poles. Featured are the stars of Orion (right of center), brilliant Venus rising (left) as bright star Sirius rises in the south (bottom center), and a polar orbiting Iridium satellite (upper left). Beautiful dawn sky colors seem painted along the horizon. This remarkable picture was constructed from 477 consecutive 30 second digital exposures recorded over 4.3 hours and later combined.

Image credit: Koen van Gorp
The Fairy of Eagle Nebula (2007-12-09)

The dust sculptures of the Eagle Nebula are evaporating.  As powerful starlight whittles away these cool cosmic mountains, the statuesque pillars that remain might be imagined as mythical beasts.  Pictured above is one of several striking dust pillars of the Eagle Nebula that might be described as a gigantic alien fairy.   This fairy, however, is ten light years tall and spews radiation much hotter than common fire.  The greater Eagle Nebula, M16, is actually a giant evaporating shell of gas and dust inside of which is a growing cavity filled with a spectacular stellar nursery currently forming an open cluster of stars.  The above image in scientifically re-assigned colors was  released as part of the fifteenth anniversary celebration of the launch of the Hubble Space Telescope.
A Jet from the Sun (2007-12-10)

What powers the solar wind? Our Sun is known to emit a powerful wind of particles with gusts that can even affect astronauts and satellites orbiting Earth. The cause of the solar wind has been debated for decades but is thought to be rooted in Alfv�n waves generated by the ever changing magnetic field of the Sun.  Newly released images from the Japanese Hinode satellite appear to bolster this hypothesis, imaging an average of 240 daily plasma jets that are excellent candidates to fuel the outwardly moving Alfv�n waves.  The jets and waves are themselves ultimately created by magnetic reconnection events, rapid events where lines of constant magnetic field suddenly move extremely rapidly, dragging electrons and protons along with them.  On the image left, one such jet is visible in X-ray light.  Bright spots show relatively energetic regions elsewhere on the Sun.

Image credit: Hinode
The Universe Nearby (2007-12-11)

What does the universe nearby look like? This plot shows over one and a half million of the brightest stars and galaxies in the nearby universe detected by the Two Micron All Sky Survey (2MASS) in infrared light. The resulting image is an incredible tapestry of stars and galaxies that provides limits on how the universe formed and evolved.  Across the center are stars that lie in the plane of our own Milky Way Galaxy. Away from the Galactic plane, vast majority of the dots are galaxies, color coded to indicate distance, with blue dots representing the nearest galaxies in the 2Mass survey, and red dots indicating the most distant survey galaxies that lie at a redshift near 0.1.  Named structures are annotated.  Many galaxies are gravitationally bound together to form clusters, which themselves are loosely bound into superclusters, which in turn are sometimes seen to align over even larger scale structures.

Image credit: 2MASS, 
T. H. Jarrett, J. Carpenter, & R. Hurt
Mars Rover Races to Survive (2007-12-12)

The Martian rover Spirit is now in the race of its life.  The rolling robot is trying to reach an outpost to spend the winter, but it keeps getting bogged down in soft sand on Mars. Earth scientists hope that Spirit can reach a slope on the northern edge of the unusual feature dubbed Home Plate, before the end of this month when northern winter will be phasing in on Mars.   Reaching this slope will likely allow the rover to tilt enough toward the Sun to create a needed increase in the efficiency of its energy-absorbing solar panels.  This map shows the path of Spirit from July 2004 until just last month.
T Tauri and Hind's Variable Nebula (2007-12-13)

The orange star centered in this remarkable telescopic skyview is T Tauri, prototype of the class of T Tauri variable stars. Nearby it is a dusty yellow cosmic cloud historically known as Hind's Variable Nebula (NGC 1555/1554). Over 400 light-years away, at the edge of a molecular cloud, both star and nebula are seen to vary significantly in brightness but not necessarily at the same time, adding to the mystery of the intriguing region. T Tauri stars are now generally recognized as young (less than a few million years old), sun-like stars still in the early stages of formation. To further complicate the picture, infrared observations indicate that T Tauri itself is part of a multiple system and suggest that the associated Hind's Nebula may also contain a very young stellar object. The dramatic color image spans about 4 light-years at the estimated distance of T Tauri.

Image credit: Don Goldman
Apollo 17: Shorty Crater Panorama (2007-12-14)

In December of 1972, Apollo 17 astronauts Eugene Cernan and Harrison Schmitt spent about 75 hours on the Moon in the Taurus-Littrow valley, while colleague Ronald Evans orbited overhead. This sharp panorama is digitally stitched together from pictures taken by Cernan as he and Schmitt roamed the valley floor. Starting with a view of the imposing South Massif, scrolling the panorama to the right will reveal Schmitt and the lunar rover at the edge of Shorty Crater, near the spot where geologist Schmitt discovered orange lunar soil. The Apollo 17 crew returned with 110 kilograms of rock and soil samples, more than was returned from any of the other lunar landing sites. Now thirty five years later, Cernan and Schmitt are still the last to walk on the Moon.
Mountains of Creation (2007-12-15)

This fantastic skyscape lies at the eastern edge of giant stellar nursery W5, about 7,000 light-years away in the constellation Cassiopeia. An infrared view from the Spitzer Space Telescope, it features interstellar clouds of cold gas and dust sculpted by winds and radiation from a hot, massive star outside the picture (just above and to the right). Still swaddled within the cosmic clouds, newborn stars are revealed by Spitzer's penetrating gaze, their formation also triggered by the massive star. Fittingly dubbed "Mountains of Creation", these interstellar clouds are about 10 times the size of the analogous Pillars of Creation in M16, made famous in a 1995 Hubble Space Telescope view. W5 is also known as IC 1848 and together with IC 1805 it is part of a complex region popularly dubbed the Heart and Soul Nebulae. The Spitzer image spans about 70 light-years at the distance of W5.
The Holographic Principle (2007-12-16)

Is this picture worth a thousand words?  According to the Holographic Principle, the most information you can get from this image is about 3 x 1065 bits for a normal sized computer monitor.  The Holographic Principle, yet unproven, states that there is a maximum amount of information content held by regions adjacent to any surface.  Therefore, counter-intuitively, the information content inside a room depends not on the volume of the room but on the area of the bounding walls.  The principle derives from the idea that the Planck length, the length scale where quantum mechanics begins to dominate classical gravity, is one side of an area that can hold only about one bit of information.  The limit was first postulated by physicist Gerard 't Hooft in 1993.  It can arise from generalizations from seemingly distant speculation that the information held by a black hole is determined not by its enclosed volume but by the surface area of its event horizon.  The term "holographic" arises from a hologram analogy where three-dimension images are created by projecting light though a flat screen.  Beware, other people looking at the above image may not claim to see 3 x 1065 bits -- they might claim to see a teapot.

Image credit: E. Winfree, 
K. Fleischer, A. Barr 
et al. 
(Caltech)
Saturn's Ancient Rings (2007-12-17)

How old are Saturn's rings?  No one is quite sure.  One possibility is that the rings formed relatively recently in our Solar System's history, perhaps only about 100 million years ago when a moon-sized object broke up near Saturn.  Evidence for a young ring age includes a basic stability analysis for rings, and the fact that the rings are so bright and relatively unaffected by numerous small dark meteor impacts.  New evidence, however, raises the possibility that some of Saturn's rings may be billions of years old and so almost as old as Saturn itself.   Inspection of images by the Saturn-orbiting Cassini spacecraft indicates that some of Saturn's ring particles temporarily bunch and collide, effectively recycling ring particles by bringing fresh bright ices to the surface.  Seen here, Saturn's rings were imaged in their true colors by the robotic Cassini in late October.  Icy bright Tethys, a moon of Saturn likely brightened by a sandblasting rain of ice from sister moon Enceladus, is visible in front of the darker rings.    APOD presents: Astronomy Pictures of the Year for 2007