juliensimon/nasa-apod huggingface dataset:
An Anomalous SETI Signal (2011-02-06)

No one knows for sure what caused this signal.  There is a slight possibility that it just might originate from an extraterrestrial intelligence. The bright colors on the blue background indicate that an anomalous signal was received here on Earth by a radio telescope involved in a Search for Extraterrestrial Intelligence (SETI).  A search for these signals is ongoing by  several groups including volunteer members of the SETI League.  Time labels the vertical axis of the above plot, and frequency marks the horizontal axis.  Although  this strong signal was never positively identified, astronomers have identified in it many attributes characteristic of a more mundane and ultimately terrestrial origin.  In this case, a leading possibility is that the signal originates from an unusual modulation between a GPS satellite and an unidentified Earth-based source. Many unusual signals from space remain unidentified.  No signal has yet been strong enough or run long enough to be unambiguously identified as originating from an extraterrestrial intelligence.

Image credit: SETI League
Sun 360: STEREO Captures Views of the Entire Sun (2011-02-07)

For the first time, the entire Sun is being imaged all at once. This has become possible because the two STEREO satellites orbiting and monitoring the Sun are now on opposite sides of the Sun. The two satellites have been drifting apart, as expected, since their launch in 2006, since one satellite orbits slightly closer to the Sun than the other.  The above image shows nearly the entire Sun as it appeared one day last week, a few days before maximum exposure.  Yesterday, the dark gap in the center closed completely, and STEREO was able to beam back to Earth full 360 degree images of the closest star.  Full solar images are useful scientifically for a number of reasons, including catching rapidly evolving flares, coronal mass ejections, tsunamis, and filaments, no matter where they occur on the Sun, as well as monitoring days-long sunspots and active regions without losing them as they rotate out of view. Even though the STEREO satellites will continue to drift apart at about 44 degrees per year, Sun-staring instruments on or near the Earth will augment them to provide a full view of the Sun for the next several years.    Students (young and old): See free lectures on astronomy
Iridescent Clouds from the Top of the World Highway (2011-02-08)

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 last year from the Top of the World Highway outside Dawson City, in the Yukon Territory in Northern Canada.

Image credit: Charles Stankievech & Sophie Springer
NGC 2174: Stars Versus Mountains (2011-02-09)

It's stars versus gas mountains in NGC 2174 and the stars are winning. More precisely, the energetic light and winds from massive newly formed stars are evaporating and dispersing the dark stellar nurseries in which they formed. The structures of NGC 2174 are actually much thinner than air and only appear as mountains due to relatively small amounts of opaque interstellar dust.  A lesser known sight in the nebula-rich constellation Orion, NGC 2174 can be found with binoculars near the head of the celestial hunter.  About 6,400 light-years distant, the entire glowing cosmic cloud covers an area larger than the full Moon and surrounds loose open clusters of young stars.  The above image from the Hubble Space Telescope shows a dense interior region which spans only about three light years while adopting a color map that portrays otherwise red hydrogen emission in green hues and emphasizes sulfur emission in red and oxygen in blue. Within a few million years, the stars will likely win out completely and the entire dust mountain will be dispersed.
Hanny's Voorwerp (2011-02-10)

Hanny's Voorwerp, Dutch for "Hanny's Object", is enormous, about the size of our own Milky Way Galaxy. Glowing strongly in the greenish light produced by ionized oxygen atoms, the mysterious voorwerp is below spiral galaxy IC 2497 in this view from the Hubble Space Telescope. Both lie at a distance of some 650 million light-years in the faint constellation Leo Minor. In fact, the enormous green cloud is now suspected to be part of a tidal tail of material illuminated by a quasar inhabiting the center of IC 2497. Powered by a massive black hole, the quasar suddenly turned off, leaving only galaxy and glowing voorwerp visible in telescopes at optical wavelengths. The sharp Hubble image also resolves a star forming region in the voorwerp, seen in yellow on the side near IC 2497. That region was likely compressed by an outflow of gas driven from the galaxy's core. The remarkable mystery object was discovered by Dutch schoolteacher Hanny van Arkel in 2007 while participating online in the Galaxy Zoo project.  Galaxy Zoo enlists the public to help classify galaxies found in the Sloan Digital Sky Survey, and more recently in deep Hubble imagery.
Star Colors in Orion (2011-02-11)

What determines a star's color? Its temperature. Red stars are cool, with surface temperatures of around 3,000 kelvins (K), while blue stars are hotter and can have temperatures over 30,000 K. Our own lovely "yellow" Sun's temperature is a comforting 6,000 K. Differences in star colors are particularly easy to see in this intriguing composite view of the constellation Orion, made while experimenting with a star trail step-focus technique. In it, a series of 35 consecutive exposures were combined to produce trails of stars moving left to right through the frame, while changing focus in steps. Beginning and ending with the camera out of focus produced a sharply focused exposure near the middle of the series and blurs the star trails into a bow tie shape. For the brighter stars, blurring produces more saturated colors in the images. At the upper left, Orion's cool red supergiant Betelgeuse stands out from the other, hotter, bluish stars composing the body of the constellation. Not a star at all, the Orion Nebula contributes a pinkish tint below center. Also remarkable in the field, the fainter step focus trail of cool, deep red carbon star W Orionis is near the center right edge, its red hue enhanced by a carbon-rich composition.

Image credit: Jens Hackmann
Simeis 147: Supernova Remnant (2011-02-12)

It's easy to get lost following the intricate filaments in this detailed mosaic image of faint supernova remnant Simeis 147. Also cataloged as Sh2-240 and seen towards the constellation Taurus, it covers nearly 3 degrees (6 full moons) on the sky. That corresponds to a width of 150 light-years at the stellar debris cloud's estimated distance of 3,000 light-years. The remarkable composite includes image data taken through narrow-band filters to highlight emission from hydrogen and oxygen atoms tracing regions of shocked, glowing gas. This supernova remnant has an estimated age of about 40,000 years - meaning light from the massive stellar explosion first reached Earth 40,000 years ago. But this expanding remnant is not the only aftermath. The cosmic catastrophe also left behind a spinning neutron star or pulsar, all that remains of the original star's core.

Image credit: Nobuhiko Miki
Ice Fishing for Cosmic Neutrinos (2011-02-13)

Scientists are melting holes in the bottom of the world. In fact, almost 100 holes melted near the South Pole are now being used as astronomical observatories.  Astronomers with the IceCube Neutrino Observatory lowered into each vertical lake a long string knotted with basketball-sized light detectors.  The water in each hole soon refreezes.  The detectors attached to the strings are sensitive to blue light emitted in the surrounding clear ice. Such light is expected from ice collisions with high-energy neutrinos emitted by objects or explosions out in the universe.  Late last year, the last of IceCube's 86 strings was lowered into the freezing abyss, pictured above, making IceCube the largest neutrino detector yet created. Data from a preliminary experiment, AMANDA, has already been used to create the first detailed map of the high-energy neutrino sky.  Experimental goals of the newer IceCube include a search for cosmic sources of neutrinos, a search for neutrinos coincident with nearby supernova and distant gamma-ray bursts, and, if lucky, a probe of exotic physical concepts such as unseen spatial dimensions and faster-than-light travel.
The Rosette Nebula (2011-02-14)

Would the Rosette Nebula by any other name look as sweet? The bland New General Catalog designation of NGC 2237 doesn't appear to diminish the appearance of  this flowery emission nebula. Inside the nebula lies an open cluster of bright young stars designated NGC 2244. These stars formed about four million years ago from the nebular material and their stellar winds are clearing a hole in the nebula's center, insulated by a layer of dust and hot gas. Ultraviolet light from the hot cluster stars causes the surrounding nebula to glow. The Rosette Nebula spans about 100 light-years across, lies about 5000 light-years away, and can be seen with a small telescope towards the constellation of the Unicorn (Monoceros).

Image credit: Brian Lula
The North America Nebula in Infrared (2011-02-15)

The North America Nebula can do what most North Americans cannot -- form stars.  Precisely where in the nebula these stars are forming has been mostly obscured by some of the nebula's thick dust that is opaque to visible light. However, a new view of the North America Nebula in infrared light by the orbiting Spitzer Space Telescope has peered through much of the dust and uncovered thousands of newly formed stars. Rolling your cursor over the above scientifically-colored infrared image will bring up a corresponding optical image of the same region for comparison.  The new infrared image neatly captures young stars in many stages of formation, from being imbedded in dense knots of gas and dust, to being surrounded by disks and emitted jets, to being clear of their birth cocoons. The North America Nebula (NGC 7000) spans about 50 light years and lies about 1,500 light years away toward the constellation of the Swan (Cygnus). Still, of all the stars known in the North America Nebula, which massive stars emit the energetic light that gives the ionized  red glow is still debated.   Images coming in: Stardust spacecraft's close pass of Comet Tempel 1 last night
Comet Tempel 1 from Stardust-NeXT Spacecraft (2011-02-16)

No comet has ever been visited twice before. Therefore, the unprecedented pass of the Stardust-NeXT spacecraft near Comet Tempel 1 earlier this week gave humanity a unique opportunity to see how the nucleus of a comet changes over time.  Changes in the nucleus of Comet Tempel 1 were of particular interest because the comet was hit with an impactor from the passing Deep Impact spacecraft in 2005.  Pictured above is one digitally sharpened image of Comet Tempel 1 near the closest approach of Stardust-NeXT. Visible are many features imaged in 2005, including craters, ridges, and seemingly smoother areas. Few firm conclusions are yet available, but over the next few years astronomers who specialize in comets and the understanding the early Solar System will be poring over these images looking for new clues as to how Comet Tempel 1 is composed, how the 2005 impact site now appears, and how general features of the comet have evolved.
X-Class Flare (2011-02-17)

On Valentine's Day (ET) the Sun unleashed one of its most powerful explosions, an X-class flare. The blast was the largest so far in the new solar cycle. Erupting from active region AR1158 in the Sun's southern hemisphere, the flare is captured here in this extreme ultraviolet image from the Solar Dynamics Observatory (SDO). The intense burst of electromagnetic radiation momentarily overwhelmed pixels in SDO's detectors causing the bright vertical blemish. This X-class flare was also accompanied by a coronal mass ejection (CME), a massive cloud of charged particles traveling outward at nearly 900 kilometers per second. Skywatchers at high latitudes should be alert for aurorae tonight.
Planetary Nebula Project (2011-02-18)

Cast off by dying sunlike stars, planetary nebulae are a brief but glorious final phase of stellar evolution. The gaseous shrouds are ionized by an extremely hot central source, the shrinking core of a star running out of fuel for nuclear fusion. Shining in the cosmic night, their simple symmetries are fascinating and have inspired this planetary nebula poster project. In it, nine planetaries are displayed for comparison in a 3x3 grid. Of course, planetary nebula fans should be able to pick out the bright Messier objects M27 - the Dumbbell Nebula, M76 - the Little Dumbbell, and M57 - the Ring Nebula, as well as NGC 6543, aka the Cat's Eye Nebula. Lesser known nebulae include the Medusa and the Bug. All the images were made with detailed narrow band data and are shown at the same angular scale, spanning 20 arc minutes (1/3 degree). At that scale, the grey circle represents the apparent size of the Full Moon. These planetary nebulae hint at the fate of our own Sun as its core runs out of nuclear fuel in another 5 billion years.

Image credit: Astro Anarchy
Spiral Galaxy NGC 2841 Close Up (2011-02-19)

A mere 46 million light-years distant, spiral galaxy NGC 2841 can be found in the northern constellation of Ursa Major. This sharp view of the gorgeous island universe shows off a striking yellow nucleus and galactic disk. Dust lanes, small, pink star-forming regions, and young blue star clusters are embedded in the patchy, tightly wound spiral arms. In contrast, many other spirals exhibit grand, sweeping arms with large star-forming regions.  NGC 2841 has a diameter of over 150,000 light-years, even larger than our own Milky Way, but this close-up Hubble image spans about 34,000 light-years along the galaxy's inner region. X-ray images suggest that resulting winds and stellar explosions create plumes of hot gas extending into a halo around NGC 2841.
Mammatus Clouds Over Olympic Valley (2011-02-20)

What's happened to these clouds? Normal cloud bottoms are flat because moist warm air that rises and cools will condense into water droplets at a very specific temperature, which usually corresponds to a very specific height.  After water droplets form that air becomes an opaque cloud.  Under some conditions, however, cloud pockets can develop that contain large droplets of water or ice that fall into clear air as they evaporate.  Such pockets may occur in turbulent air near a thunderstorm, being seen near the top of an anvil cloud, for example.  Resulting mammatus clouds can appear especially dramatic if sunlit from the side.  These mammatus clouds were photographed last August over Olympic Valley, California, USA.
Milky Way Over Switzerland (2011-02-21)

What's visible in the night sky during this time of year? To help illustrate the answer, a beautiful land, cloud, and skyscape was captured earlier this month over Neuch�tel, Switzerland. Visible in the foreground were the snow covered cliffs of the amphitheater shaped Creux du Van, as well as distant trees, and town-lit clouds. Visible in the night sky (at midnight) were galaxies including the long arch of the central band of our Milky Way Galaxy, the Andromeda galaxy (M31), and the Triangulum galaxy (M33). Star clusters visible included NGC 752, M34, M35, M41, the double cluster, and the Beehive (M44). Nebulas visible included the Orion Nebula (M42), NGC 7822, IC 1396, the Rosette Nebula, the Flaming Star Nebula, the California Nebula, the Heart and Soul Nebulas, and the Pacman Nebula. Rolling your cursor over the above image will bring up labels for all of these. But the above wide angle sky image captured even more sky wonders. What other nebulas can you find in the above image?
Star Size Comparisons (2011-02-22)

How big is our Sun compared to other stars? In a dramatic and popular video featured on YouTube, the relative sizes of planets and stars are shown from smallest to largest. The above video starts with Earth's Moon and progresses through increasingly larger planets in our Solar System.  Next, the Sun is shown along as compared to many of the brighter stars in our neighborhood of the Milky Way Galaxy. Finally, some of the largest stars known spin into view. Note that the true sizes of most stars outside of the Sun and Betelgeuse are not known by direct observation, but rather inferred by measurements of their perceived brightness, temperature, and distance. Although an inspiring learning tool that is mostly accurate, APOD readers are encouraged to complete the learning experience -- and possibly help make future versions more accurate -- by pointing out slight inaccuracies in the video.

Image credit: morn1415 (YouTube)
The Solar System from MESSENGER (2011-02-23)

If you looked out from the center of the Solar System, what would you see? Nearly such a view was taken recently from the MESSENGER spacecraft currently orbiting the Sun from the distance of Mercury. The Sun's planets all appear as points of light, with the closest and largest planets appearing the brightest. The planets all appear to orbit in the same direction and are (nearly) confined to the same great circle around the sky -- the ecliptic plane. Mercury, Venus, Earth, Mars, Jupiter, and Saturn are all visible in the above horizontally compressed image, while the positions of Uranus and Neptune are labeled even though they are too faint to make out.  Pluto, which has had its planetary status recently called into question, is much too faint to see. Earth's Moon is visible, however, as are the Galilean moons of Jupiter. The above image is the reverse of one taken from the outside of the Solar System in 1990 by Voyager 1. MESSENGER, which has flown by Mercury three times now, is on schedule to enter orbit around the Solar System's innermost planet next month.
NGC 1999: South of Orion (2011-02-24)

South of the large star-forming region known as the Orion Nebula, lies bright blue reflection nebula NGC 1999. Also at the edge of the Orion molecular cloud complex some 1,500 light-years distant, NGC 1999's illumination is provided by the embedded variable star V380 Orionis. The nebula is marked with a dark sideways T-shape near center in this broad cosmic vista that spans over 10 light-years. The dark shape was once assumed to be an obscuring dust cloud seen in silhouette against the bright reflection nebula. But recent infrared images indicate the shape is likely a hole blown through the nebula itself by energetic young stars. In fact, this region abounds with energetic young stars producing jets and outflows that create luminous shock waves. Cataloged as Herbig-Haro (HH) objects, named for astronomers George Herbig and Guillermo Haro, the shocks appear bright red in this view that includes HH1 and HH2 just below NGC 1999. The stellar jets and outflows push through the surrounding material at speeds of hundreds of kilometers per second.

Image credit: Adam Block
NGC 4449: Close-up of a Small Galaxy (2011-02-25)

Grand spiral galaxies often seem to get all the glory. Their young, blue star clusters and pink star forming regions along sweeping spiral arms are guaranteed to attract attention. But small irregular galaxies form stars too, like NGC 4449, about 12 million light-years distant. Less than 20,000 light-years across, the small island universe is similar in size, and often compared to our Milky Way's satellite galaxy, the Large Magellanic Cloud (LMC). This remarkable Hubble Space Telescope close-up of the well-studied galaxy was reprocessed to highlight the telltale reddish glow of hydrogen gas. The glow traces NGC 4449's widespread star forming regions, some even larger than those in the LMC, with enormous interstellar arcs and bubbles blown by short-lived, massive stars. NGC 4449 is a member of a group of galaxies found in the constellation Canes Venatici. Interactions with the nearby galaxies are thought to have influenced star formation in NGC 4449.
Shell Galaxies in Pisces (2011-02-26)

This colorful cosmic skyscape features a peculiar system of galaxies cataloged as Arp 227 some 100 million light-years distant. Swimming within the boundaries of the constellation Pisces, Arp 227 consists of the two galaxies prominent on the left; the curious shell galaxy NGC 474 and its blue, spiral-armed neighbor NGC 470. The faint, wide arcs or shells of NGC 474 could have been formed by a gravitational encounter with neighbor NGC 470. Alternately the shells could be caused by a merger with a smaller galaxy producing an effect analogous to ripples across the surface of a pond. Remarkably, the large galaxy on the right hand side of the deep image, NGC 467, appears to be surrounded by faint shells too, evidence of another interacting galaxy system. Intriguing background galaxies are scattered around the field that also includes spiky foreground stars. Of course, those stars lie well within our own Milky Way Galaxy. The field of view spans 25 arc minutes or about 1/2 degree on the sky.

Image credit: Stephen Leshin
Saturn's Hyperion: A Moon with Odd Craters (2011-02-27)

What lies at the bottom of Hyperion's strange craters?  Nobody's sure.  To help find out, the robot Cassini spacecraft now orbiting Saturn swooped past the sponge-textured moon in 2005 and 2010 and took images of unprecedented detail.  An image from the 2005 pass, shown above in false color, shows a remarkable world strewn with strange craters and a generally odd surface.   The slight differences in color likely show differences in surface composition.   At the bottom of most craters lies some type of unknown dark material.  Inspection of the image shows bright features indicating that the dark material might be only tens of meters thick in some places.   Hyperion is about 250 kilometers across, rotates chaotically, and has a density so low that it might house a vast system of caverns inside.   Students (of all ages): See free astronomy lectures online
Red Snow Moon Over Edmonton (2011-02-28)

What's hovering between those buildings? The Moon.  The above image was taken two weeks ago as the full Snow Moon started to rise above Edmonton, Alberta, Canada. The odd coincidence between the angular size of the far distant Moon and the angular width of nearby buildings created a striking juxtaposition. Backing away from the buildings so to reduce their angular size was a key to planning the image. The temperature was so low, -25 C, that plumes of steam rose from neighboring oil refineries. The above image was taken during a momentary break in the plumes. The rising Moon appears red here for the same reason that a setting Sun appears red -- because blue light is preferentially scattered away by intervening air. In this case, the shimmering steam plumes likely also caused the Moon to appear slightly compressed. The next full moon, the full Worm Moon, will occur in mid-March.   Professional Astronomers: Browse the Astrophysics Source Code Library

Image credit: Luca Vanzella