juliensimon/nasa-apod huggingface dataset:
A Pileus Iridescent Cloud Over Ethiopia (2011-08-24)

Yes, but how many dark clouds have a multicolored lining? Pictured, behind this darker cloud, is a pileus iridescent cloud, a group of water droplets that have a uniformly similar size and so together diffract different colors of sunlight by different amounts.  The above image was taken just after the picturesque sight was noticed by chance by a photographer in Ethiopia.  A more detailed picture of the same cloud shows not only many colors, but unusual dark and wavy bands whose origins are thought related to  wave disturbances in the cloud.

Image credit: Esther Havens
(Light the World)
Portrait of NGC 281 (2011-08-25)

Look through the cosmic cloud cataloged as NGC 281 and it's almost easy to miss stars of open cluster IC 1590. But, formed within the nebula, that cluster's young, massive stars ultimately power the pervasive nebular glow. The eye-catching shapes looming in this portrait of NGC 281 are sculpted columns and dense dust globules seen in silhouette, eroded by intense, energetic winds and radiation from the hot cluster stars. If they survive long enough, the dusty structures could also be sites of future star formation. Playfully called the Pacman Nebula because of its overall shape, NGC 281 is about 10,000 light-years away in the constellation Cassiopeia. This composite image was made through narrow-band filters, but combines emission from the nebula's hydrogen, sulfur, and oxygen atoms in a visible spectrum palette. It spans over 80 light-years at the estimated distance of NGC 281.

Image credit: Astro Anarchy
A Young Supernova in the Nearby Pinwheel Galaxy (2011-08-26)

A nearby star has exploded and telescopes all over the world are turning to monitor it.  The supernova, dubbed PTF 11kly, was discovered by computer only two days ago as part of the Palomar Transient Factory (PTF) sky survey utilizing the wide angle 1.2-meter Samuel Oschin Telescope in California.  Its rapid recovery makes it one of the supernovas caught most soon after ignition.  PTF 11kly occurred in the photogenic Pinwheel galaxy (M101), which, being only about 21 million light years away, makes it one of the closest supernovas seen in decades.  Rapid follow up observations have already given a clear indication that PTF 11kly is a Type Ia supernova, a type of white dwarf detonation that usually progresses in such a standard manner than it has helped to calibrate the expansion history of the entire universe.  Studying such a close and young Type Ia event, however, may yield new and unique clues. If early indications are correct, PTF 11kly should brighten to about visual magnitude 10 in the coming weeks, making it possible to monitor with even moderately sized telescopes.   APOD Retrospective: The best of the spiral galaxy M101
Hurricane Irene Forms (2011-08-27)

How does a hurricane form?  Although a complete picture is still being researched, insight into this process might be gleaned by watching the above time lapse movie of the formation of Hurricane Irene, a large storm system currently threatening the eastern seaboard of the USA. Starting as a slight pressure difference visible as nondescript clouds on the lower right, Hurricane Irene is shown growing into large spiraling storm system of low pressure off the coast of South Carolina.  A hurricane is powered by evaporating ocean water, and so typically gains strength over warm water and loses strength over land. Besides Earth, other planets that have hurricane-like storm systems include Venus, Saturn, Jupiter, Uranus, and Neptune.  Much remains unknown about hurricanes and cyclones, including the exact path they will take.
A Jet from Galaxy M87 (2011-08-28)

What's causing a huge jet to emanate from the center of galaxy M87? Although the unusual jet was first noticed early in the twentieth century, the exact cause is still debated.  The above picture taken by the Hubble Space Telescope in 1998 shows clear details, however. The most popular hypothesis holds that the jet is created by energetic gas swirling around a massive black hole at the galaxy's center.  The result is a 5000 light-year long blowtorch where electrons are ejected outward at near light-speed, emitting eerily blue light during a magnetic spiral.  M87 is a giant elliptical galaxy residing only 50 million light-years away in the Virgo Cluster of Galaxies.  The faint dots of light surrounding M87's center are large ancient globular clusters of stars.
Hickson 44 in Leo (2011-08-29)

Scanning the skies for galaxies, Canadian astronomer Paul Hickson and colleagues identified some 100 compact groups of galaxies, now appropriately called Hickson Compact Groups. The four prominent galaxies seen in this intriguing telescopic skyscape are one such group, Hickson 44, about 100 million light-years distant toward the constellation Leo. The two spiral galaxies in the center of the image are edge-on NGC 3190 with its distinctive, warped dust lanes, and S-shaped NGC 3187. Along with the bright elliptical, NGC 3193 at the right, they are also known as Arp 316. The spiral in the upper left corner is NGC 3185, the 4th member of the Hickson group. Like other galaxies in Hickson groups, these show signs of distortion and enhanced star formation, evidence of a gravitational tug of war that will eventually result in galaxy mergers on a cosmic timescale. The merger process is now understood to be a normal part of the evolution of galaxies, including our own Milky Way. For scale, NGC 3190 is about 75,000 light-years across at the estimated distance of Hickson 44.   APOD Retrospective: Small Groups of Galaxies

Image credit: Stephen Leshin
The Coldest Brown Dwarf (2011-08-30)

This cosmic snapshot composed with image data from NASA's Wide-field Infrared Survey Explorer (WISE) satellite captures a multitude of faint stars and distant galaxies toward the constellation Lyra at wavelengths longer than visible light. But the object circled at the center is not quite a star. Cataloged as WISE 1828+2650, it lies within 40 light-years of the Sun and is currently the coldest brown dwarf known. A brown dwarf begins like a star, with the gravitational collapse of a dense cloud of gas and dust, but is not massive enough to achieve the core temperatures and densities that trigger hydrogen fusion, the stable source of a star's energy. Instead the failed star ultimately cools and emits most of its light at infrared wavelengths. Remarkably, brown dwarfs are roughly the size of the planet Jupiter. How cold is WISE 1828+2650? While brown dwarfs have measured surface temperatures of up to 1,400 degrees C (2,600 degress F), this brown dwarf , assigned to spectral class Y, has the estimated temperature of a warm room, less than about 27 degrees C (80 degrees F).
Roll Cloud Over Wisconsin (2011-08-31)

What kind of cloud is this?  A type of arcus cloud called a roll cloud.  These rare long clouds may form near advancing cold fronts.   In particular, a downdraft from an advancing storm front can cause moist warm air to rise, cool below its dew point, and so form a cloud.  When this happens uniformly along an extended front, a roll cloud may form.  Roll clouds may actually have air circulating along the long horizontal axis of the cloud.  A roll cloud is not thought to be able to morph into a tornado.  Unlike a similar shelf cloud, a roll cloud is completely detached from their parent  cumulonimbus cloud.  Pictured above, a roll cloud extends far into the distance as a storm approached in 2007 in Racine, Wisconsin, USA.
M27: Not a Comet (2011-09-01)

While hunting for comets in the skies above 18th century France, astronomer Charles Messier diligently kept a list of the things he encountered that were definitely not comets. This is number 27 on his now famous not-a-comet list. In fact, 21st century astronomers would identify it as a planetary nebula, but it's not a planet either, even though it may appear round and planet-like in a small telescope. Messier 27 (M27) is an excellent example of a gaseous emission nebula created as a sun-like star runs out of nuclear fuel in its core. The nebula forms as the star's outer layers are expelled into space, with a visible glow generated by atoms excited by the dying star's intense but invisible ultraviolet light. Known by the popular name of the Dumbbell Nebula, the beautifully symmetric interstellar gas cloud is over 2.5 light-years across and about 1,200 light-years away in the constellation Vulpecula. This impressive color composite highlights details within the well-studied central region and fainter, seldom imaged features in the nebula's outer halo. It incorporates broad and narrowband images recorded using filters sensitive to emission from sulfur, hydrogen and oxygen atoms.

Image credit: Martin Pugh
Herschel Views the Milky Way (2011-09-02)

With a 3.5 meter diameter mirror, larger than the Hubble Space Telescope, ESA's Herschel Space Observatory explores the Universe at infrared wavelengths. Herschel is named for German-born British astronomer Frederick William Herschel who discovered infrared light over 200 years ago. Herschel's sensitive cameras have combined to deliver this spectacular skyscape looking toward the constellation of the Southern Cross. Spanning some 2 degrees the premier, false-color, far-infrared view captures our galaxy's cold dust clouds in extreme detail, showing a remarkable, connected maze of filaments and star-forming regions. Such observations are intended to unravel mysteries of star formation by surveying broad areas of the galactic plane.
Comet Garradd Passes Ten Thousand Stars (2011-09-03)

Comet Garradd continues to brighten as it drifts across the northern sky. Last week the comet, visible with binoculars and discernible by its green coma, passed nearly in front of globular cluster M71.  M71 was once thought to be an open cluster, but is now known to be an older globular cluster containing over 10,000 stars. The photogenic duo was captured with a standard digital camera in a 10-minute, wide-angle exposure toward the northern constellation of the Arrow (Sagitta).  The stars Sham (alpha Sagittae), beta Sagittae, gamma Sagittae, and the double star delta Sagittae are all visible in a diagonal band running down from the upper left. Comet C/2009 P1 (Garradd), will remain visible in northern skies for months and will reach its closest approach to the Sun in December.
In the Shadow of Saturn (2011-09-04)

In the shadow of Saturn, unexpected wonders appear. The robotic Cassini spacecraft now orbiting Saturn drifted in giant planet's shadow for about 12 hours in 2006 and looked back toward the eclipsed Sun.  Cassini saw a view unlike any other.  First, the night side of Saturn is seen to be partly lit by light reflected from its own majestic ring system.  Next, the rings themselves appear dark when silhouetted against Saturn, but quite bright when viewed away from Saturn, slightly scattering sunlight, in this exaggerated color image. Saturn's rings light up so much that new rings were discovered, although they are hard to see in the image.  Seen in spectacular detail, however, is Saturn's E ring, the ring created by the newly discovered ice-fountains of the moon Enceladus and the outermost ring visible above.  Far in the distance, at the left, just above the bright main rings, is the almost ignorable pale blue dot of Earth.
HH 47: A Young Star Jet Expands (2011-09-05)

Stars remain where they are. Nebulas appear the same. Day after day. Year after year. Given the vast distances in astronomy, even fast moving objects will not appear to change their appearance in a human lifetime. Typically. A recent spectacular exception to this, however, is the supersonic jet in the star forming Herbig Haro 47. HH 47 is so close -- and the jets are moving so fast -- that images from the Hubble Space Telescope from 1994 to 2008 have been combined into a time-lapse movie that actually shows a powerful jet expanding.  Visible above, jets of plasma extending over 10,000 times the Earth-Sun distance shoot out from a forming star at speeds in excess of 150 kilometers per second. Studying how these jets evolve gives clues not only to how the star in HH 47 is forming, but how stars like our Sun formed billions of years ago. HH 47 is located about 1,500 light years away toward the constellation of Sails of a Ship (Vela).
M6: The Butterfly Cluster (2011-09-06)

To some, the outline of the open cluster of stars M6 resembles a butterfly.  M6, also known as NGC 6405, spans about 20 light-years and lies about 2,000 light years distant.  M6 can best be seen in a dark sky with binoculars towards the constellation of Scorpius, coving about as much of the sky as the full moon.  Like other open clusters, M6 is composed predominantly of young blue stars, although the brightest star is nearly orange.  M6 is estimated to be about 100 million years old.  Determining the distance to clusters like M6 helps astronomers calibrate the distance scale of the universe.    APOD Retrospective: Open Star Clusters

Image credit: Sergio Eguivar
Buenos Aires Skies
SDSS J102915+172927: A Star That Should Not Exist (2011-09-07)

Why does this star have so few heavy elements? Stars born in the generation of our Sun have an expected abundance of elements heavier than hydrogen and helium mixed into their atmospheres. Stars born in the generation before our Sun, Population II stars, the stars that created most of the heavy elements around us today, are seen to have some, although fewer, elements heavier than H and He. Furthermore, even the elusive never-seen first stars in the universe, so-called Population III stars, are predicted to have a large mass and a small but set amount of heavy elements.  Yet low-mass Milky Way star SDSS J102915+172927, among others, appears to have fewer metals than ever predicted for any stars, including at least 50 times less lithium than came out of the Big Bang.  The unusual nature of this star, initially cataloged by the Sloan Digital Sky Survey (SDSS) and pictured above, was discovered by detailed spectroscopic observations by a large VLT telescope in Chile. Many models of star formation indicate that such a star should not even form.  Research is ongoing, however, with one leading hypothesis holding that fragile primordial lithium was destroyed in the star's hot core.
Apollo 17 Site: A Sharper View (2011-09-08)

This view of the Apollo 17 landing site in the Taurus-Littrow valley was captured last month by the Lunar Reconnaissance Orbiter (LRO), the sharpest ever recorded from space. The high resolution image data was taken during a period when LRO's orbit was modified to create a close approach of about 22 kilometers as it passed over some of the Apollo landing sites. That altitude corresponds to only about twice the height of a commercial airline flight over planet Earth. Labeled in this image are Apollo 17 lunar lander Challenger's descent stage (inset), the lunar rover (LRV) at its final parking spot, and the Apollo Lunar Surface Experiments Package (ALSEP) left to monitor the Moon's environment and interior. Clear, dual lunar rover tracks and the foot trails left by astronauts Eugene Cernan and Harrison Schmitt, the last to walk on the lunar surface, are also easily visible at the Apollo 17 site.
Comet Garradd and the Coat Hanger (2011-09-09)

Sweeping through planet Earth's night sky, last weekend Comet Garradd (C/2009 P1) visited this lovely star field along the Milky Way in the constellation Vulpecula. Suggestively oriented, the colorful skyscape features stars in the asterism known as the Coat Hanger with the comet's tail pointing toward the southeast. Also known as Al Sufi's Cluster, the Coat Hanger itself is likely just a chance alignment and not a cluster of related stars. But compact open star cluster NGC 6802 does grace the field of view just right of the Coat Hanger, near the edge of the frame. Below naked eye visibility but approaching 7th magnitude in brightness, Comet Garradd has been a good target for binoculars and small telescopes. Still, bright moonlit skies this week will make the comet harder to spot.

Image credit: Rogelio
Bernal Andreo
Rollback from GRAIL's Rocket (2011-09-10)

This dramatic time-lapse photo traces a 20 minute long, late evening rollback of the lighted Mobile Service Tower at Cape Canaveral Air Force Station Launch Complex 17. Twin spacecraft are snug inside the 13 story tall Delta 2 rocket poised for launch. The duo will journey to the Moon on NASA's Gravity Recovery and Interior Laboratory mission, GRAIL, using precision formation flying to map the Moon's gravity field. The scene was captured prior to the first launch attempt, canceled due to upper level winds, on September 8. Further launch opportunities for GRAIL begin this morning, September 10.  The GRAIL launch is the last one planned from Complex 17 on Cape Canaveral's oldest still active pad, in use since 1957.

Image credit: Ben
Cooper
On the Origin of Gold (2011-09-11)

Where did the gold in your jewelry originate?  No one is completely sure.  The relative average abundance in our Solar System appears higher than can be made in the early universe, in stars, and even in typical supernova explosions.  Some astronomers have recently suggested that neutron-rich heavy elements such as gold might be most easily made in rare neutron-rich explosions such as the collision of neutron stars.  Pictured above is an artist's illustration depicting two neutron stars spiraling in toward each other, just before they collide.  Since neutron star collisions are also suggested as the origin of short duration gamma-ray bursts, it is possible that you already own a souvenir from one of the most powerful explosions in the universe.
Tisdale 2 Rock Formation on Mars (2011-09-12)

Why does this Martian rock have so much zinc? Roughly the size and shape of a tilted coffee-table, this oddly flat, light-topped rock outcropping was chanced upon a few weeks ago by the robotic Opportunity rover currently rolling across Mars. Early last month Opportunity reached Endeavour crater, the largest surface feature it has ever encountered, and is now exploring Endeavour's rim for clues about how wet Mars was billions of years ago. Pictured above and named Tisdale 2, the unusual rock structure was probed by Opportunity last week and is now thought to be a remnant thrown off during the impact that created nearby Odyssey crater. The resulting chemical analysis of Tisdale 2, however, has shown it to have a strangely high amount of the element zinc. The reason for this is currently unknown, but might turn out to be a clue to the history of the entire region. Opportunity is already finding rocks older than any previously studied and will continue to explore several other intriguing rock formations only now glimpsed from a distance.
Great Orion Nebulae (2011-09-13)

The Great Nebula in Orion, also known as M42, is one of the most famous nebulas in the sky. The star forming region's glowing gas clouds and hot young stars are on the right in this sharp and colorful image that includes the smaller nebula M43 near center and dusty, bluish reflection nebulae NGC 1977 and friends on the left. Located at the edge of an otherwise invisible giant molecular cloud complex, these eye-catching nebulae represent only a small fraction of this galactic neighborhood's wealth of interstellar material. Within the well-studied stellar nursery, astronomers have also identified what appear to be numerous infant planetary systems. The gorgeous skyscape spans nearly two degrees or about 45 light-years at the Orion Nebula's estimated distance of 1,500 light-years.

Image credit: Jes�s Vargas  
(Astrogades) & Maritxu Poyal (Maritxu)
The Bubble and M52 (2011-09-14)

To the eye, this cosmic composition nicely balances the Bubble Nebula at the lower right with open star cluster M52. The pair would be lopsided on other scales, though. Embedded in a complex of interstellar dust and gas and blown by the winds from a single, massive O-type star, the Bubble Nebula, also known as NGC 7635, is a mere 10 light-years wide.  On the other hand, M52 is a rich open cluster of around a thousand stars. The cluster is about 25 light-years across. Seen toward the northern boundary of Cassiopeia, distance estimates for the Bubble Nebula and associated cloud complex are around 11,000 light-years, while star cluster M52 lies nearly 5,000 light-years away. The wide telescopic field of view spans about 1.5 degrees on the sky or three times the apparent size of the Full Moon.

Image credit: Lóránd Fényes
NGC 3521: Galaxy in a Bubble (2011-09-15)

Gorgeous spiral galaxy NGC 3521 is a mere 35 million light-years away, toward the constellation Leo. Relatively bright in planet Earth's sky, NGC 3521 is easily visible in small telescopes but often overlooked by amateur imagers in favor of other Leo spiral galaxies, like M66 and M65. It's hard to overlook in this colorful cosmic portrait, though. Spanning some 50,000 light-years the galaxy sports characteristic patchy, irregular spiral arms laced with dust, pink star forming regions, and clusters of young, blue stars. Remarkably, this deep image also finds NGC 3521 embedded in gigantic bubble-like shells. The shells are likely tidal debris, streams of stars torn from satellite galaxies that have undergone mergers with NGC 3521 in the distant past.

Image credit: R Jay Gabany
September's Harvest Moon (2011-09-16)

A Full Moon rising can be a dramatic celestial sight, and Full Moons can have many names. For example, Monday's Full Moon was the one nearest this year's autumnal equinox for the northern hemisphere, traditionally called the Harvest Moon. According to lore the name is a fitting one because farmers could work late into the night at the end of the growing season harvesting crops by moonlight. This serene telephoto image captures this September's harvest moonrise from Turin, Italy. In silhouette against an orange lunar disk is Turin's hilltop Basilica of Superga.   APOD Retrospective: Harvest and Hunter Moons

Image credit: Stefano De Rosa
Spitzer's Orion (2011-09-17)

Few cosmic vistas excite the imagination like the Orion Nebula, an immense stellar nursery some 1,500 light-years away. This stunning false-color view spans about 40 light-years across the region, constructed using infrared data from the Spitzer Space Telescope. Compared to its visual wavelength appearance, the brightest portion of the nebula is likewise centered on Orion's young, massive, hot stars, known as the Trapezium Cluster. But the infrared image also detects the nebula's many protostars, still in the process of formation, seen here in red hues. In fact, red spots along the dark dusty filament to the left of the bright cluster include the protostar cataloged as HOPS 68, recently found to have crystals of the silicate mineral olivine within its protostellar envelope.
A Sharp View of the Sun (2011-09-18)

Here is one of the sharper views of the Sun ever taken. This stunning image shows remarkable details of a dark sunspot across the image bottom and numerous boiling granules which appear like kernels of corn across the top.  Taken in 2002, the picture was made using the Swedish Solar Telescope operating on the Canary Island of La Palma. The high resolution image was achieved using sophisticated adaptive optics, digital image stacking, and other processing techniques to counter the blurring effect of Earth's atmosphere. Currently a sunspot group is crossing the Sun that is so large it can be easily seen by the cautious observer even without magnification.