juliensimon/nasa-apod huggingface dataset:
Andromeda Island Universe (2005-12-22)

The most distant object easily visible to the unaided eye is M31, the great Andromeda Galaxy some two million light-years away. But without a telescope, even this immense spiral galaxy - spanning over 200,000 light years - appears as a faint, nebulous cloud in the constellation Andromeda. In contrast, a bright yellow nucleus, dark winding dust lanes, gorgeous blue spiral arms and star clusters are recorded in this stunning telescopic digital mosaic with a cumulative exposure of over 90 hours. While even casual skygazers are now inspired by the knowledge that there are many distant galaxies like M31, astronomers seriously debated this fundamental concept only 80 years ago. Were these "spiral nebulae" simply outlying components of our own Milky Way Galaxy or were they instead "island universes" -- distant systems of stars comparable to the Milky Way itself? This question was central to the famous Shapley-Curtis debate of 1920, which was later resolved by observations of M31 in favor of Andromeda, island universe.

Image credit: Robert Gendler
Hydrogen and Dust in the Rosette Nebula (2005-12-23)

At the edge of a large molecular cloud in Monoceros, some 3,000 light years away, dark filaments of dust are silhouetted by luminous hydrogen gas. The close up view of the Rosette Nebula dramatically suggests that star formation is an on going process in the region, with dark filaments sculpted by winds and radiation from hot, young stars. Ultraviolet radiation from the young stars also strips electrons from the surrounding hydrogen atoms. As electrons and atoms recombine they emit longer wavelength, lower energy light in a well known characteristic pattern of bright spectral lines. At visible wavelengths, the strongest emission line in this pattern is in the red part of the spectrum and is known as "Hydrogen-alpha" or just H-alpha. Part of IPHAS, a survey of H-alpha emission in our Milky Way Galaxy, this image spans about 25 light-years.
Earthrise (2005-12-24)

In December of 1968, the Apollo 8 crew flew from the Earth to the Moon and back again. Frank Borman, James Lovell, and William Anders were launched atop a Saturn V rocket on December 21, circled the Moon ten times in their command module, and returned to Earth on December 27. The Apollo 8 mission's impressive list of firsts includes: the first humans to journey to the Earth's Moon, the first manned flight using the Saturn V, and the first to photograph the Earth from deep space.  As the Apollo 8 command module rounded the farside of the Moon, the crew could look toward the lunar horizon and see the Earth appear to rise, due to their spacecraft's orbital motion. The famous picture that resulted, of a distant blue Earth above the Moon's limb, was a marvelous gift to the world.
The Mysterious Cone Nebula (2005-12-25)

Sometimes the simplest shapes are the hardest to explain.  For example, the origin of the mysterious cone-shaped region seen on the far left remains a mystery.   The interstellar formation, dubbed the Cone Nebula, is located about 2700 light years away. Other features in the image include red emission from diffuse  interstellar hydrogen, wispy filaments of dark dust, and bright star S Monocerotis, visible on the far right.  Blue reflection nebulae surround the brighter stars.  The dark Cone Nebula region clearly contains much dust which blocks light from the emission nebula and open cluster NGC 2264 behind it.  One hypothesis holds that the Cone Nebula is formed by wind particles from an energetic source blowing past the Bok Globule at the head of the cone.

Image credit: R Jay GaBany 
(Cosmotography.com)
SN 1006: Supernova Remnant in X-Rays (2005-12-26)

This huge puff ball was once a star.  One thousand years ago, in the year 1006, a new star was recorded in the sky that today we know was really an existing star exploding.  The resulting expanding gas from the supernova is still visible with telescopes today, continues to expand, and now spans over 70 light years.  SN 1006 glows in every type of light.  The above image of SN 1006 was captured by the orbiting Chandra Observatory in X-ray light.  Even today, not everything about the SN 1006 is understood, for example why particle shocks that produce the bright blue filaments are only visible at some locations.  SN 1006 is thought to have once been a white dwarf that exploded when gas being dumped onto it by its binary star companion caused it to go over the Chandrasekhar limit.  Foreground stars are visible that have nothing to do with the supernova.
IC 2118: The Witch Head Nebula (2005-12-27)

Double, double toil and trouble; Fire burn, and cauldron bubble -- maybe Macbeth should have consulted the Witch Head Nebula.  This suggestively shaped reflection nebula is associated with the bright star Rigel in the  constellation Orion.  More formally known as IC 2118, the Witch Head Nebula glows primarily by light reflected from Rigel, located just outside the top right corner of the above image. Fine dust in the nebula reflects the light.  The blue color is caused not only by Rigel's blue color but because the dust grains reflect blue light more efficiently than red.  The same physical process causes  Earth's daytime sky to appear blue, although the scatterers in Earth's atmosphere are molecules of nitrogen and oxygen.  The nebula lies about 1000 light-years away.

Image credit: Bernhard Hubl
Smooth Sections on Asteroid Itokawa (2005-12-28)

Why are parts of this asteroid's surface so smooth?  No one is yet sure, but it may have to do with the dynamics of an asteroid that is a loose pile of rubble rather than a solid rock.  The unusual asteroid is currently being visited by the Japanese spacecraft Hayabusa that is documenting its unusual structure and mysterious lack of craters.  Last month, Hayabusa actually touched down on one of the smooth patches, dubbed the MUSES Sea, and collected soil samples that will eventually be returned to Earth for analysis.  Unfortunately, the robot Hayabusa craft has been experiencing communications problems and so its departure for Earth has been delayed until 2007.   Computer simulations show that 500-meter asteroid Itokawa may impact the Earth within the next few million years.

Image credit: ISAS,
JAXA
The Iris Nebula from CFHT (2005-12-29)

Like delicate cosmic petals, these clouds of interstellar dust and gas have blossomed 1,300 light-years away in the fertile star fields of the constellation Cepheus. Sometimes called the Iris Nebula and dutifully cataloged as NGC 7023, this is not the only nebula in the sky to evoke the imagery of flowers. Still, this beautiful digital image shows off the Iris Nebula's range of colors and symmetries in impressive detail. Within the Iris, dusty nebular material surrounds a massive, hot, young star in its formative years. Central filaments of cosmic dust glow with a reddish photoluminescence as some dust grains effectively convert the star's invisible ultraviolet radiation to visible red light. Yet the dominant color of the nebula is blue, characteristic of dust grains reflecting starlight. Dark, obscuring clouds of dust and cold molecular gas are also present and can lead the eye to see other convoluted and fantastic shapes. Infrared observations indicate that this nebula may contain complex carbon molecules known as PAHs. As shown here, the Iris Nebula is about 6 light-years across.

Image credit: Jean-Charles Cuillandre
The Perseus Cluster of Galaxies (2005-12-30)

Here is one of the largest objects that anyone will ever see on the sky.  Each of these fuzzy blobs is a galaxy, together making up the Perseus Cluster, one of the closest clusters of galaxies.  The cluster is seen through a foreground of faint stars in our own Milky Way Galaxy.  Near the cluster center, roughly 250 million light-years away, is the cluster's dominant galaxy NGC 1275, seen here just left of picture center. A prodigious source of x-rays and radio emission, NGC 1275 accretes matter as gas and galaxies fall into it. The Perseus Cluster of Galaxies is part of the Pisces-Perseus supercluster spanning over 15 degrees and containing over 1,000 galaxies. At the distance of NGC 1275, this view covers about 1.5 million light-years.

Image credit: Ken Crawford  
(Rancho Del Sol Observatory)
A Year at Saturn (2005-12-31)

Arriving at Saturn in July of 2004, the Cassini spacecraft has now spent a year and a half exploring the magnificent rings and moons of the distant gas giant. The year 2005 began with Cassini's Huygens probe landing on Saturn's large moon Titan. Cassini's continuing series of close flybys also revealed details and discoveries across the surface of the smog shrouded moon. In fact, with a ringside seat throughout 2005, Cassini's cameras have made spectacular pictures of Titan along with Saturn's other moons and rings almost common place. But often, Saturn itself provided the most dramatic backdrop. In this view, Saturn's moon Dione lies in front of edge-on rings and the gas giant's cloud tops draped with broad ring shadows. Dione is 1,118 kilometers across and lies about 300,000 kilometers from the ring's edge.
The Largest Rock in the Solar System (2006-01-01)

There, that faint dot in the center - that's the largest rock known in our Solar System.  It is larger than every known asteroid, moon, and comet nucleus.  It is larger than any other local rocky planet. (Nobody knows for sure what size rocks lie at the cores of Jovian planets, or orbit other stars.)  It used to be the largest rock of any type known until earlier last year.  The Voyager 1 spacecraft took the above picture of the giant space rock in 1990 from the outer Solar System.  This rock is so large its gravity makes it nearly spherical, and holds heavy gases near its surface. Today, this rock starts another orbit around its parent star, for roughly the 5 billionth time, spinning over 350 times during each trip.  Happy Gregorian Calendar New Year to all the human inhabitants of this rock we call Earth.
A Sun Pillar Over Maine (2006-01-02)

Have you ever seen a sun pillar?  When the air is cold and the Sun is rising or setting, falling ice crystals can reflect sunlight and create an unusual column of light.  Ice sometimes forms flat, six-sided shaped crystals as it falls from high-level clouds.  Air resistance causes these crystals to lie nearly flat much of the time as they flutter to the ground.  Sunlight reflects off crystals that are properly aligned, creating the sun-pillar effect.  In the above picture taken late last month, a sun-pillar reflects light from a Sun setting over Bangor, Maine, USA.

Image credit: Lucy Orloski
Dark Terrain on Saturn's Iapetus (2006-01-03)

Why are vast sections of Iapetus as dark as coal?   No one knows for sure. Iapetus, the third largest moon of Saturn, was inspected again as the Saturn-orbiting robot Cassini spacecraft swooped past the enigmatic world again late last year.  The dark material covers most of the surface visible in the above image, while the small portion near the top that appears almost white is of a color and reflectance more typical of Saturn's other moons. The unknown material covers about half of the 1,500 kilometer wide moon. The material is so dark that it reflects less than five percent of incident sunlight, yet overlays craters indicating that it was spread after the craters were formed.  Iapetus has other unexplained features. The bright part of Iapetus is covered with unexplained long thin streaks.  The orbit of Iapetus is also unusual, being tilted to the plane of Saturn's orbit by an unusually high fifteen degrees. A strange ridge about 13 kilometers high crosses much of Iapetus near the equator and is visible near the bottom.  Oddly, this ridge is almost exactly parallel with Iapetus' equator. The exact shape of Iapetus remains undetermined, but images indicate that it is quite strange -- something like a walnut.   Research into the formation and history of mysterious Iapetus is active and ongoing.
Hauoli Makahiki Hou (2006-01-04)

Fading sunlight, a young crescent Moon, and brilliant Venus shared the western sky in this view of 2005's final sunset from the top of Mount Haleakala, on Maui, Hawaii. Also known as the Sacred House of the Sun, Haleakala, is Maui's dormant volcano. At 10,000 feet the summit is an ideal site for astronomical observatories, and this scene also features the silhouette of the northern hemisphere Faulkes Telescope. Of particular interest to students the Faulkes Telescope is a 2-meter diameter instrument, dedicated to astronomy education, that can be remotely operated over the internet. The project is a joint effort between the Dill Faulkes Educational Trust and the University of Hawaii Institute for Astronomy. Of course on Haleakala, "Happy New Year" would be "Hauoli Makahiki hou" (how-oh-lee ma-ka-hee-key ho).

Image credit: Rob Ratkowski
New Year Mars Panorama (2006-01-05)

According to an Earth-based calendar, the Spirit rover spent the first day of 2006 gathering data to complete this panoramic view from Gusev crater on Mars. That day corresponded to Spirit's 710th Martian day or sol on the Red Planet. Scrolling right the view spans 160 degrees, looking up a slope and across rippled sand deposits in a dark field dubbed "El Dorado". The Spirit rover is traveling in a down hill direction after reaching the summit of Husband Hill. This month, both Spirit and Opportunity rovers will celebrate two years of Mars exploration, a remarkable achievement considering their original 90 day warranty. During that time Spirit has traveled over 3.5 miles and Opportunity over 4 miles across the Martian surface.
The Tarantula Nebula (2006-01-06)

First cataloged as a star, 30 Doradus is actually an immense star forming region in nearby galaxy The Large Magellanic Cloud. The region's spidery appearance is responsible for its popular name, the Tarantula Nebula, except that this tarantula is about 1,000 light-years across, and 180,000 light-years away in the southern constellation Dorado. If the Tarantula Nebula were at the distance of the Orion Nebula (1,500 light-years), the nearest stellar nursery to Earth, it would appear to cover about 30 degrees on the sky or 60 full moons. The spindly arms of the Tarantula Nebula surround NGC 2070, a cluster that contains some of the intrinsically brightest, most massive stars known. Intriguing details of the nebula's core can be seen in this remarkable skyscape, a composite of 31 hours of exposure time. This cosmic Tarantula also lies near the site of the closest recent supernova.

Image credit: Brad Moore
Apollo 17's Moonship (2006-01-07)

Awkward and angular looking, Apollo 17's lunar module Challenger was designed for flight in the vacuum of space. This picture from command module America, shows Challenger's ascent stage in lunar orbit. Small reaction control thrusters are at the sides of the moonship with the bell of the ascent rocket engine underneath. The hatch allowing access to the lunar surface is seen at the front, with a round radar antenna at the top. Mission commander Gene Cernan is just visible through the dark, triangular window. This spaceship performed gracefully, landing on the Moon and returning the Apollo astronauts to the orbiting command module in December of 1972. So where is Challenger now? Its descent stage remains at the Apollo 17 landing site, Taurus-Littrow. The ascent stage was intentionally crashed nearby after being jettisoned from the command module prior to the astronauts' return to planet Earth. Apollo 17's mission was the sixth and last time astronauts have landed on the Moon.
Arp 188 and the Tadpole's Tidal Tail (2006-01-08)

In this stunning vista recorded with the Hubble Space Telescope's Advanced Camera for Surveys, distant galaxies form a dramatic backdrop for disrupted spiral galaxy Arp 188, the Tadpole Galaxy. The cosmic tadpole is a mere 420 million light-years distant toward the northern constellation Draco. Its eye-catching tail is about 280 thousand light-years long and features massive, bright blue star clusters. One story goes that a more compact intruder galaxy crossed in front of Arp 188 - from left to right in this view - and was slung around behind the Tadpole by their gravitational attraction. During the close encounter, tidal forces drew out the spiral galaxy's stars, gas, and dust forming the spectacular tail. The intruder galaxy itself, estimated to lie about 300 thousand light-years behind the Tadpole, can be seen through foreground spiral arms at the upper left. Following its terrestrial namesake, the Tadpole Galaxy will likely lose its tail as it grows older, the tail's star clusters forming smaller satellites of the large spiral galaxy.
M45: The Pleiades Star Cluster (2006-01-09)

Perhaps the most famous star cluster on the sky, the Pleiades can be seen without binoculars from even the depths of a light-polluted city.  Also known as the Seven Sisters and M45, the Pleiades is one of the brightest and closest open clusters.   The Pleiades contains over 3000 stars, is about 400 light years away, and only 13 light years across.  Quite evident in the above photograph are the blue reflection nebulae that surround the brighter cluster stars.  Low mass, faint, brown dwarfs have also been found in the Pleiades. (Editors' note:  The prominent diffraction spikes are caused by the telescope itself and may be either distracting or provide aesthetic enhancement, depending on your point of view.)

Image credit: Robert Gendler
The Phases of Venus (2006-01-10)

Venus goes through phases.  Just like our Moon, Venus can appear as full as a disk or as a thin as a crescent.  Venus, frequently the brightest object in the post-sunset or pre-sunrise sky, appears so small, however, that it usually requires binoculars or a small telescope to clearly see its current phase.  The above time-lapse sequence, however, was taken over the course of many months and shows not only how Venus changes phase but how it's apparent angular size also changes.  In the middle negative image, Venus is in a new phase, the same phase that occurred during its rare partial eclipse of the Sun in 2004.

Image credit: Wah!
The Tadpoles of IC 410 (2006-01-11)

This close-up view shows a portion of otherwise faint emission nebula IC 410 in striking false-colors. It also shows two remarkable denizens of the glowing gas cloud at the right - the "tadpoles" of IC 410. The picture is a composite of images taken through narrow band filters intended to trace atoms in the nebula. Emission from sulfur atoms is shown in red, hydrogen atoms in green, and oxygen in blue. Partly obscured by foreground dust, the nebula itself surrounds NGC 1893, a young galactic cluster of stars that energizes the glowing gas. Composed of denser, cooler gas and dust, the tadpoles are around 10 light-years long. Sculpted by wind and radiation from the cluster stars, their tails trail away from the cluster's central region. IC 410 lies some 12,000 light-years away, toward the constellation Auriga. At that distance, this image spans about 70 light-years.

Image credit: Ken Crawford  
(Rancho Del Sol Observatory)
Infrared Helix (2006-01-12)

Over six hundred light years from Earth, in the constellation Aquarius, a sun-like star is dying. Its last few thousand years have produced the Helix Nebula (NGC 7293), a well studied and nearby example of a Planetary Nebula, typical of this final phase of stellar evolution. Emission in this infrared Spitzer Space Telescope image of the Helix comes mostly from the nebula's molecular hydrogen gas. The gas appears to be clumpy, forming thousands of comet-shaped knots each spanning about twice the size of our solar system. Bluer, more energetic radiation is seen to come from the heads with redder emission from the tails, suggesting that they are more shielded from the central star's winds and intense ultraviolet radiation. The nebula itself is about 2.5 light-years across. The Sun is expected to go through its own Planetary Nebula phase ... in another 5 billion years.
Stars of the Galactic Center (2006-01-13)

The center of our Milky Way Galaxy is hidden from the prying eyes of optical telescopes by clouds of obscuring dust and gas. But in this stunning vista, the Spitzer Space Telescope's infrared cameras, penetrate much of the dust revealing the stars of the crowded galactic center region. A mosaic of many smaller snapshots, the detailed, false-color image shows older, cool stars in bluish hues. Reddish glowing dust clouds are associated with young, hot stars in stellar nurseries. The galactic center lies some 26,000 light-years away, toward the constellation Sagittarius. At that distance, this picture spans about 900 light-years.
Lunokhod: Moon Robot (2006-01-14)

On November 17, 1970 the Soviet Luna 17 spacecraft landed the first roving remote-controlled robot on the Moon. Known as Lunokhod 1, it weighed just under 2,000 pounds and was designed to operate for 90 days while guided in real-time by a five person team at the Deep Space Center near Moscow, USSR, Planet Earth.  The futuristic looking eight wheeler rode on top of a descent module that extended ramps from both sides offering alternative routes to the surface in case one side was blocked by boulders. Lunokhod 1 toured the lunar Mare Imbrium (Sea of Rains) for 11 months in one of the greatest successes of the Soviet lunar exploration program. This Lunokhod's operations officially ceased on October 4, 1971, 14 years after the launch of Sputnik.
The Sombrero Galaxy from HST (2006-01-15)

Why does the Sombrero Galaxy look like a hat?  Reasons include the Sombrero's unusually large and extended central bulge of stars, and dark prominent dust lanes that appear in a disk that we see nearly edge-on.  Billions of old stars cause the diffuse glow of the extended central bulge.  Close inspection of the bulge in the above photograph shows many points of light that are actually globular clusters. M104's spectacular dust rings harbor many younger and brighter stars, and show intricate details astronomers don't yet fully understand.  The very center of the Sombrero glows across the electromagnetic spectrum, and is thought to house a large black hole.  Fifty million-year-old light from the Sombrero Galaxy can be seen with a small telescope towards the constellation of Virgo.
Stardust Capsule Returns to Earth (2006-01-16)

A flying saucer from outer space crash-landed in the Utah desert yesterday after being tracked by radar and chased by helicopters and airplanes.  Like last time, no space aliens were involved.  The saucer, the Stardust return capsule, is carrying bits of Comet Wild 2 captured two years ago during a rendezvous between Stardust and the ancient comet.  The capsule is pictured above entering a temporary clean room in Utah before part of it is shipped to NASA's Johnson Space Center in Houston, Texas, USA.  In the inset on the lower right, an artist depicts the capsule as it likely looked after it parachuted to a landing in the Utah desert.  The upper right insert shows an image of the streaking capsule taken yesterday by a DC-8 chase plane.  The bits of Comet Wild 2 stored in the Stardust return capsule are likely older than the Sun and will be inspected over the next few years for clues about the early years of our Solar System. You, too, can help look for dust grains in the Stardust aerogel!
A Roll Cloud Over Missouri (2006-01-17)

What kind of cloud is this?  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 in the summer of 2005 above Albany, Missouri, USA.

Image credit: Dan Bush
(Missouri Skies)