juliensimon/nasa-apod huggingface dataset:
Sharpless 212 in Hydrogen and Sulfur (2001-12-18)

Where do the most massive stars form? Observational evidence indicates that the outskirts of developing open clusters of stars are primary locations.  Pictured above is one such open cluster: Sharpless 212.  Visible in the image center are massive stars in the open cluster.  The energetic light from these stars ionizes surrounding hydrogen atoms creating an HII region.   As the hydrogen atoms re-acquire electrons, they emit the red light highlighted.  Sharpless 212 also contains small amounts of dust and heavy atoms such as Sulfur.  The dust efficiently absorbs light, while emission from Sulfur is highlighted in blue.  Particularly striking and well-defined boundaries that separate the ionized material from surrounding neutral material are visible at the edge of the HII region.  Sharpless 212 spans about 20 light years and lies about 25,000 light years away.

Image credit: Lise Deharveng 
(Universite de Provence) 
et al., 
CNRS, 
OHP
Finding Dark Matter (2001-12-19)

Where is dark matter?  Galaxies rotate and move in clusters as if a tremendous amount of unseen matter is present.  But does dark matter exist in the greater universe too -- and if so, where?   The answer can be found by comparing the distribution of galaxies observed with numerical simulations.  This comparison became much more accurate recently when over 100,000 galaxy observations from the 2-Degree Field Galactic Redshift Survey were used.  In the above frame from a computer simulation of our universe, a 300 million light-year slice shows dark matter in gray and galaxies as colored circles.  The red box indicates the location of a rich cluster of galaxies, while the green box shows a more typical cross-section of our universe.  Analyses indicate that the immense gravity of the pervasive dark matter pulls normal matter to it, so that light matter and dark matter actually cluster together.

Image credit: A. J. Benson 
(Caltech) et al., 
U. Durham, 
PPARC
Jupiter and Saturn Pas de Deux (2001-12-20)

Viewed from Earth, the solar system's planets do a cosmic dance that is hard to appreciate on any single night. But consider this well planned animated sequence combining 23 pictures taken at approximately 2 week intervals from June 2000 through May 2001. It reveals the graceful looping or retrograde motion of bright wanderers Jupiter (leftmost) and Saturn. Loitering among the background stars are the familiar Pleiades (above right) and V-shaped Hyades (below left) star clusters. The planets didn't actually loop by reversing the direction of their orbits, though. Their apparent retrograde motion is a reflection of the motion of the Earth itself. Retrograde motion can be seen each time Earth overtakes and laps planets orbiting farther from the Sun, Earth moving more rapidly through its own relatively close-in orbit. Astronomer Tunc Tezel captured Jupiter and Saturn's "paired" retrograde loop in this remarkable series made after the close alignment of these gas giants in May 2000. The next opportunity to see these two planets dance such a pas de deux will be in the year 2020.

Image credit: Tunc Tezel
Partial Eclipse, Cloudy Day (2001-12-21)

Welcome to the December Solstice, first day of winter in the north and summer in the southern hemisphere of planet Earth. Today the Sun reaches its southernmost declination in the sky at 19:21 Universal Time. Just a short week ago, as the Sun approached the end of its annual journey south, it was eclipsed by the Moon. Observers in Costa Rica witnessed a fleeting annular eclipse with the Moon surrounded by a dramatic bright ring as it covered about 96 percent of the visible solar surface during the maximum phase. But from most of the Americas this eclipse was partial ... and skies were often partially cloudy! Public Television Engineer Stan Richard captured this view near Des Moines, Iowa, USA. Taken close to eclipse maximum for his location, the sharp, silhouetted edge of the Moon is visible through the clouds in the lower left quadrant of the solar disk.

Image credit: Stan Richard
Hot Stars in the Southern Milky Way (2001-12-22)

Hot blue stars, red glowing hydrogen gas, and dark, obscuring dust clouds are strewn through this dramatic region of the Milky Way in the southern constellation of Ara (the Altar). About 4,000 light-years from Earth, the stars at the left are young, massive, and energetic. Their intense ultraviolet radiation is eating away at the nearby star forming cloud complex - ionizing the hydrogen gas and producing the characteristic red "hydrogen-alpha" glow. At right, visible within the dark dust nebula, is small cluster of newborn stars. This beautiful color picture is a composite of images made through blue, green, and hydrogen-alpha filters.
Saturn Aurora (2001-12-23)

he second largest planet in the Solar System, Saturn's Rings are one of the most spectacular sights for earthbound telescopes. This image from the orbiting Hubble Space Telescope's STIS instrument, offers a striking view of another kind of ring around Saturn - pole encircling rings of ultraviolet aurora. Towering more than 1,000 miles above the cloud tops, these Saturnian auroral displays are analogous to Earth's. Energetic charged particles in the Solar Wind are funneled by the planet's magnetic field into polar regions where they interact with atmospheric gases. Following the ebb and flow of Saturn's aurora, researchers can remotely explore the planet's atmosphere and magnetic field. In this false color image, the dramatic red aurora identify emission from atomic hydrogen, while the more concentrated white areas are due to hydrogen molecules. In 2004, NASA plans to begin making close-up studies of the Saturnian system with the Cassini Spacecraft.
Asteroid 1998 WT24 Passes Near Earth (2001-12-24)

Last week, an asteroid approached unusually close to the Earth.  Passing well outside the orbit of our Moon, Asteroid 1998 WT24 posed no danger, but became bright enough to see with binoculars and to track with radar.  Pictured above, the kilometer-sized asteroid was imaged crossing the sky on December 14, two days before closest approach.  Every few years, an asteroid will actually pass inside the orbit of the Moon.  Large impact features on the Earth are testaments to asteroids or comets that actually impacted the Earth in the distant past.  Astronomers continue to discover, track, and study potentially hazardous asteroids with a goal of making planet Earth a safer place.

Image credit: S. Gajdos & J. Toth 
(Comenius U. Bratislava), 
Modra Observatory
Star Forming Region Hubble-V (2001-12-25)

How did stars form in the early universe?  Astronomers are gaining insight by studying NGC 6822, a nearby galaxy classified as irregular by modern standards but appearing more typical of galaxies billions of years ago.   Inspection of NGC 6822 shows several bright star groups, including two dubbed Hubble-X and Hubble-V.  Pictured above, the Hubble Space Telescope has resolved Hubble V into the energetic stars that are lighting up the surrounding gas.  Each star in the central dense knot of Hubble V shines brighter than 100,000 Suns.   The Hubble V gas cloud spans about 200 light years and lies about 1.5 million light-years away toward the constellation Sagittarius.
Himalayan Horizon From Space (2001-12-26)

This stunning aerial view shows the rugged snow covered peaks of a Himalayan mountain range in Nepal. The seventh-highest peak on the planet, Dhaulagiri, is the high point on the horizon at the left while in the foreground lies the southern Tibetan Plateau of China. But, contrary to appearances, this picture wasn't taken from an airliner cruising at 30,000 feet. Instead it was taken with a 35mm camera and telephoto lens by the Expedition 1 crew aboard the International Space Station -- orbiting 200 nautical miles above the Earth. The Himalayan mountains were created by crustal plate tectonics on planet Earth some 70 million years ago, as the Indian plate began a collision with the Eurasian plate. Himalayan uplift still continues today at a rate of a few millimeters per year.
The Incredible Expanding Crab (2001-12-27)

The Crab Nebula is cataloged as M1, the first on Charles Messier's famous list of things which are not comets. In fact, the Crab is now known to be a supernova remnant, an expanding cloud of debris from the explosion of a massive star. The violent birth of the Crab was witnessed by astronomers in the year 1054. Roughly 10 light-years across today, the nebula is still expanding at a rate of over 1,000 kilometers per second. Flipping between two images made nearly 30 years apart, this animation clearly demonstrates the expansion. The smaller Crab was recorded as a photographic image made in 1973 using the Kitt Peak National Observatory 4-meter telescope in 1973. The expanded Crab was made this year with the Kitt Peak Visitor Center's 0.4-meter telescope and digital camera. Background stars were used to register the two images.
Starlight Reflections (2001-12-28)

Interstellar dust grains often find themselves in a reflective "mood". Near a bright star, clouds of these dust particles scatter short wavelengths of visible starlight more readily than long wavelengths, producing lovely blue reflection nebulae. Nine of the more spectacular examples of these dusty, blue stellar neighborhoods have been assembled here by astrophotographer Rob Gendler. From left to right starting with the top row are NGC 1977 in Orion, IC2118 (the Witch Head), and M78 also in Orion. Across the middle row are, M20 (Trifid), NGC 2264 in Monoceros, and IC405 (Flaming Star Nebula). Along the bottom are NGC 2023 (near the Horsehead), NGC 7023 (Iris Nebula), and finally bright star Merope surrounded by a veil of dust (NGC 1435). Merope is one of the seven sisters of the Pleiades.

Image credit: Robert Gendler
The Annotated Galactic Center (2001-12-29)

The sky toward the center of our Galaxy is filled with a wide variety of celestial wonders. Many are easily visible with binoculars.   Constellations near the galactic center include Sagittarius, Libra, Scorpius, Scutum, and Ophiuchus.  Nebulae include Messier Objects M8, M16, M17, M20 and the Pipe Nebula.  Open star clusters include M6, M7, M18, M21, M23, M24, M25.   Globular star clusters include M9, M22, M28, M54, M69, M70.  And don't forget Baade's Window.  Click on the photo to get the un-annotated version.
Trifid Pillars & Jets (2001-12-30)

Dust pillars are like interstellar mountains.  They survive because they are more dense than their surroundings, but they are being slowly eroded away by a hostile environment.  Visible in the above picture is the end of a huge gas and dust pillar in the Trifid Nebula, punctuated by a smaller pillar pointing up and an unusual jet pointing to the left.  The pink dots are newly formed low-mass stars. A star near the small pillar's end is slowly being stripped of its accreting gas by radiation from a tremendously brighter star situated off the above picture to the upper right.  The jet extends nearly a light-year and would not be visible without external illumination.  As gas and dust evaporate from the pillars, the hidden stellar source of this jet will likely be uncovered, possibly over the next 20,000 years.
A Year of Dark Cosmology (2001-12-31)

We live in the exciting time when humanity discovers the nature of our entire universe.  During this year, in particular, however, the quest for cosmological understanding appears to have astronomers groping in the dark.  Dark matter and dark energy are becoming accepted invisible components of our universe, much like oxygen and nitrogen have become established invisible components of Earth-bound air.  In comprehending the nature and origin of the formerly invisible, however, we are only just exiting the cosmological dark age.  Relatively unexplored concepts such as higher spatial dimensions, string theories of fundamental particles, quintessence, and new forms of inflation all vie for cornerstone roles in a more complete theory.  As understanding invisible air has led to such useful inventions as the airplane and the oxygen mask, perhaps understanding dark matter and dark energy can lead to even more spectacular and useful inventions.  Pictured above, three of the largest optical telescopes (Keck I, Keck II, and Subaru) prepare to peer into the dark and distant universe.
The Secret of the Black Aurora (2002-01-01)

What causes black aurora?  These gaps in normal bright aurora are frequently recorded but rarely questioned.  Recent research using data from four Cluster spacecraft orbiting the Earth has now likely found the secret: black auroras are actually anti-auroras.   In normal auroras, electrons and/or predominantly negatively charged particles fall toward Earth along surfaces of constant magnetic field.  They ionize the Earth's atmosphere on impact, causing the bright glows.  In black anti-auroras, however, negatively charged particles are sucked out from the Earth's ionosphere along adjoining magnetic field lines.  These dark anti-auroras can climb to over 20,000 kilometers and last for several minutes.  Pictured above, a black aurora is seen dividing bright auroras over Fairbanks, Alaska, USA.

Image credit: Jan Curtis
International Space Station Over Earth (2002-01-02)

High above a cloudy Earth, the International Space Station (ISS) orbits silently.  The Space Shuttle Endeavor Crew took the above picture as they departed the space station in mid-December.  Endeavor brought up three new astronauts to occupy the ISS and carried home the members of Expedition Three, a trio that has been housed in the ISS since August.  Highlights of this Endeavor mission included fixing a solar panel and maneuvering the station to avoid a large piece of passing space junk.  Visible in the above picture are the space station's robot manipulator arm as well as several modules and solar arrays.
M16: Stars, Pillars, and the Eagle's EGGs (2002-01-03)

The Hubble Space Telescope's 1995 image of pillars of dust and gas, light-years long, within the Eagle Nebula (M16) was sensational. The three prominent pillars in that close-up visible light picture also appear below center in this wide-field mosaic along with massive, bright, young stars of cluster NGC 6611 (upper right), whose winds and radiation are shaping the dusty pillars. Made in near infrared light with the European Southern Observatory's 8.2-meter Antu telescope, this wide-field image makes the pillars seem more transparent, as the longer wavelengths partially penetrate the obscuring dust. While the Hubble image showed the pillars' startling surface details - over 70 opaque, finger-shaped lumps of material dubbed evaporating gaseous globules or EGGs, the near infrared view has allowed astronomers to peer inside. Comparing the two views reveals that nearly a dozen of the EGGs do indeed have stars embedded near their tips. More stars within EGGs may be detected if longer wavelength observations of the region are made. But which came first, the stars or the EGGs?
M16: Infrared Star Hunt (2002-01-04)

The head of an interstellar gas and dust cloud is shown here in false-color, a near-infrared view recorded by astronomers hunting for stars within M16's Eagle Nebula. Made famous in a 1995 Hubble image of the 7,000 light-year distant star forming region, the pillar-shaped cloud's surface was seen to be covered with finger-like evaporating gaseous globules (EGGs). The near-infrared image penetrates the obscuring dust cloud's edges. But the cloud's core appears dark and opaque, even at these relatively long wavelengths. Still, this image, made with ESO's Antu telescope, reveals a massive, bright yellow star not directly detected in the visible light Hubble data. This very young star lights up the small bluish nebula with a dark, twisted central stripe, just above it. Below and to its right are several much fainter, less massive stars also not seen in visible light - newborn stars which lie within the Eagle's EGGs. These newborn stars may have already been collapsing, forming from material inside the nebula before the intense radiation from other, nearby, emerging hot stars eroded and sculpted the dramatic pillars and EGGs. In any event, as the dusty clouds are eroded away, stars still forming will be cutoff from their reservoir of star stuff. Further growth and even the development of planetary systems will likely be seriously affected.
Apollo 17's Moonship (2002-01-05)

Awkward and angular looking, Apollo 17's lunar module Challenger was designed for flight in the vacuum of space. This sharp picture from the 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 itself underneath. The hatch allowing access to the lunar surface is visible in the front and a round radar antenna appears at the top. This spaceship performed gracefully, landing on the moon and returning the Apollo astronauts to the orbiting command module in December of 1972 - but 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. Editor's note: Eric Jones, Apollo Lunar Surface Journal editor, comments; "If you look at the [... large, dark] triangular window, you'll see a bright rectangular area - which is the rendezvous window - beneath it, a bright arc. After much discussion, my team of volunteers and I concluded that the bright arc is the top of [mission commander] Gene Cernan's bubble helmet lit by sunlight ..."
M2-9: Wings of a Butterfly Nebula (2002-01-06)

Are stars better appreciated for their art after they die?  Actually, stars usually create their most artistic displays as they die.  In the case of low-mass stars like our Sun and M2-9 pictured above, the stars transform themselves from normal stars to white dwarfs by casting off their outer gaseous envelopes.  The expended gas frequently forms an impressive display called a planetary nebula that fades gradually over thousand of years.  M2-9, a butterfly planetary nebula 2100 light-years away shown in representative colors, has wings that tell a strange but incomplete tale.  In the center, two stars orbit inside a gaseous disk 10 times the orbit of Pluto.  The expelled envelope of the dying star breaks out from the disk creating the bipolar appearance.  Much remains unknown about the physical processes that cause planetary nebulae.
The Mysterious Cone Nebula (2002-01-07)

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: Robert Gendler
Thackeray's Globules (2002-01-08)

Rich star fields and glowing hydrogen gas silhouette dense, opaque clouds of interstellar gas and dust in this Hubble Space Telescope close-up of IC 2944, a bright star forming region in Centaurus, 5,900 light-years away. The largest of these dark globules, first spotted by South African astronomer A. D. Thackeray in 1950, is likely two separate but overlapping clouds, each more than one light-year wide. Combined the clouds contain material equivalent to about 15 times the mass of the Sun, but will they actually collapse to form massive stars? Along with other data, the sharp Hubble images indicate that Thackeray's globules are fractured and churning as a result of intense ultraviolet radiation from young, hot stars already energizing and heating the bright emission nebula. These and similar dark globules known to be associated with other star forming regions may ultimately be dissipated by their hostile environment -- like cosmic lumps of butter in a hot frying pan. The chevron shape of the picture outlines the detectors of the Hubble's WFPC2 camera.
Blue Flash (2002-01-09)

Difficult to observe, the momentary green flash above the rising or setting sun has been documented as a phenomenon caused by the atmospheric bending or refraction of sunlight. Like a weak prism, the Earth's atmosphere breaks white sunlight into colors, bending red colors slightly and green and blue colors through increasingly larger angles. When the sky is clear, a green flash just above the sun's edge can sometimes be seen for a second or so, when the sun is close to a distant horizon. A blue flash is even harder to see though, because the atmosphere must be extraordinarily clear to avoid scattering and diminishing the refracted blue sunlight. Still, from a site near Roques de los Muchachos (altitude 2,400 meters) on La Palma in the Canary Islands, astrophotographer Mario Cogo captured this dramatic telescopic image of a blue flash on color film in October of 2001. The image of the setting Sun with large sunspot groups on its surface is heavily distorted by atmospheric layers. A lingering green rim is just visible under the tantalizing blue flash.

Image credit: Mario Cogo
X-Ray Milky Way (2002-01-10)

If you had x-ray vision, the center regions of our Galaxy would not be hidden from view by immense cosmic dust clouds opaque to visible light. Instead, the Milky Way toward Sagittarius might look something like this stunning mosaic of images from the orbiting Chandra Observatory. Pleasing to look at, the gorgeous false-color representation of the x-ray data shows high energy x-rays in blue, medium energies in green, and low energies in red. Hundreds of white dwarf stars, neutron stars, and black holes immersed in a fog of multimillion-degree gas are included in the x-ray vista. Within the white patch at the image center lies the Galaxy's central supermassive black hole. Chandra's sharp x-ray vision will likely lead to a new appreciation of our Milky Way's most active neighborhood and has already indicated that the hot gas itself may have a temperature of a mere 10 million degrees Celsius instead of 100 million degrees as previously thought. The full mosaic is composed of 30 separate images and covers a 900 by 400 light-year swath at the galactic center.
Sunbather (2002-01-11)

Intense and overwhelming, the direct glare of the Sun is blocked by the smooth disk centered in this image from the sun-staring SOHO spacecraft. Taken on January 8, the picture shows streamers of solar wind billowing radially outward for millions of kilometers above the Sun's surface indicated by the white circle. Below and right is inner planet Venus, so bright that its image is marred by a sharp horizontal stripe, a digital imaging artifact. Also impressively bright is a periodic visitor to the inner Solar System, sunbathing comet 96/P Machholz 1 (above and left). This comet is definitely not a member of the more suicidal sungrazer comet family often spotted approaching the Sun by SOHO. Seen here only 18 million kilometers from the Sun (about one eighth the Earth-Sun distance) with a substantial coma and foreshortened tail, Machholz 1 has now passed perihelion and is outbound in its orbit, to return again in just over 5 years.