juliensimon/nasa-apod huggingface dataset:
Hubble Holiday (1999-12-24)

How would you like to spend your holiday in low earth orbit? That's what the crew of the space shuttle Discovery is doing as they deliver six new gyros and a faster main computer to the orbiting Hubble Space Telescope. The telescope has been holding in safe mode since November 13 and will use the timely "gifts" to resume its exploration of the distant Universe.  This mission, STS-103, is the third mission to service the famous space observatory which was placed in orbit by Discovery on April 25, 1990. Seen here in a 1997 picture from STS-82, the second servicing mission, the space telescope, flanked by its gold-colored solar panels, hangs above the shuttle payload bay. The Earth's bright limb is in the background. Discovery closed with the Hubble on Tuesday and crew members are conducting space walks to install the new equipment. Discovery is scheduled to return to Earth after Christmas, with a December 27 landing at Kennedy Space Center - but you can e-mail season's greetings to the orbiting crew.
An Earth Ornament (1999-12-25)

The Apollo 8 astronauts spent the 1968 Christmas Season orbiting the Moon, returning with striking images of both Moon and Earth from space - pictures which inspired the world. While in lunar orbit in 1994, the prospecting Clementine spacecraft also turned its cameras toward the home world and the result was this mosaic of 70 high resolution images of our planet from a cosmic perspective. The swirling clouds and dramatic colors give the Earth the appearance of a delicate, painted ornament hanging in space. Best Wishes and Seasons Greetings from Astronomy Picture Of The Day!
West Of The Great Red Spot (1999-12-26)

The turbulent region West of Jupiter's Great Red Spot is highlighted in this picture constructed from data recorded by the Galileo spacecraft. The image is color coded to show cloud height and thickness; white clouds are high and thick, light blue clouds are high and thin, and reddish clouds are low. The edge of the Red Spot itself appears blue here (lower right) and spans about  10,000 kilometers along the curving limb of the planet (north is up). Westward winds, deflected north by the circulation within the Great Red Spot, collide with Eastward winds at higher latitudes and generate the roiling, Turbulent structures. The largest eddies near the Northwestern edge of the Red Spot are bright, suggesting upward convection and high altitude cloud formation are taking place there.
Solar Moss (1999-12-27)

Discovered in recent close-up pictures of the Sun from NASA's Transition Region And Coronal Explorer (TRACE) spacecraft, this spongy-looking stuff has a temperature of 2 million degrees Fahrenheit ... and has been dubbed "Solar Moss". The false-color TRACE image above was recorded in extreme ultraviolet light on October 18. It shows the solar moss associated with hot magnetic plasma loops arching above one of the Sun's active regions. The moss is the dark blue and white fluff that seems to cover areas between the bright white bases of the loops. Solar moss has been seen to spread, typically persisting for tens of hours, and may form rapidly following a solar flare. Solar moss isn't found growing only on the Sun's north side, but as a rule seems to lie above the photosphere or visible surface, in the transition region of the solar atmosphere. Complex and previously unknown, this feature may provide a clue to the long sought mystery mechanism responsible for heating the Sun's outer atmosphere.
A Year Of New Perspectives (1999-12-28)

Fittingly, 1999 saw a decade of astronomical discoveries to an end with portents of things to come - embodied in new spacecraft, telescopes, and perspectives to explore the distant Universe across the electromagnetic spectrum. X-ray astronomy in particular will likely flourish in coming years, judging from this year's successful launch of the triple-barrelled X-ray Multi-Mirror satellite and spectacular first results from the orbiting Chandra X-ray Observatory. Ground-based astronomy will flourished too as very large telescopes and new instruments have come online or near completion. Radio astronomers also achieved an observational milestone this year with the record breaking VLBI observations from a network of radio telescopes as large as planet Earth. But the APOD editors' favorite astronomical screensaver of 1999 has leveraged the phenomenonal growth of the internet and the personal computer boom to support the Search for ExtraTerrestrial Intelligence in the SETI@home project - which has now likely involved more computer power than any other project in history.   News: APOD Home Switched During Y2K Transition
The Decade that Defined Star System (1999-12-29)

As the 1990s began, the only planetary star system known was our own Solar System.  The first extra-solar star system was discovered orbiting a pulsar in 1991.  Slight changes in the precise arrival times of the pulses from the central small dense neutron star gave evidence of orbiting planets.  By mid-decade Jupiter-like planets around Sun-like stars were being found by a slight wobble detected in the motion of these stars.  Pictured above is an artist's sketch of a planet discovered orbiting HD217107.  As the decade comes to a close, over two dozen planetary star systems have been identified, one recently confirmed by the detection of a slight eclipse by the planet itself.  An unambiguous discovery of Sun-like star systems containing Earth-like planets still remains a goal.

Image credit: Lynette Cook
The Century that Defined Galaxy (1999-12-30)

As the twentieth century dawned a debate raged over whether the Milky Way was the entire universe.  The 1920s brought observations of spiral-nebulae that housed familiar but faint variable stars, and hence placed these nebulae much farther than anything else ever found.  The Milky Way, apparently, is itself a spiral nebula seen from the inside.  Soon interstellar dust was identified as the opaque material that blocks our view of distant spiral arms, confirming that our Sun was well away from the center.  As the twentieth century closes, galaxies are seen forming across the universe, and our Milky Way is referred to as a spiral galaxy quite similar to NGC 1288, as pictured above.
The Millennium that Defined Earth (1999-12-31)

When the second millennium began, people generally knew that the Earth was round, but few saw much of it beyond their local village.  As the millennium progressed, humans mapped the continents, circumnavigated the globe, and determined the composition of the Earth.  The Earth started as the center of everything, but became a planet placed in the Solar System, which became placed in a Galaxy, which became placed in the Local Group of Galaxies, which became placed in an expanse so vast we call it just the Universe.  As millennium two ends people generally know what Earth looks like from afar, and how it is that all of humanity is confined to the surface of this fragile and watery globe.
The Millennium that Defines Universe (2000-01-01)

Welcome to the millennial year at the threshold of millennium three. During millennium two, humanity continually redefined its concept of "Universe": first as spheres centered on the Earth, in mid-millennium as the Solar System, a few centuries ago as the Galaxy, and within the last century as the matter emanating from the Big Bang. During millennium three humanity may hope to discover alien life, to understand the geometry and composition of our present concept of Universe, and even to travel through this Universe. Whatever our accomplishments, humanity will surely find adventure and discovery in the space above and beyond, and possibly define the surrounding Universe in ways and colors we cannot yet imagine by the threshold of millennium four.
The Largest Rock Known (2000-01-02)

There, that faint dot in the center - that's the largest rock known. It is larger than every known asteroid, moon, and comet nucleus. It is larger than any other rocky planet. (Nobody knows for sure what size rocks lie at the cores of Jovian planets, or orbit other stars.) The Voyager 1 spacecraft took this picture 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. Yesterday, this rock started 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.
Cas A Supernova Remnant in X-Rays (2000-01-03)

The complex shell of a star seen to explode 300 years ago is helping astronomers to understand how that star exploded. The above recently released image of supernova remnant Cassiopeia A (Cas A) shows unprecedented detail in three X-ray colors. The relationship between brightness, color, and position of material in the image indicates where in the star this material was just before the explosion. Bright knots on the left, for example, contain little iron, and so are hypothesized to originated from a higher layer than outer red filaments, which are iron rich. The blue region on the right is seen through absorbing dust, and so appears depleted of low-energy X-rays. It takes light ten years to cross the gas shell of the Cas A supernova remnant, which is 10,000 light-years distant. Most of the elements that make people and planets were produced in supernova explosions.
Galaxies Cluster Toward the Great Attractor (2000-01-04)

Galaxies dot the sky like jewels in the direction of a mass so large it is known simply as the Great Attractor.  The galaxies pictured above are part of a cluster of galaxies called ACO 3627 near the center of the Great Attractor.  Previously, this cluster of galaxies, also known as the Norma Cluster, was largely unstudied because dust in the disk of our own Galaxy obscured much of its light.  The Great Attractor is a diffuse mass concentration fully 250 million light-years away, but so large it pulls our own Milky Way Galaxy and millions of other galaxies towards it.  Many of the galaxies in ACO 3627 are slowly heading towards collisions with each other.
Earth, Moon, Hubble (2000-01-05)

Tomorrow's picture: Millennial Mars  < Archive | Index | Search | Calendar | Glossary | Education | About APOD >  Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA) NASA Technical Rep.: Jay Norris. Specific rights apply. A service of: LHEA at NASA/ GSFC & Michigan Tech. U.
NGC 4214: Star Forming Galaxy (2000-01-07)

Dazzling displays of star formation abound across the face of galaxy NGC 4214, a mere 13 million light-years away in the northern constellation Canes Venatici. While this 1997 Hubble Space Telescope image shows the numerous faint, older stars of NGC 4214, the most eye-catching features are the galaxy's bright young star clusters surrounded by fluorescent gas clouds. Sculpted into bubbles and filamentary shapes by energetic explosions and stellar winds from massive cluster stars, the clouds fluoresce in the intense stellar ultraviolet radiation. The colorful spectacle of massive young star forming clusters and distinguished presence of a fainter, older stellar population indicate that NGC 4214 has experienced star formation episodes spanning billions of years.
Albert Einstein Describes Space and Time (2000-01-08)

Albert Einstein (1879-1955) is considered by many the greatest astrophysicist and single most significant Person of the 20th Century. He is pictured here in the Swiss Patent Office where he did much of his defining work.  Einstein's many visionary scientific contributions include the equivalence of mass and energy (E=mc^2), how the maximum speed limit of light affects measurements of time and space (special relativity), and a more accurate theory of gravity based on simple geometric concepts (general relativity). One reason Einstein was awarded the 1921 Nobel Prize in Physics was to make the prize more prestigious.
Galaxy Dwingeloo 1 Emerges (2000-01-09)

Sometimes you can't see the forest for the trees.  But if you look closely at the center of the above photograph, you will see a whole spiral galaxy behind the field of stars.  Named Dwingeloo 1, this nearby galaxy was only discovered recently (1994) because much of its light was obscured by dust, gas and bright stars of our own Milky Way Galaxy. In fact, all the individually discernible stars in the above photograph are in our Galaxy.  Dwingeloo 1 turned out to be a large galaxy located only five times as distant as the closest major galaxy - M31.
Brown Sun Bubbling (2000-01-10)

Our Sun may look like all soft and fluffy, but its not.  Our Sun is an extremely large ball of bubbling hot gas, mostly hydrogen gas.  The above picture was taken in a specific color of light emitted by hydrogen gas called Hydrogen-alpha.  Granules cover the solar photosphere surface like shag carpet, interrupted by bright regions containing dark sunspots.  Visible at the left edge is a solar prominence.  Our Sun glows because it is hot, but it is not on fire.  Fire is the rapid acquisition of oxygen, and there is very little oxygen on the Sun.  The energy source of our Sun is the nuclear fusion of hydrogen into helium deep within its core.  Astronomers are still working to understand, however, why so few neutrinos are measured from the Sun's core.

Image credit: Robert Gendler
The Rosette Nebula in Hydrogen, Oxygen, and Sulfur (2000-01-11)

The Rosette Nebula is a large emission nebula located 3000 light-years away.   The great abundance of hydrogen gas gives NGC 2237 its red color in most photographs.  The wind from the open cluster of stars known as NGC 2244 has cleared a hole in the nebula's center.  The above photograph, however, was taken in the light emitted by three elements of the gas ionized by the energetic central stars.  Here green light originating from oxygen and blue light originating from sulfur supplements the red from hydrogen.  Filaments of dark dust lace run through the nebula's gases.  The origin of recently observed fast-moving molecular knots in the Rosette Nebula remains under investigation.
NGC 6791: An Old, Large Open Cluster (2000-01-12)

NGC 6791 is one of the oldest and largest open clusters of stars known.  But how did it get so dirty?  Open star clusters usually contain a few hundred stars each less than a billion years old.  Open star cluster NGC 6791, however, contains thousands of stars recently measured to be about 8 billion years old.  What's really confusing, though, is that the stars of NGC 6791 are relatively dirty - the minuscule amounts of heavy elements (generically called metals) are high relative to most other star clusters.  Older stars are supposed to be metal poor, since metals have only been slowly accumulating in our Milky Way Galaxy.  This enigma makes NGC 6791, pictured above, one of the most studied open clusters and a possible example of how stars might evolve in the centers of galaxies.
A Skygazer's Full Moon (2000-01-13)

This dramatically sharp picture of the full moon was recorded on 22 December, 1999 by astroimager Rob Gendler. Big, beautiful, bright, and evocative, it was the last full moon of the Y1.9Ks, pleasing and inspiring even casual skygazers. December's moon was special for another reason, as the full phase occurred on the day of the winter solstice and within hours of lunar perigee. The first full moon of the year 2000 will bring a special treat as well, presenting denizens of planet Earth with a total lunar eclipse. On Thursday evening, January 20, the moon will encounter the dark edge of Earth's shadow at 10:01 PM Eastern Time with the total eclipse phase beginning at 11:05 PM and lasting for 77 minutes. This lunar eclipse will be visible from North and South America and Western Europe (total phase begins at 4:05 AM GMT January 21).

Image credit: Robert Gendler
Chandra Resolves the Hard X-Ray Background (2000-01-14)

It is everywhere but nobody knew why.  In every direction at all times, the sky glows in X-rays.  The X-ray background phenomenon was discovered over 35 years ago, soon after the first X-ray satellites were launched, and has since gone unexplained.  Yesterday results were released using data from the recently launched Chandra X-Ray Observatory that appears to have resolved much of this mystery.  The above photograph shows that about 80 percent of the apparently diffuse hard X-ray background can be resolved into very many very faint sources.  The new question is now what are these sources?  Early speculation, much of which predates these observations, holds that many of these sources are the active centers of distant galaxies, probably involving massive black holes.  Still other sources may be of origins currently unknown.
The Sun Also Rises (2000-01-15)

Sunrise seen from low Earth orbit can be very dramatic indeed (and the authors don't apologize to Hemingway for using his title!). In this breathtaking view from the space shuttle Endeavor, the Sun is just visible peaking over towering anvil-shaped storm clouds. The silhouetted cloud tops mark the upper boundary of the troposphere, the lowest layer of planet Earth's atmosphere. Sunlight filtering through suspended dust causes this dense layer of air to appear red. In contrast, the blue stripe marks the stratosphere, the tenuous upper atmosphere, which preferentially scatters blue light.
The Compton Gamma Ray Observatory (2000-01-16)

The Compton Gamma Ray Observatory (CGRO) was the most massive instrument ever launched by a NASA Space Shuttle in 1991 and continues to revolutionize gamma-ray astronomy.  Before Compton loses more stabilizing gyroscopes, NASA is considering firing onboard rockets to bring it on a controlled reentry into the ocean. This orbiting observatory sees the sky in gamma-ray photons - light so blue humans can't see it.  These photons are blocked by the Earth's atmosphere from reaching the Earth's surface.  Results from CGRO, pictured above, have shown the entire universe to be a violent and rapidly changing place - when viewed in gamma-rays.  Astronomers using CGRO data continue to make monumental discoveries, including identifying mysterious gamma-ray bursts that uniquely illuminate the early universe, discovery of a whole new class of QSOs, and discovery of objects so strange that astronomers can't yet figure out what they are.
V4641 Sgr: The Closest Black Hole Candidate (2000-01-17)

An object many astronomers believe is a black hole has been found only 1500 light-years from Earth, making it the closest black hole candidate.  Although dramatic explosions emanate from the object, it is far enough away so that we are in no danger.  Pictured above, V4641 was imaged just after emitting an outburst in the radio band.  Jets, which lasted only minutes, are visible.  V4641 is the fourth known microquasar, a miniature version of the massive, matter spewing black holes thought to exist in the centers of galaxies.  The explosions are not thought to emanate from within the black hole, a location where neither matter nor information can escape, but from around the black hole, where matter from its companion star may be heating up as it falls in.  Astronomers are working to understand why V4641 acts strangely even for a black hole, as the explosions it creates fade within minutes, and appear at different times in different bands of light.
NGC 7635: The Bubble Nebula (2000-01-18)

What created this huge space bubble?  A massive star that is not only bright and blue, but also emitting a fast stellar wind of ionized gas.  The Bubble Nebula is actually the smallest of three bubbles surrounding massive star BD+602522, and part of gigantic bubble network S162 created with the help of other massive stars.  As fast moving gas expands off BD+602522, it pushes surrounding sparse gas into a shell.  The energetic starlight then ionizes the shell, causing it to glow.  The above picture taken with the Hubble Space Telescope and released last week shows many details of the Bubble Nebula never seen before and many still not understood.  The nebula, also known as NGC 7635, is about six light-years across and visible with a small telescope towards the constellation of Cassiopeia.
A Big Black Hole Floats By (2000-01-19)

A black hole glides silently through space.  Is there any way to know it's there?  Until last week, all objects that might be black holes in our Galaxy were part of binary star systems.  There, gas from the companion star was hypothesized to swirl around the black hole, heat up, and emit X-rays before falling in.  Last week, however, analysis was released of a mysteriously dark object that floated in front of a distant background star, dramatically increasing this star's light by the lens effect of its gravity, as pictured above. The high mass and low light of this strange lens have astronomers guessing they might just have detected a lone black hole, weighing in at six times the mass of our Sun.  The existence of isolated black holes is not in itself surprising, as they should be the end result of the cores of massive stars.
X-Rays From The Galactic Center (2000-01-20)

ring quasars and active galaxies in the distant universe, astronomers have come to believe that most galaxies have massive black holes at their centers. Swirling stars and a strong, variable radio source offer convincing evidence that even our own Milky Way galaxy's center harbors such a bizarre object, a mere 30,000 light-years away. Still, it has long been realized that if a massive black hole lurks there it should produce X-rays - which have not previously been identified. Now, though relatively faint, the missing X-ray source may have been found. Taking advantage of the sensitive Chandra Observatory astronomers have recorded this false-color X-ray image of the Galactic Center. Embedded in a diffuse cloud of X-ray hot gas, the white dot at the center corresponds to an X-ray source at exactly the position of the strong radio source and suspected black hole. Other individual X-ray sources are also present in the picture which spans about 10 light-years at the distance of the galactic center. With radio and X-ray emission generated by infalling material, the Milky Way's central black hole is thought to have a mass of over 2 million suns.   News: Lunar Eclipse Tonight!
X For Andromeda (2000-01-21)

A big beautiful spiral galaxy 2 million light-years away, Andromeda (M31) has long been touted as an analog to the Milky Way, a distant mirror of our own galaxy. The popular 1960s British sci-fi series, A For Andromeda, even postulated that it was home to another technological civilization that communicated with us. Using the newly unleashed observing power of the orbiting Chandra X-ray telescope, astronomers have now imaged the center of our near-twin island universe, finding evidence for an object so bizarre it would have impressed many 60s science fiction writers (and readers). Like the Milky Way, Andromeda's galactic center appears to harbor an X-ray source characteristic of a black hole of a million or more solar masses. Seen above, the false-color X-ray picture shows a number of X-ray sources, likely X-ray binary stars, within Andromeda's central region as yellowish dots. The blue source located right at the galaxy's center is coincident with the position of the suspected massive black hole. While the X-rays are produced as material falls into the black hole and heats up, estimates from the X-ray data show Andromeda's central source to be surprisingly cool - only a million degrees or so compared to the tens of millions of degrees indicated for Andromeda's X-ray binaries.