juliensimon/nasa-apod huggingface dataset:
Composing the Omega Nebula (2000-12-11)

The Omega Nebula is a massive, complex cloud of dust and gas from which new stars are continually forming.  The similarity to the Greek letter capital Omega gives the molecular cloud its popular name, but the nebula is also known as the Swan Nebula, the Horseshoe Nebula, and M17.  Detailed features such as thin filaments of emission by diffuse dust and dark clouds of absorption by dense dust are visible in this recently released picture.  The image highlights infrared light emitted by large molecules known as polycyclic aromatic hydrocarbons (PAHs), a gas similar to car exhaust that traces carbon and interstellar dust.  PAHs may be an intermediate step between smaller molecules and large interstellar dust grains.  The origin of PAHs is currently unknown but thought by some astronomers to form in the cool atmospheres of young carbon stars and to be dispersed by their stellar winds.

Image credit: K. M. Merrill
Jupiter Eyes Ganymede (2000-12-12)

Who keeps an eye on the largest moon in the Solar System?  This moon, visible on the lower right, is Ganymede, and the planet it orbits, Jupiter, seems to be keeping a watchful eye, as its Great Red Spot appears serendipitously nearby. This recently released enhanced-contrast image from the robot spacecraft Cassini captures new details of the incredible intricacies of Jupiter's complex cloud patterns.  Features as small as 250 kilometers can be seen.  Counter-clockwise rotating high-pressure white ovals that are similar to the Great Red Spot appear in the red band below the spot.  Between these spots are darker low-pressure systems that rotate clockwise. The hydrogen and helium that compose most of Jupiter's clouds is nearly invisible - the trace chemicals that give Jupiter these colors remain unknown. The Cassini spacecraft is using Jupiter to pull it toward Saturn, where it is scheduled to arrive in 2004.
Manicouagan Impact Crater on Earth (2000-12-13)

The Manicouagan Crater in northern Canada is one of the oldest impact craters known.  Formed during a surely tremendous impact about 200 million years ago, the present day terrain supports a 70-kilometer diameter hydroelectric reservoir in the telltale form of an annular lake.  The crater itself has been worn away by the passing of glaciers and other erosional processes. Still, the hard rock at the impact site has preserved much of the complex impact structure and so allows scientists a leading case to help understand large impact features on Earth and other Solar System bodies.  Also visible above is the vertical fin of the Space Shuttle Columbia from which the picture was taken in 1983.
International Space Station Trail (2000-12-14)

Still under construction, the International Space Station is becoming one of the brightest, fastest moving "stars" in the heavens. Despite illuminated clouds and bright light from a nearly full moon (lower left), this 5 minute time exposure easily captures the Space Station's trail as it arcs through early evening skies above Palm Beach Gardens, Florida, USA on December 9. At the time, the Space Shuttle Endeavour had undocked and moved away from the orbiting platform, the shuttle crew having just completed the installation of large solar panels to power the Space Station's systems. Sunlight glinting off the large, shiny panels is likely the source of the brief flare visible along the track. Astrophotographer Doug Murray and colleague report that both Shuttle and Space Station were visible separately and on close inspection of this image they do produce distinct, parallel arcs. At the extreme right hand edge of the picture, the trails pass very near the brightest "star" in the night sky, Venus.

Image credit: Doug Murray
IC443's Neutron Star (2000-12-15)

Using x-ray data from the orbiting Chandra Observatory along with radio data from the Very Large Array, a team of researchers has discovered evidence for a new example of one of the most bizarre objects known to modern astrophysics -- a neutron star. Embedded within supernova remnant IC443, the suspected neutron star appears as the reddish source at the lower right in this false-color x-ray image. Perhaps 20 kilometers across but with more mass than the Sun, this ultracompact object is the collapsed core of a massive star. The core collapsed when the star, located a reassuring 5,000 light-years away in the constellation Gemini, exploded long ago. How long ago? Judging from the characteristic bow wave shape of the x-ray nebula the researchers have estimated the speed of the neutron star as it plows away from the explosion site. Comparing the speed to the measured distance traveled from the center of IC443, the team, three high school students and a teacher from the North Carolina School for Science and Mathematics, calculated that the light from the supernova explosion arrived at Earth about 30,000 years ago.
Degas Ray Crater on Mercury (2000-12-16)

Like the Earth's Moon, Mercury is scarred with craters testifying to an intense bombardment during the early history of the Solar System. In 1974, the Mariner 10 spacecraft surveyed this innermost planet up close, producing the only detailed images of its tortured surface. In the above mosaic the bright rays emanating from the 45 kilometer wide Degas crater almost appear to be painted on. The rays consist of light colored material blasted out during the crater's formation. Craters older than Degas are covered by the ray material while younger craters are seen superimposed on the rays. Mercury's gravity and density are about twice that of Earth's Moon so such bright ray craters on the lunar surface tend to be much larger. NASA plans to launch MESSENGER to the least explored terrestrial planet in 2004.
M2-9: Wings of a Butterfly Nebula (2000-12-17)

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.
Oceans Under Jupiter's Moon Ganymede? (2000-12-18)

The search for extraterrestrial life came back into our own Solar System last week with the announcement that there may be liquid oceans under the surface of Jupiter's moon Ganymede.  Ganymede now joins Callisto and Europa as moons of Jupiter that may harbor seas of liquid water under layers of surface ice.  The ocean hypothesis surfaced as an explanation for Ganymede's unusually strong magnetic field.  Ganymede, the largest moon in the Solar System, also has the largest measured magnetic field of any moon.  Some exobiologists hypothesize that life may be able to emerge in such an ocean, much as it did in the oceans of ancient Earth. Above, a frame from a computer simulation shows what it would look like to fly over the surface of Ganymede, as extrapolated from photographs of the grooved moon taken by the robot spacecraft Galileo currently orbiting Jupiter.
A Close-Up of Aurora on Jupiter (2000-12-19)

Jupiter has aurorae.  Like Earth, the magnetic field of the gas giant funnels charged particles released from the Sun onto the poles.  As these particles strike the atmosphere, electrons are temporarily knocked away from existing gas molecules.  Electric force attracts these electrons back.  As the electrons recombine to remake neutral molecules, auroral light is emitted.  In the above recently released photograph by the Hubble Space Telescope taken in ultraviolet light, the aurorae appear as annular sheets around the pole.  Unlike Earth's aurorae, Jupiter's aurorae include several bright streaks and dots.  These marks are caused by magnetic flux tubes connecting Jupiter to its largest moons.  Specifically, Io caused the bright streak on the far left, Ganymede caused the bright dot below center, and Europa caused the dot to its right.
Sgr A*: Fast Stars Near the Galactic Center (2000-12-20)

Why are these stars moving so fast?  Shown above is a time-lapse movie in infrared light detailing how stars in the central light-year of our Galaxy have moved over the past eight years.  The yellow mark at the image center represents the location of a peculiar radio source named Sgr A*.  If these fast stars are held to the Galactic Center by gravity, then the central object exerting this gravity must be both compact and massive.  Analysis of the stellar motions indicates that over one million times the mass of our Sun is somehow confined to a region less than a fifth of a light-year across.  Astronomers interpret these observations as strong evidence that the center of our Galaxy is home to a very massive black hole.
Solstice And Season's Eclipse (2000-12-21)

Today the Sun reaches its southernmost point in planet Earth's sky at 13:37 UT. This celestial event is known as a solstice, marking the beginning of Summer in the Southern Hemisphere and Winter in the North. But this year, the solstice will be followed, on December 25th, by another geocentric celestial event -- the last eclipse of the millennium! The Christmas day eclipse will only be a partial one as the silhouetted disk of the Moon obscures the Sun's edge. Visible from much of Canada, The United States and Mexico, the appearance of the partially eclipsed Sun might remind you of the last holiday cookie you took a bite from. Still, the exact timing and degree of the eclipse will depend very much on your location. This image, from an annular eclipse in 1994, shows the lunar disk covering around 55% of the Sun's diameter. It is representative of what could be seen from Washington D. C. during the December 25 eclipse maximum which, for that location, occurs at 12:41 PM ET. As always, if you view the eclipse be extremely careful to protect your eyes.

Image credit: Sebastien Gauthier
Simulated Supergiant Star (2000-12-22)

Looking for that perfect holiday gift for an astronomer? Consider this "star in a box". Of course, the box is actually a computational box consisting of a three dimensional grid of points, and the star is a virtual one whose physical properties and internal dynamics are numerically simulated at the points on the grid. While computers and software capable of a totally realistic numerical simulation of a complete star don't presently exist, researchers have been making progress. This picture is a movie frame from a recent numerical simulation of a supergiant star with properties intended to approximate the real star Betelgeuse. The single frame shows large convection cells and bright spots mottling the virtual supergiant's surface. Simulation movies show these surface features changing substantially with time. Encouragingly, telescopic observations indicate that the surface of Betelgeuse does indeed have prominent large scale features and the well-known star's brightness variations are detectable with the unaided eye. The real supergiant Betelgeuse is some 2,500 degrees cooler than, and 620 times the size of the Sun.
Summer at the South Pole (2000-12-23)

The December solstice brings the beginning of Winter to Earth's Northern Hemisphere and Summer time to the South! This view of Earth's Southern Hemisphere near the beginning of Summer was created using images from the Galileo spacecraft taken during its  December 1990 flyby of our fair planet. Dramatically centered on the South Pole, this mosaic was constructed by piecing together images made over a 24 hour period so that the entire hemisphere appears to be in sunlight. South America (middle left), Africa (upper right), and Australia (lower right), are visible as dark masses while Antarctica gleams brightly in the center. Swirling clouds marking regularly spaced major weather systems are also prominent.
NGC 1850: Gas Clouds and Star Clusters (2000-12-24)

There's nothing like it in our own Galaxy.  Globular clusters as young as NGC 1850 don't exist here.  Globular clusters only 40 millions of years old can still be found in the neighboring LMC galaxy, though, but perhaps none so unusual as NGC 1850.  Close inspection of the above photograph will reveal two clusters.  Below and right of the main group of stars known as NGC 1850A is a smaller, still younger group dubbed NGC 1850B.  This cluster is made of stars only about four million years old.  The large red cloud of gas surrounding the clusters may have been predominantly created by supernovae explosions of stars in the younger cluster.  The red supernova remnant N57D is visible on the upper left.
The Eclipse Tree (2000-12-25)

If you look closely at the shadow of this tree, you will see something quite unusual: it is composed of hundreds of images of a partially eclipsed Sun. Early today, trees across North America will be casting similar shadows as a partial eclipse of the Sun takes place.  In a partial eclipse, the Moon does not cover the entire Sun.  The above effect is created by small spaces between leaves and branches acting as pinhole lenses.  Looking at shadows involving eclipse light is relatively safe - looking directly at the Sun, even during an eclipse, is dangerous and proper precautions should be taken.  The above picture was taken in 1994 on the campus of Northwestern University.

Image credit: E. Israel
Jupiter, Io, and Shadow (2000-12-26)

Just as planets orbit our Sun, Jupiter's Moons orbit Jupiter.  Pictured above is the closest of Jupiter's Galilean Satellites, Io, superposed in front of the giant planet it circles.  To the left of Io is a dark spot that is its own shadow.  The tremendous complexities that can be seen in Jupiter's banded, swirling atmosphere are being studied and may provide insight as to how Earth's atmosphere behaves.  The above true-color contrast-enhanced image was  taken two weeks ago by the robot spacecraft Cassini, currently passing Jupiter and on its way to Saturn in 2004.  Engineers continue to study the Cassini spacecraft itself to understand why it required more force than normal to turn one of its maneuvering wheels.
The Dust and Ion Tails of Comet Hale Bopp (2000-12-27)

Tomorrow's picture: Moon Mare and Montes  < | 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.

Image credit: John Gleason 
(Celestial Images)
Moon Mare and Montes (2000-12-28)

Tomorrow's picture: Cosmic Cloud  < | 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.

Image credit: Peter Armstrong
The Dark Horsehead Nebula (2000-12-29)

While drifting through the cosmos this magnificent interstellar dust cloud, sculpted by stellar winds and radiation, has chanced to assume a recognizable shape. Fittingly named The Horsehead Nebula it is embedded in the immense complex of star forming gas and dust surrounding the Orion Nebula some 1,500 light-years distant. The dark nebula is cataloged as Barnard 33 and is visible only because its obscuring dust is silhouetted against the bright emission nebulae which lie behind it. In fact, the prominent horse head portion of the nebula is really just part of a larger cloud of dust which can be seen extending off the bottom of the picture. A potentially rewarding but difficult object to view with a small instrument, this gorgeous color image is a composite of exposures made with a digital camera and astronomical BVR filters using a 1-meter diameter telescope.
A Year of Resolving Backgrounds (2000-12-30)

No matter which direction you look, no matter what type of light you see, the sky glows - but why?  The sources of many of these background radiations have remained long-standing puzzles, but this millennial year brought some partial resolutions.  In X-ray light the recently launched spacecraft Chandra and XMM resolved much of the seemingly uniform X-ray background into many discrete sources, many of which appear to be black holes at the centers of galaxies accreting matter.  In microwave light, the BOOMERANG and MAXIMA-1 missions resolved with new clarity the seemingly uniform microwave background.  The size and distribution of these spots indicates a geometrically flat universe, which, when combined recent supernovae results, indicate a universe with an accelerating expansion rate filled with dark matter and dark energy. Pictured above, a map spanning ten degrees of the microwave sky resolves the microwave background into hot and cold spots, as indicated in microkelvins.
The Millennium that Defined Earth (2000-12-31)

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 (2001-01-01)

Welcome to 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.
Jupiter, Europa, and Callisto (2001-01-02)

As the robot Cassini spacecraft rounds Jupiter on its way toward Saturn, it has taken a sequence of images of the gas giant with its four largest moons.  Previously released images have highlighted Ganymede and Io.  Pictured above are the two remaining Galilean satellites: Europa and Callisto.  Europa is the bright moon superposed near Jupiter's Great Red Spot, while Callisto is the dark moon near the frame edge.  Callisto is so dark that it would be hard to see here if its brightness was not digitally enhanced.  Recent evidence indicates that both moons hold salt-water seas under surface ice that might be home to extra-terrestrial life.  By noting the times that moons disappeared and reappeared behind Jupiter in 1676, Ole Roemer was able to make the first accurate estimation of the speed of light.
M8: In the Center of the Lagoon Nebula (2001-01-03)

In the center of the Lagoon Nebula one finds glowing gas, star clusters, and dense knots of gas and dust just now forming stars.  The young open cluster of stars, designated NGC 6523, can be seen in the center of the above image.  These stars emit energetic light that ionizes the surrounding hydrogen gas.  As this gas reacquires electrons, it emits red light.  The Lagoon Nebula lies about 5000 light-years away and spans about 100 light-years across.   The nebula occupies an area on the sky larger than a full moon, and can be seen even without binoculars from a dark location towards the constellation of Sagittarius.

Image credit: Yuugi Kitahara
Third Millennium, First Eclipse (2001-01-04)

The first eclipse of the third millennium is coming up! A total lunar eclipse mainly visible from Europe, Asia, and Africa, will occur on January 9th as the full Moon glides through the long, but not so dark shadow of planet Earth. Why not so dark? Sunlight is actually scattered and refracted into Earth's shadow by the atmosphere, a circumstance which can help create some striking photographs of the eclipsed Moon. For example, this image is a composite of photographs taken during the July 2000 total lunar eclipse. Early and late phases of the eclipse flank a deep exposure made during totality. The feeble sunlight still shining on the lunar surface gives the Moon a dramatic dark red cast. While the July 2000 eclipse was not the first or last of any millennium, it was remarkable for being the longest eclipse for about the next 1,000 years, totality lasting an impressive one hour and 47 minutes. Totality for the January 9th eclipse will last just over one hour.

Image credit: Fred Espenak
(courtesy of
www.MrEclipse.com)
Second Millennium, Last Eclipse (2001-01-05)

Christmas Day 2000 featured the final eclipse of the Second Millennium -- a partial solar eclipse visible from much of North America. Astrophotographer Phil Rau recorded the entire event on a single image as the Sun and Moon arced through winter skies above Cary, North Carolina, USA. Using a well positioned, tripod mounted camera and a solar filter, Rau made a short exposure every 10 minutes from 11:00 am to 1:30 pm EST, covering the local duration of the eclipse. The resulting image beautifully illustrates the steady progress of the dark new Moon as it appears to take a bite out of the golden solar disk. From his location, at eclipse maximum (near picture center) just less than 50 percent of the Sun's diameter was covered by the Moon.

Image credit: Philipp Rau
Apollo 17's Moonship (2001-01-06)

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.
Tycho Brahe Measures the Sky (2001-01-07)

Tycho Brahe was the most meticulous astronomical observer of his time.  Brahe, who lived between 1546 and 1601, set out to solve the day's most pressing astronomical problem: to determine whether the Earth or the Sun was at the center of the Solar System.  To do this Brahe and his assistants created the first major astronomical observatory where they devised and used the most accurate pre-telescopic astronomical instruments.  Tycho Brahe thus compiled tables of precise measurements of the positions and brightnesses of planets and stars.  Brahe never solved the Solar System problem himself - but left data so impressively accurate his assistant Johannes Kepler was able to develop definitive laws. Brahe is also remembered for witnessing a supernova in 1572, showing that the Great Comet of 1577 was not an atmospheric phenomena, and for his metal nose.