juliensimon/nasa-apod huggingface dataset:
Runaway Star (1999-11-27)

Runaway stars are massive stars traveling rapidly through interstellar space. Like a ship plowing through the interstellar medium, runaway star HD 77581 has produced this graceful arcing bow wave or "bow shock" - compressing the gaseous material in its path. Located near the centre of this European Southern Observatory photograph, HD 77581 itself is so bright that it saturates the sensitive camera and produces the spiky cross shape. This star is over 6,000 light-years away in the constellation Vela, and appears to move at over 50 miles per second. What force could set this star in motion? A clue to the answer may lie in its optically invisible companion star, an X-ray bright pulsar known as Vela X-1. This pulsar is clearly the remnant of a supernova explosion ... which seems to have given this massive star and its companion a mighty kick!
Beneath Venus' Clouds (1999-11-28)

If the thick clouds covering Venus were removed, how would the surface appear?  Using an imaging radar technique, the Magellan spacecraft was able to lift the veil from the Face of Venus and produce this spectacular high resolution image of the planet's surface.  Red, in this false-color map, represents mountains, while blue represents valleys.  This 3-kilometer resolution map is a composite of Magellan images compiled between 1990 and 1994.  Gaps were filled in by the Earth-based Arecibo Radio Telescope. The large yellow/red area in the north is Ishtar Terra featuring Maxwell Montes, the largest mountain on Venus.  The large highland regions are analogous to continents on Earth.  Scientists are particularly interested in exploring the geology of Venus because of its similarity to Earth.
Arcs and Jets in Herbig Haro 34 (1999-11-29)

Tomorrow's picture: Superbubble  < 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.
Henize 70: A Superbubble in the LMC (1999-11-30)

Tomorrow's picture: Three Days From 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.
Landing At The Martian South Pole (1999-12-01)

The latest invader from Earth, the Mars Polar Lander (MPL) spacecraft, is scheduled to touch down on the Red Planet's surface on Friday, December 3rd. If all goes well the robotic explorer will open a parachute, deploy landing legs, fire thrusters, and come to rest amidst rolling plains of intriguing layered terrain near the Martian South Pole. In this orbital image, a grid overlays the landing region with the targeted site indicated by a purple oval about 200 kilometers long by 20 kilometers wide. A secondary landing site is shown as a white oval while at the upper left an inset shows the area in relation to the pole. This polar landing region's layered terrain will hopefully allow MPL's instruments to view a record of climate changes in Martian history, like reading tree rings or layers in an ice core. Before landing, MPL will drop off two microprobes intended to penetrate the soil and look for water ice. Mars Polar Lander will also carry a small Mars Microphone.
1999 Leonid Fireball (1999-12-02)

Most Leonid meteoroids, the bits of comet debris which produce the annual Leonid meteor shower, range from a mere millimeter to a centimeter in diameter. Yet these cosmic grains of sand and gravel can put on quite a spectacular show. How can something so small generate so much light? The answer is their astronomical speed, as these particles enter Earth's atmosphere at around 71 kilometers per second. In the high-speed collisions with air molecules, electrons are stripped from atoms as meteroid material is blasted away. When the electrons recombine with the atoms, light is emitted. This dramatic example of a brilliant 1999 Leonid meteor was photographed while tracking the stars in partly foggy skies on November 18, from a location near Dagali, Norway. The two bright reddish-orange stars visible are the familiar giant stars Betelgeuse (left) and Aldebaran.

Image credit: Arne Danielsen
Southern Mars (1999-12-03)

This topographical map of the southern hemisphere of Mars was generated using data from the Mars Orbiter Laser Altimeter (MOLA). Flying on the Mars Global Surveyor spacecraft, MOLA has bounced a laser beam off the Martian surface over 200,000,000 times producing a wealth of detailed elevation measurements. The MOLA measurements have been color-coded so, for example, the white areas at left are the highest elevations in the southern Tharsis region and not snow-covered peaks. These areas are more than 6 kilometers above the hypothetical Martian "sea-level". Likewise, deep blues and purples are not water oceans but correspond to the lowest elevations (more than 4 kilometers below "sea-level"), like those found within the giant Hellas impact basin at right. In fact, liquid water is not present on Mars' surface today, but may have been in the past. NASA's Mars Polar Lander spacecraft is scheduled to embark on an investigation of the role of water in the climate history of the Red Planet. The lander is targeted to touch down within the long, thin ellipse indicated here just below the Martian South Pole today at 20:00 UTC.
Mars Polar Lander Target Ellipse (1999-12-04)

Tomorrow's picture: Saturnian Moon  < 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.
Rhea: Saturn's Second Largest Moon (1999-12-05)

Rhea is the second largest moon of Saturn, behind Titan, and the largest without an atmosphere.   It is composed mostly of water ice, but has a small rocky core.  Rhea's rotation and orbit are locked together (just like Earth's Moon) so that one side always faces Saturn.  A consequence of this is that one side always leads the other.  Rhea's leading surface is much more heavily cratered than its trailing surface.  The above photograph was taken with the Voyager 1 spacecraft in 1980.

Image credit: Calvin J. Hamilton
M83: The Southern Pinwheel Galaxy from VLT (1999-12-06)

M83 is one of the closest and brightest spiral galaxies on the sky.  Visible with binoculars in the constellation of Hydra, majestic spiral arms have prompted its nickname as the Southern Pinwheel.  Although discovered 250 years ago, only in this century was it appreciated that M83 was not a gas cloud but a barred spiral galaxy much like our own Milky Way Galaxy.  M83, pictured above in a recently released photograph from a Very Large Telescope, is a prominent member of a group of galaxies that includes Centaurus A and NGC 5253, all of which lie about 15 million light years distant.  To date, six supernova explosions have been recorded in M83.  An unusual double circumnuclear ring has recently been discovered at the center of M83 and is still being investigated.
The Cat's Paw Nebula (1999-12-07)

As soon as we find out whose cat did this . . .  Nebulae are as famous for being identified with familiar shapes as perhaps cats are for getting into trouble.  No cat, though, could have created the vast Cat's Paw Nebula visible in Scorpius.  The Cat's Paw Nebula is the glowing red nebula near the top of the above picture -- the lower nebula is NGC 6357.  At 5500 light years distant, Cat's Paw is an emission nebula with a red color that originates from an abundance of ionized hydrogen atoms.  Alternatively known as the Bear Claw Nebula or NGC 6334, stars nearly ten times the mass of our Sun have been born there in only the past few million years.

Image credit: Jason Ware
Moon Struck (1999-12-08)

Craters produced by ancient impacts on the airless Moon have long been a familiar sight. But now observers have seen elusive optical flashes on the lunar surface - likely the fleeting result of impacting meteoroids. Orchestrated by David Dunham, president of the International Occultation Timing Association (IOTA), video recordings made with modest equipment and visual telescopic observations have, for the first time, detected and confirmed a total of six flashes on the Moon's dark side. The flashes, some initially as bright as a third magnitude star, were all seen within hours of the peak of this year's Leonid meteor shower. Their locations are indicated by the red Xs on this projection of the Moon as it appeared on the night of November 18. Similar flashes would have been difficult to see if viewed against the Moon's brightly lit portion. It has been estimated that the brightest flashes were made by meteoroids weighing around a tenth of a kilogram, resulting in lunar craters about one meter across. And ... the next chance to observe lunar impact flashes is coming up! Enterprising astronomers interested in long distance lunar prospecting should be monitoring the dark side of a nearly first quarter Moon during the Geminids meteor shower which will peak around December 13.
X-ray Hot Supernova Remnant in the SMC (1999-12-09)

The Q-shaped cloud seen in this false-color X-ray image from the orbiting Chandra Observatory is big ... about 40 light-years across. It's hot too, as its X-ray glow is produced by multi-million degree gas. Cataloged as E0102-72, this cosmic Q is likely a several thousand year old supernova remnant, the result of the death explosion of a massive star. A supernova can dramatically affect its galactic environment, triggering star formation and enriching the local interstellar medium with newly synthesized elements. This supernova remnant is located about 210,000 light-years away in our neighboring galaxy, the Small Magellanic Cloud (SMC), so the detailed Chandra X-ray image is impressive - particularly as it reveals what appear to be strange spoke-like structures radiating from the remnant's center.
Spot The Planet (1999-12-10)

OK, it's a picture of the Sun (duh!), but can you spot the planet? Of course, most of the spots you've spotted are sunspots, as large or larger than planet Earth itself. The sunspots are regions of strong surface magnetic fields which are dark in this picture only because they are relatively cool compared to their surroundings. Over the past few years, the number of sunspots has been steadily increasing as the Sun approaches the maximum in its 11 year activity cycle. But also visible in this photograph from November 15, is planet Mercury. At just over 1/3 Earth's size, Mercury is passing in front of the Sun, its silhouette briefly creating a diminutive dark spot drifting across an enormous solar disk. While "transits" of Mercury do occur 13 times a century, this one was additionally a very rare grazing transit of our Solar System's innermost planet. Spotted Mercury yet? Click on the picture for a hint.

Image credit: Rick Scott and Joe Orman
A Close Encounter Of The Stellar Kind (1999-12-11)

The unassuming star centered in this sky view will one day be our next door stellar neighbor. The faint 9th magnitude red dwarf, currently 63 light-years away in the constellation Ophiucus, was recently discovered to be approaching our Solar System. Known in catalogs of nearby stars as Gliese 710 it is predicted to come within 1 light-year of the Sun ... a million years from now. At that distance this star, presently much too faint to be seen by the naked eye, will blaze at 0.6 magnitude - rivaling the apparent brightness of the mighty red giant Antares. Ultimately Gliese 710 poses no direct collision danger itself although its gravitational influence will likely scatter comets out of the Solar System's reservoir, the Oort cloud, sending some inbound. This future stellar encounter was discovered by researchers Joan Garcia-Sanchez and Robert Preston (JPL), and collaborators while studying stars in the solar neighborhood using data from the Hipparcos Astrometry Satellite. The star field shown is based on the Palomar Digitized Sky Survey and is 1/4 degree wide (about half the diameter of the full moon).
NGC 4314: A Nuclear Starburst Ring (1999-12-12)

Is this old galaxy up to new tricks? The barred spiral galaxy NGC 4314 is billions of years old, but its appearance has changed markedly over just the past few millions of years.  During that time, a nuclear ring of bright young stars has been evolving.  The inset picture of NGC 4314 taken by McDonald Observatory shows the whole galaxy and boxes the small region around the core imaged by the Hubble Space Telescope.  This inner region appears much like a miniature spiral galaxy itself, complete with dust lanes and spiral arms, even though it is only a few thousand light-years across.  Further study of NGC 4314 might help astronomers understand how the inner and outer parts of this galaxy interact, and what caused this unusual ring of star formation.
A Magellanic Starfield (1999-12-13)

Stars of many types and colors are visible in this Hubble Space Telescope vista of the Large Magellanic Cloud (LMC).  Over 10,000 stars are visible -- the brightest of which are giant stars.  Were our Sun 170,000 light-years distant and among these stars, it would hardly be discernable.  By contrast, only a few thousand stars are individually visible at night with the unaided eye, and many of these lie within only a few hundred light-years.  Typically, the light we see from nearby stars left during the age of our great-grand-parents, while light from LMC stars started its journey before the dawn of recorded human history.
High Velocity Clouds and the Milky Way (1999-12-14)

Where are these gas clouds going so quickly?  High velocity clouds (HVCs) of gas have been seen for decades but their origins and destinations have remained mysterious.  Recent measurements have now placed at least one of these clouds in the halo of our Milky Way Galaxy, while other measurements have determined the relative abundance of cloud elements.  Of the two clouds measured, each appears to have a chemical abundance consistent with a different origin.  One HVC has been measured to have very few heavy elements ("low metallically") compared to neighboring stars, while another HVC has been inferred to have a heavy element abundance more typical of neighboring stars.  Hypotheses are therefore being investigated that some HVCs are local gas remnants being pushed away from our Galaxy by supernova explosions, while other HVCs are ancient dwarf galaxy remnants falling toward our Galaxy.  The latter possibility is particularly interesting as it might help explain how our Galaxy can continue to make stars at the observed rate. Fast moving HVCs are circled in the above false-color mosaic.
A Nova In Aquila (1999-12-15)

On December 1st, experienced observers patroling the night sky with binoculars noticed what seemed to be a new star in the constellation of Aquila (The Eagle). It wasn't really a new star though. A comparison with detailed skymaps revealed the amazing truth, there was a known star at that position in the sky ... its brightness had simply increased by about 70,000 times.  The star, now fondly known to variable star observers as Nova V1494 Aquilae, continued to grow brighter for several days, becoming easily visible to the unaided eye before starting to slowly fade away. Its position within the constellation is indicated on this wide-angle picture taken on December 4th, near the time it was brightest. What would cause a star to undergo such a cataclysmic change? This "new star" appears to be a classical nova. Classical novae are thought to be interacting binary star systems in which one of the pair is a dense, hot white dwarf. Material from the companion falls onto the surface of the white dwarf, building up until it triggers a thermonuclear blast. A stunning increase in brightness and an expanding shell of debris result - but the binary system is likely not destroyed! Classical novae are believed to recur as the flow of material resumes and produces another outburst in perhaps hundreds of years time.

Image credit: Till Credner
Supernova Remnant In M82 (1999-12-16)

This false-color radio wavelength picture of an expanding stellar debris cloud is the product of one of the largest radio astronomy experiments ever. Combining the output of 20 radio telescopes scattered around planet Earth, astronomers have produced this amazingly detailed image of a supernova remnant just over 1.5 light-years across in M82, an intense star forming galaxy 12 million light-years away. The radio astronomy technique for creating the earth-sized array of telescopes is known as VLBI (very long baseline interferometry). The individual telescopes of the array are too far apart for all their signals to be combined in "real time", so their output was recorded on magnetic tapes which were brought to a single location and correlated. This technique has produced the highest resolution astronomical observations currently possible and allows the exploration of such distant, violent galactic environments in unprecedented detail.
Hot Gas In Hydra A (1999-12-17)

The Hydra A galaxy cluster is really big. In fact, such clusters of galaxies are the largest gravitationally bound objects in the Universe. But individual galaxies are too cool to be recorded in this false-color Chandra Observatory X-ray image which shows only the 40 million degree gas that permeates the Hydra A cluster. Astronomers have discovered that such X-ray hot gas clouds, millions of light-years across, are common in galaxy clusters. They expected the gas to be cooling and smoothly flowing into the clusters' central regions to form new galaxies and stars. Instead, the Chandra image shows details around the X-ray bright cluster core which suggest that magnetic fields and explosive events disturb the flow, deflecting the gas into loops and long structures and possibly inhibiting the formation of more cluster galaxies and stars.
Irregular Galaxy Sextans A (1999-12-18)

Grand spiral galaxies often seem to get all the glory. Their newly formed, bright, blue star clusters found along beautiful, symmetric spiral arms are guaranteed to attract attention. But small irregular galaxies form stars too, like this lovely, gumdrop-shaped galaxy, Sextans A. A member of the local group of galaxies which includes the massive spirals Andromeda and our own Milky Way, Sextans A is about 10 million light years distant. The bright Milky Way foreground stars appear yellowish in this view. Beyond them lie the stars of Sextans A with tantalizing young blue clusters clearly visible.
Accretion Disk Binary System (1999-12-19)

Our Sun is unusual in that it is alone - most stars occur in multiple or binary systems.  In a binary system, the higher mass star will evolve faster and will eventually become a compact object - either a white dwarf star, a neutron star, or black hole.  When the lower mass star later evolves into an expansion phase, it may be so close to the compact star that its outer atmosphere actually falls onto the compact star.  Such is the case diagrammed above. Here gas from a blue giant star is shown being stripped away into an accretion disk around its compact binary companion.  Gas in the accretion disk swirls around, heats up, and eventually falls onto the compact star.  Extreme conditions frequently occur on the surface of the compact star as gas falls in, many times causing detectable X-rays, gamma-rays, or even cataclysmic novae explosions.  Studying the extreme conditions in these systems tells us about the inner properties of ordinary matter around us.
Lava Fountain on Jupiter's Io (1999-12-20)

A lava fountain shooting over a kilometer high has been discovered on Jupiter's moon Io.  The robot Galileo spacecraft orbiting Jupiter photographed the volcanic eruption during its close flyby of the moon late last month.  The fountain is visible in the above mosaic of images from the flyby.  The lava plume was so bright it saturated Galileo's camera, and so has been digitally reconstructed in false color.  The hot lava appears to be shooting from a 20-kilometer long canyon on Io's active surface.
XMM Launched (1999-12-21)

X-ray astronomy entered a golden age earlier this month with the successful launch of the X-ray Multi-Mirror (XMM) satellite.  XMM's three huge telescope barrels each hold 58 concentric cylindrical mirrors, together totaling a surface area rivaling a tennis court.  Each mirror has been gold plated to less than one-millimeter thickness to reflect normally penetrating X-rays.  ESA's XMM joins NASA's Chandra X-ray Observatory as leading observatories in X-ray astronomy.  The XMM satellite also carries a small optical and ultraviolet telescope.  XMM's unusually elliptical orbit around the Earth peaks nearly one-third of the way to the Moon.  XMM's observing program during its planned two-year mission includes monitoring the hot surroundings of black holes, the fiery regions surrounding the centers of galaxies, the mysterious X-ray background light that appears to come from all directions, and the hot gas that glows between galaxies and stars.
Perigee Moon, Apogee Moon (1999-12-22)

Tonight, those blessed with clear skies can enjoy a glorious full moon, the last full moon of the "Y1.9K"s. In fact, tonight's moon will be a full-perigee-solstice moon, reaching its full phase and perigee (the closest point in its orbit) on the solstice, the first day of northern hemisphere winter. Anticipation of this celestial event has generated a flood of e-mail chasing the rumor that the full-perigee-solstice moon will be intensely bright. However, calculations show it will only be a little brighter than usual, and certainly not dramatically so. Why would it be brighter? Mostly because the moon appears bigger and brighter at perigee. The above illustration, based on Galileo spacecraft images, shows the approximate difference in apparent size between a full moon at perigee (left) and a full moon at apogee, the farthest point in the lunar orbit. The Earth and moon are also a few percent closer to the sun during northern winter, but the combined effects will cause tonight's full moon to appear only about 20% brighter than an average full moon. That difference is not even likely to be noticeable to the human eye.
Unusual Aurora During Solar Wind Dropout (1999-12-23)

On May 10, for some unknown reason, the Solar Wind virtually stopped. Normally our Sun emits a wind of between five and ten energetic particles per cubic centimeter moving outward at about 500 kilometers per second.  Late on May 10, however, this gale was reduced to a mere breeze of one particle per every five cubic centimeters.  The Sun's Corona was suddenly able to flow out into the Solar System relatively unimpeded, creating beams of energetic electrons.  One such beam apparently reached Earth's North Magnetic Pole, and was seen as the unusual X-ray aurora digitally reconstructed in the above false-color image.  Our atmosphere absorbed the electrons.  This display gave direct evidence, however, that Earth's North Magnetic Pole was connected to the Sun, while the Earth's South Magnetic Pole connected to the distant Solar System.  The Solar Wind returned to normal on May 12.