juliensimon/nasa-apod huggingface dataset:
Tails Of Comet LINEAR (2000-07-27)

Comet C/1999 S4 LINEAR is only one of many comets discovered with the Lincoln Near Earth Asteroid Research (LINEAR) telescope operating near Soccoro, New Mexico, USA. Traveling steadily southward through Earth's night sky, C/1999 S4 passed perihelion (closest approach to the Sun) yesterday on what is likely its first trip through the inner solar system. Now fading, comet LINEAR became no brighter than about 6th magnitude, but is still easily visible with binoculars in northern hemisphere skies. While the memorable comets Hale-Bopp and Hyakutake were much brighter, comet LINEAR is displaying delightful tails evident in this false-color composite image from the Crni Vrh Observatory in Slovenia. The combined series of exposures made on July 22nd are registered on the comet. In the resulting picture, stars appear as rows of dots, but the faint structures in the comet's tail are beautifully recorded. Presently seen moving from Ursa Major to Leo this comet LINEAR will begin to shine in southern hemisphere skies in August.

Image credit: Jure Skvarc,
Bojan Dintinjana,
Herman Mikuz
(Crni Vrh
Observatory, Slovenia)
Moon And Venus Share The Sky (2000-07-28)

Tomorrow's picture: Star Cluster  < | 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: Tunc Tezel
NGC1850: Star Cluster in the LMC (2000-07-29)

NGC1850 is a large cluster of stars located a mere 166,000 light-years from Earth in our neighboring galaxy the Large Magellanic Cloud (LMC). The colors in this beautiful Hubble Space Telescope composite image of the cluster reveal different populations of stars. Yellowish stars are the main cluster stars, sun-like main sequence hydrogen burners about 50 million years old. The white stars are massive, hotter, and younger, about 4 million years old. Radiating strongly in ultraviolet light, they represent a loose cluster themselves, perhaps within 200 light-years of the main cluster. Massive stars which formed in the older main cluster have long since disappeared, ending their lives in spectacular supernova explosions. Did expanding debris from these supernovae trigger the formation of the nearby younger cluster? Probably so. In any event, a few million years from now a similar fate awaits the massive stars of the younger cluster - burning brightly but briefly before they explode sending new clouds of stellar debris into space.
NGC 2440: Cocoon of a New White Dwarf (2000-07-30)

Like a butterfly, a white dwarf star begins its life by casting off a cocoon that enclosed its former self.  In this analogy, however, the Sun would be a caterpillar and the ejected shell of gas would become the prettiest of all! The above cocoon, the planetary nebula designated NGC 2440, contains one of the hottest white dwarf stars known. The white dwarf can be seen as the bright dot near the photo's center.  Our Sun will eventually become a "white dwarf butterfly", but not for another 5 billion years.  The above false color image and was post-processed by Forrest Hamilton.
Comet LINEAR Breaks Up (2000-07-31)

Unexpectedly, Comet LINEAR is breaking up.  In retrospect, clues of its demise have been surfacing all month as the new comet has been approaching the Sun and brightening with dramatic flares.  Above, the Hubble Space Telescope captured Comet C/1999 S4 LINEAR early this month blowing off a large piece of its crust.  Recent speculation holds that the nucleus completely disrupted on or about July 24.  If true, the elongated train of material should continue to ablate and orbit the Sun, but may now fade much more quickly.  The break up of a bright comet is unusual but not unprecedented, as Comet Shoemaker-Levy 9 broke up before it struck Jupiter in 1994, and Comet Bennett broke apart as it neared the Sun in 1974.  Future observations will tell if Comet LINEAR's first trip into the inner Solar System is its last.
X-Rays from Comet LINEAR (2000-08-01)

Why do comets emit X-rays?  First discovered during the passing of Comet Hyakutake in 1996, the reason a cold comet would produce hot X-rays has since remained a mystery.  On July 14, however, the orbiting Chandra X-ray Observatory was able to provide an image of passing Comet LINEAR, shown above, in enough detail to unravel the mystery.  The key to the solution turns out to be the unusual wind of fast ions emitted by our Sun.  These ions apparently collide with gas recently emitted by the comet and cause some ions to acquire a new electron.  An electron that starts in a high-energy state will emit an X-ray as it falls in closer to the ion nucleus.  As other comets move into the inner Solar System, this discovery should allow future study of the continually evolving gas cloud that surrounds comets as well as the composition of the solar wind.
At the Edge of the Crescent Nebula (2000-08-02)

The Crescent Nebula is a rapidly expanding shell of gas surrounding a dying star.  In this recently released image by the Hubble Space Telescope, a bright dynamic part of the nebula three light-years across is shown in representative color.  The Crescent Nebula began to form about 250,000 years ago as central Wolf-Rayet star WR 136 began to shed its outer envelope in a strong stellar wind, expelling the equivalent of our Sun's mass every 10,000 years.  This wind has been impacting surrounding interstellar gas, compacting it into a series of complex shells, and lighting it up.  The Crescent Nebula, also known as NGC 6888, lies about 4,700 light-years away in the constellation of Cygnus and can only be seen through a telescope.  Star WR 136 will probably undergo a supernova explosion sometime in the next million years.
22 Miles From Eros (2000-08-03)

Last month the NEAR Shoemaker spacecraft swooped closer to Eros, orbiting only 22 miles (36 kilometers) from the center of the asteroid. These two images taken on July 19 (left) and July 24 (right) reveal the diminutive world's pocked and mottled surface in amazing detail, showing features as small as 19 feet (6 meters) across. Eros is thought to be a primordial, undifferentiated asteroid based on X-ray and gamma-ray studies of its surface composition. In the left picture, its surface layer or regolith is seen to be laced with bright and dark regions while in the right hand image dark regolith appears to have filled in some crater floors. The left and right images span an area about 2,600 feet (800 meters) and 3,000 ft (900 meters) wide respectively. On July 31, NEAR Shoemaker returned to its familiar 31 mile (50 kilometer) orbit, circling Eros serenely at about 6 miles per hour.
M15: Dense Globular Star Cluster (2000-08-04)

Life might get dull at the core of M15 but the sky would always be bright with stars! In fact, only 40,000 light-years away in the constellation Pegasus, M15 is one of the most densely packed globular star clusters in our Milky Way Galaxy. This stunning Hubble Space Telescope image of M15 shows thousands of individual stars across the central 10 or so light-years of the cluster, also cataloged as NGC 7078. Yet even the Hubble's sharp vision can't clearly separate the stars at this cluster's core. Globular star clusters harbor from a hundred thousand up to a million stars and roam the Milky Way halo. Like most globulars, M15 is filled with ancient stars, about 12 billion years old compared to the Sun's estimated 4.5 billion years. Its cool red giant stars appear yellowish in this color composite image. Unlike most globulars, M15 displays a planetary nebula, the briefly visible gaseous shroud of a dying star. Can you pick it out? Cataloged as Kuestner 648, M15's planetary nebula is the round pinkish cloud at the upper left.
Halley's Nucleus: An Orbiting Iceberg (2000-08-05)

What does a comet nucleus look like? Formed from the primordial stuff of the solar system, it is thought to resemble a very dirty iceberg. But for active comets, telescopic images only reveal the surrounding cloud of gas and dust, the comet's coma, and the characteristic cometary tails. In 1986, the European spacecraft Giotto encountered the nucleus of Halley's comet as it approached the sun. Data from Giotto's camera was used to generate this enhanced image of the potato shaped nucleus which measures roughly 15 kilometers across. It shows surface features on the dark nucleus against the bright background of the coma as the icy material is vaporized by the Sun's heat. Every 76 years Comet Halley returns to the inner solar system and each time the nucleus sheds about a 6 meter deep layer of its ice and rock into space. This debris composes Halley's tails and leaves an orbiting trail responsible for the Orionids meteor shower.

Image credit: MPAE
The Coma Cluster of Galaxies (2000-08-06)

Almost every object in the above photograph is a galaxy.  The Coma Cluster of Galaxies pictured above is one of the densest clusters known - it contains thousands of galaxies.  Each of these galaxies houses billions of stars - just as our own Milky Way Galaxy does.  Although nearby when compared to most other clusters, light from the Coma Cluster still takes hundreds of millions of years to reach us.  In fact, the Coma Cluster is so big it takes light millions of years just to go from one side to the other!  Most galaxies in Coma and other clusters are ellipticals, while most galaxies outside of clusters are spirals.  The nature of Coma's X-ray emission is still being investigated.

Image credit: O. Lopez-Cruz  
(INAOEP) et al., 
AURA,
NOAO, 
NSF
Nearby Star Episilon Eridani Has a Planet (2000-08-07)

A planet has been found orbiting a Sun-like star only 10 light-years away.  No direct picture of the planet was taken - the planet was discovered by the gravitational wobble it created on its parent star, Epsilon Eridani.  The discovery marks the closest Sun-like star yet found to house an extra-solar planet.  Pictured above, the star Epsilon Eridani is visible near the belt of Orion to the unaided eye.  The detected planet is thought to have a mass like Jupiter but orbit slightly closer in.  The elliptical nature of the planet's orbit raises questions about whether the nearly circular orbits of planets in our own Solar System are relatively uncommon. It is unknown whether other planets exist around Epsilon Eridani.

Image credit: J. B. Kaler 
(UIUC)
Comet LINEAR Disperses (2000-08-08)

What's happened to the nucleus of Comet LINEAR?  The brightest comet this year has unexpectedly broken up into many smaller pieces.  The break-up occurred on or about July 25 and was noted by many astronomers around the world with particularly pioneering work by Mark Kidger (IAC). Since then astronomers had been searching in vain to find any fragments left of the nucleus, and watching to see how fast the remaining debris fades.  Just three days ago the Hubble Space Telescope was maneuvered to photograph the region and recovered some of the disintegrating fragments that used to compose Comet LINEAR's nucleus.  The above image covers only the very tip of an elongated diffuse train of slowly dispersing gas, dust, ice fragments, and gravel. The largest bits remaining of the badly fractured nucleus appear to be less than 30 meters across. This debris train will not collide with the Earth and so will not cause a meteor shower.  Interested astronomers are now theorizing why Comet LINEAR's nucleus disintegrated into such small pieces.
A Solar Filament Lifts Off (2000-08-09)

Hot gas frequently erupts from the Sun.  One such eruption produced the glowing filament pictured above, which was captured on July 19 by the Earth-orbiting TRACE satellite.  The filament, although small compared to the overall size of the Sun, measures over 100,000 kilometers in height, so that the entire Earth could easily fit into its outstretched arms.  Gas in the filament is funneled by the complex and changing magnetic field of the Sun.  After lifting off from the Sun's surface, most of the filamentary gas will eventually fall back.  More powerful solar eruptions emit particles that reach the Earth and can disrupt manmade satellites.  The cause and nature of solar eruptions are the topic of much research.
Other Worlds and HD 38529 (2000-08-10)

After the latest round of discovery announcements, the list of known worlds of distant suns has grown to 50. While extrasolar planet discoveries are sure to continue, none - so far - points clearly to another planetary system like our own. Take, for example, the newly discovered parent star HD38529. Shining in Earth's night sky at 6th magnitude, this sun-like star lies 137 light-years away in the constellation Orion. Like most of the known extrasolar planets, HD38529's planet was discovered by detecting the telltale Doppler wobble in the parent star's spectrum. The data reveal that this planet orbits once every 14.3 days at an average of only 0.13 times the Earth-Sun distance and has a minimum of 0.77 Jupiter masses (about 240 Earth masses). There is even evidence in the wobble data that HD38529, and other stars with one known planet have additional massive planets orbiting them. In this dramatic artist's vision, HD38529 and its newfound world are viewed from the moon of another massive ringed planet orbiting farther out. The ringed planet's moon is imagined to have a thin atmosphere and a surface covered with icy sheets and ridges similar to those found on Jupiter's moon Europa.
Fragments of Comet LINEAR (2000-08-11)

What do you call a bunch of comet fragments anyway ... a flock, a covey, a swarm? The question is definitely relevant to comet LINEAR (C/1999 S4 LINEAR) whose nucleus apparently fragmented late last month during its first trip through the inner solar system. This computer enhanced composite image shows faint stars as trails and the remnants of LINEAR's nucleus as a flock of "mini-comets" embedded in a cloud of gas and dust. It was recorded by astronomers using the European Southern Observatory's Antu telescope about a day after the Hubble Space Telescope (HST) was also able to image the covey of condensations. A comparison of the HST and the subsequent Antu images reveals that the swarm of cometary debris has changed markedly in 24 hours demonstrating the very dynamic behavior of comet LINEAR's remains. Astronomers intend to keep watching as comet LINEAR's fragments continue to lose dust and gas and fade from view. As a result, LINEAR's legacy may well be insight into the make-up of a primordial piece of our solar system. If pictures of comet LINEAR have piqued your curiosity about fragments of a comet, why not watch the Perseid meteor shower this weekend?
A Perseid Meteor (2000-08-12)

This weekend, the annual Perseid Meteor Shower reaches its maximum. Grains of cosmic sand and gravel shed from Comet Swift-Tuttle will streak across the sky as they vaporize during entry into Earth's atmosphere. The Perseids result from the yearly crossing of the Earth through Comet Swift-Tuttle's orbit. The Perseids are typically the most active meteor shower of the year.  In a clear dark sky, an observer might see a meteor a minute near peak times, but this year a bright moon will overwhelm the glow from many perseid meteors until moonset in the early morning hours.  Pictured above is a Perseid meteor from 1993. The colors are representative but digitally enhanced. As the meteor streaked across the night sky, different excited atoms emitted different colors of light.  The origin of the green tinge visible at the right is currently unknown, however, and might result from oxygen in Earth's atmosphere. Perseid meteors can best be seen from a relaxing position, away from lights, just before the dawn twilight.

Image credit: S. Kohle & B. Koch 
(Astron. I., U. Bonn)
Doomed Star Eta Carinae (2000-08-13)

Carinae may be about to explode.  But no one knows when - it may be next year, it may be one million years from now.  Eta Carinae's mass - about 100 times greater than our Sun - makes it an excellent candidate for a full blown supernova. Historical records do show that about 150 years ago Eta Carinae underwent an unusual outburst that made it one of the brightest stars in the southern sky.  Eta Carinae, in the Keyhole Nebula, is the only star currently thought to emit natural LASER light.  This image, taken in 1996, resulted from sophisticated image-processing procedures designed to bring out new details in the unusual nebula that surrounds this rogue star.  Now clearly visible are two distinct lobes, a hot central region, and strange radial streaks.  The lobes are filled with lanes of gas and dust which absorb the blue and ultraviolet light emitted near the center.  The streaks remain unexplained. Will these clues tell us how the nebula was formed?  Will they better indicate when Eta Carinae will explode?
Kemble's Cascade (2000-08-14)

A picturesque chain of unrelated stars is visible with strong binoculars towards the constellation of Camelopardalis.  Known as Kemble's Cascade, the asterism contains about 20 stars nearly in a row stretching over five times the width of a full moon.  Made popular by astronomy enthusiast Lucian Kemble (1922-1999), these stars appear as a string only from our direction in the Milky Way Galaxy.  The above photograph of Kemble's Cascade was made with a small telescope in New Mexico, USA.  The bright object near the bottom left is the relatively compact open cluster of stars known as NGC 1502.
The Solar Spectrum (2000-08-15)

It is still not known why the Sun's light is missing some colors.  Shown above are all the visible colors of the Sun, produced by passing the Sun's light through a prism-like device.  The above spectrum was created at the McMath-Pierce Solar Observatory and shows, first off, that although our yellow-appearing Sun emits light of nearly every color, it does indeed appear brightest in yellow-green light.  The dark patches in the above spectrum arise from gas at or above the Sun's surface absorbing sunlight emitted below.  Since different types of gas absorb different colors of light, it is possible to determine what gasses compose the Sun.  Helium, for example, was first discovered in 1870 on a solar spectrum and only later found here on Earth.  Today, the majority of spectral absorption lines have been identified - but not all.

Image credit: Nigel Sharp
Unusual Giant Galaxy NGC 1316 (2000-08-16)

Can unusual giant galaxy NGC 1316 help calibrate the universe?  Quite possibly -- if it turns out this atypical galaxy is composed of typical stars.  NGC 1316, pictured above, is most obviously strange because it has a size and shape common for an elliptical galaxy but dust lanes and a disk more commonly found in a spiral galaxy.  These attributes could be caused by interactions with another galaxy over the past billion years.  Most recently, NGC 1316 has been monitored to find novae, explosions emanating from white dwarf stars that should have a standard brightness.  Again, NGC 1316 was found atypical in that the nova rate was unexpectedly high.  If, however, the stars and white dwarfs that compose NGC 1316 are typical, then the novae observed should be just as bright as novae in other galaxies so that astronomers can use them to compute an accurate distance.  This distance can then be used to calibrate other distance indicators and result in a more accurate scale for distances throughout the universe.
Mount Megantic Magnetic Storm (2000-08-17)

Tomorrow's picture: X-ray Antennae  < | 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: Sebastien Gauthier

(Observatoire Astronomique du Mont-Megantic)
X-Rays From Antennae Galaxies (2000-08-18)

A bevy of black holes and neutron stars shine as bright, point-like sources against bubbles of million degree gas in this false-color x-ray image from the orbiting Chandra Observatory. The striking picture shows the central regions of two galaxies, NGC 4038 and NGC 4039, locked in a titanic collision some 60 million light-years distant in the constellation Corvus. In visible light images, long, luminous, tendril-like structures emanating from the wreckage lend the pair their popular moniker, the Antennae Galaxies. Galactic collisions are now thought to be fairly common, but when they happen individual stars rarely collide. Instead gas and dust clouds merge and compress, triggering furious bursts of massive star formation with thousands of resulting supernovae. The exploding stars litter the scene with bubbles of shocked hot gas and collapsed stellar cores. Transfixed by this cosmic accident astronomers watch and are beginning to appreciate the collision-driven evolution of galaxies, not unlike our own.
ROSAT Explores The X-Ray Sky (2000-08-19)

Launched in 1990, the orbiting ROSAT observatory explored the Universe by viewing the entire sky in x-rays -- photons with about 1,000 times more energy than visible light. This ROSAT survey produced the sharpest, most sensitive image of the x-ray sky to date. The all-sky image is shown with the plane of our Milky Way Galaxy running horizontally through the center. Both x-ray brightness and relative energy are represented with red, green, and blue colors indicating three x-ray energy ranges (from lowest to highest). Bright x-ray spots near the galactic plane are within our own Milky Way. The brightest region (right of center) is toward the Vela Pulsar and the Puppis supernova remnant. Bright sources beyond our Galaxy are also apparent, notably the Virgo cluster of galaxies (near top right) and the Large Magellanic Cloud (LMC). The LMC is easy to find here as several of the black stripes (blank areas caused by missing data) seem to converge on its position (lower right). Over large areas of the sky a general diffuse background of x-rays dominates.  Hot gas in our own Galaxy provides much of this background and gives rise to the grand looping structures visible in the direction of the galactic center (image center). Unresolved extragalactic sources also add to this background, particularly above and below the plane. Despite the x-ray sky's exotic appearance, a very familiar feature is visible - the gas and dust clouds which line the plane of our galaxy absorb x-rays as well as optical light and produce the dark bands running through the galactic center.
The Surface of Titan (2000-08-20)

If sailing the hydrocarbon seas of Titan, beware of gasoline rain.  Such might be a travel advisory issued one future day for adventurers visiting Titan, the largest moon of Saturn.   New images of Titan's surface were released last week from the Canada-France Hawaii Telescope featuring the finest details yet resolved.  Peering into Titan's thick smog atmosphere with infrared light, complex features interpreted as oceans, glaciers, and rock became visible.  The high-resolution infrared image pictured above was made possible using an unblurring technique called adaptive optics.  The interplanetary probe Cassini will reach Saturn and Titan in 2004 to better explore this unusual world.
A Perseid Aurora (2000-08-21)

Just after the Moon set but before the Sun rose in the early morning hours of August 12, meteors pelted the Earth from the direction of the constellation Perseus, while ions pelted the Earth from the Sun.  The meteors were expected as sub-sand grains long left behind by Comet Swift-Tuttle annually create the Perseids Meteor Shower.  The aurorae were unexpected, however, as electrons, protons, and heavier ions raced out from a large Coronal Mass Ejection that had occurred just days before on the Sun.  In the foreground is Hahn's Peak, an extinct volcano in Colorado, USA.

Image credit: Jimmy Westlake
(Colorado Mountain College)