juliensimon/nasa-apod huggingface dataset:
Aurora Iowa (2005-05-20)

rly last Sunday morning stars were not the only lights in Iowa skies. The northern lights also shone from the heavens, extending across the midwestern USA and other locations not often graced with auroral displays. The wide-ranging auroral activity was triggered as a coronal mass ejection - an energetic cloud of particles blasted outward from the Sun a few days earlier - collided with planet Earth's magnetosphere. Alerted to conditions ripe for aurora, photographer Stan Richard recorded this aparition over Saylorville Lake, near Des Moines. Bright planet Mars in the constellation Aquarius is above the horizon near the center of the eastward-looking view. While the colorful rays seem to end just above the water, they are actually at altitudes of 100 kilometers or more.

Image credit: Stan Richard
Snake in the Dark (2005-05-21)

Dark nebulae snake across a gorgeous expanse of stars in this wide-field view toward the pronounceable constellation Ophiuchus and the center of our Milky Way Galaxy. In fact, the central S-shape seen here is well known as the Snake Nebula. It is also listed as Barnard 72 (B72), one of 182 dark markings of the sky cataloged in the early 20th century by astronomer E. E. Barnard. Unlike bright emission nebulae and star clusters, Barnard's nebulae are interstellar dark clouds of obscuring gas and dust. Their shapes are visible in cosmic silhouette only because they lie in the foreground along the line of sight to rich star fields and glowing stellar nurseries near the plane of our Galaxy. Many of Barnard's dark nebulae are themselves likely sites of future star formation. Barnard 72 is a few light years across and about 650 light years away.

Image credit: Gary Stevens
The Dust and Ion Tails of Comet Hale-Bopp (2005-05-22)

Tomorrow's picture: wavemaker around saturn  < | Archive | Index | Search | Calendar | Glossary | Education | About APOD | >  Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA) NASA Web Site Statements, Warnings, and Disclaimers NASA Official: Jay Norris. Specific rights apply. A service of: EUD at NASA / GSFC & Michigan Tech. U.

Image credit: John Gleason 
(Celestial Images)
A Wave Maker Moon in Saturn's Rings (2005-05-23)

What causes small waves in Saturn's rings?  Observations of rings bordering the Keeler gap in Saturn's rings showed unusual waves.  Such waves were first noticed last July and are shown above in clear detail. The picture is a digitally foreshortened image mosaic taken earlier this month by the robot Cassini spacecraft now orbiting Saturn.  The rings, made of many small particles, were somehow not orbiting Saturn in their usual manner. Close inspection of the image shows the reason - a small moon is orbiting in the Keeler gap.  The previously unknown moon is estimated to span about seven kilometers and appears to have the same brightness as nearby ring particles.   The gravity of the small moon likely perturbs the orbits of ring particles that come near it, causing them to shimmy back and forth after the moon passes.  Since inner particles orbit more quickly than outer particles, only the leading particles of the inner rings and the trailing particles of the outer rings show the wave effect.
Swirls and Stars in IC 4678 (2005-05-24)

Swirls of gas and dust enrich this little observed starfield toward the constellation of Sagittarius.  Just to the side of the more often photographed Lagoon Nebula (M8) and the Trifid Nebula (M20) lies this busy patch of sky dubbed IC 4678.  Prominent in the above image are large emission nebulas of red glowing gas highlighted by unusually bright red filaments.  On the left, a band of thin dust preferentially reflects the blue light of a bright star creating a small reflection nebula.  On the right and across the bottom, swaths of thicker dust appear as dark absorption nebulas, blocking the light from stars farther in the distance.  IC 4678 spans about 25 light years and lies about 5,000 light years distant.

Image credit: Adam Block
Particle Sizes in Saturn's Rings (2005-05-25)

What size particles compose Saturn's rings?  To help find out, the robot Cassini spacecraft now orbiting Saturn broadcast radio waves of three different wavelengths right through the rings to Earth earlier this month.  The experiment was sensitive to ring particle sizes because ring particles much larger than a broadcast radio wavelength will reflect those radio waves away.  Three different wavelengths were used: approximately 1 centimeter, 3.5 centimeters, and 13 centimeters.  The results are coded into the above false-color digitally reconstructed image. In the above image, the color purple indicates regions populated predominantly by ring particles larger than 5 centimeters, while the color green indicates regions with a significant population of small ring particles less than even 1 centimeter.  The white center of Saturn's B-ring indicates that the density of ring particles was too high to make a good determination. Other radio observations indicate that some ring particles can be as large as several meters across. The impressive nature and clarity of the above sharp image may help determine clues about the origin of Saturn's beautiful but enigmatic ring system.
A Beautiful Trifid (2005-05-26)

The beautiful Trifid Nebula (aka M20), a photogenic study in cosmic contrasts, lies about 5,000 light-years away toward the nebula rich constellation Sagittarius. A star forming region in the plane of our galaxy, the Trifid fittingly illustrates three basic types of astronomical nebulae; red emission nebulae dominated by light from hydrogen atoms, blue reflection nebulae produced by dust reflecting starlight, and dark absorption nebulae where dense dust clouds appear in silhouette. The bright red emission nebula, roughly separated into three parts by obscuring dust lanes, lends the Trifid its popular name. In this gorgeous wide view, the red emission region is also surrounded by the telltale blue haze of reflection nebulae. Light-year long pillars and jets sculpted by newborn stars - visible here below the center of the emission nebula - appear in Hubble Space Telescope close-up images of the region.

Image credit: Dean Jacobsen
Titan's Odd Spot (2005-05-27)

Titan's odd spot could be a cloud, but if so, it's a persistent one. Peering into the thick, hazy atmosphere of Saturn's largest moon, cameras on board the Cassini spacecraft found a bright spot at the same location during Titan encounters in 2005 and 2004. Seen near Titan's upper edge in this false-color image from the VIMS instrument, the spot is almost 500 kilometers wide, and is brightest at infrared wavelengths. In addition to suggesting the uniquely colored spot is a persistent cloud possibly controlled by surface features, researchers also entertain the idea that the spot is caused by unusual surface material or extremely tall mountains. They also note the bright infrared spot could be hot. Further clues to the odd spot's nature will come during a planned encounter in July 2006 when Cassini's cameras will look at the spot during Titan's night. If it glows at night, it's hot.
Himalayan Horizon From Space (2005-05-28)

This stunning aerial view shows the rugged snow covered peaks of a Himalayan mountain range in Nepal. The seventh-highest peak on the planet, Dhaulagiri, is the high point on the horizon at the left while in the foreground lies the southern Tibetan Plateau of China. But, contrary to appearances, this picture wasn't taken from an airliner cruising at 30,000 feet. Instead it was taken with a 35mm camera and telephoto lens by the Expedition 1 crew aboard the International Space Station -- orbiting 200 nautical miles above the Earth. The Himalayan mountains were created by crustal plate tectonics on planet Earth some 70 million years ago, as the Indian plate began a collision with the Eurasian plate. Himalayan uplift still continues today at a rate of a few millimeters per year.
The Sagittarius Dwarf Tidal Stream (2005-05-29)

Is our Milky Way Galaxy out to lunch?  Recent wide field images and analyses now indicate that our home galaxy is actually still in the process of devouring one of its closer satellite neighbors.  This unfortunate neighbor, the Sagittarius Dwarf galaxy, is now seen to be part of a larger Sagittarius Tidal Stream, a loose filament of stars, gas, and possibly dark matter that entangles the Milky Way.  An artist's depiction of the stream is shown above.  Speculation also holds that the Sagittarius Dwarf was once pulled through the Milky Way disk very close to our Sun's current location.  An important resulting realization is that galaxies contain a jumble of clumps and filaments of both dim and dark matter.

Image credit: David Martinez-Delgado
(MPIA) & 
Gabriel Perez (IAC)
A Great White Spot on Rhea (2005-05-30)

What caused this great white spot on the surface of Saturn's moon Rhea?  The spot was first noticed last year by the robot Cassini spacecraft now orbiting Saturn.  Cassini's flyby of Rhea in April imaged in the spot in great detail.  Astronomers hypothesize that the light-colored spot is the result of a relatively recent impact on the surface of the icy moon.  The impact that likely created the crater also splashed light-colored material from the interior onto the darker surface.  Rhea spans 1,500 kilometers across and is the second largest moon of Saturn after Titan.  Rhea sports several other light colored surface features that are, as yet, not well understood.
The Trifid Nebula from CFHT (2005-05-31)

Unspeakable beauty and unimaginable bedlam can be found together in the Trifid Nebula. Also known as M20, this photogenic nebula is visible  with good binoculars towards the constellation of Sagittarius. The energetic processes of star formation create not only the colors but the chaos.  The red-glowing gas results from high-energy starlight striking interstellar hydrogen gas.  The dark dust filaments that lace M20 were created in the atmospheres of cool giant stars and in the debris from supernovae explosions.  Which bright young stars light up the blue reflection nebula is still being investigated.  The light from M20 we see today left perhaps 3000 years ago, although the exact distance remains unknown.  Light takes about 50 years to cross M20.

Image credit: Jean-Charles Cuillandre
White Dwarf Star Spiral (2005-06-01)

About 1,600 light-years away, in a binary star system fondly known as J0806, two dense white dwarf stars orbit each other once every 321 seconds. Interpreting x-ray data from the Chandra Observatory astronomers argue that the stars' already impressively short orbital period is steadily getting shorter as the stars spiral closer together. Even though they are separated by about 80,000 kilometers (the Earth-Moon distance is 400,000 kilometers) the two stars are therefore destined to merge. Depicted in this artist's vision, the death spiral of the remarkable J0806 system is a consequence of Einstein's theory of General Relativity that predicts the white dwarf stars will lose their orbital energy by generating gravity waves. In fact, J0806 could be one of the brightest sources of gravitational waves in our galaxy, directly detectable by future space-based gravity wave instruments.
Sculpting the South Pillar (2005-06-02)

Carinae, one of the most massive and unstable stars in the Milky Way Galaxy, has a profound effect on its environment. Found in the the South Pillar region of the Carina Nebula, these fantastic pillars of glowing dust and gas with embedded newborn stars were sculpted by the intense wind and radiation from Eta Carinae and other massive stars. Glowing brightly in planet Earth's southern sky, the expansive Eta Carinae Nebula is a mere 10,000 light-years distant. Still, this remarkable cosmic vista is largely obscured by nebular dust and only revealed here in penetrating infrared light by the Spitzer Space Telescope. Eta Carinae itself is off the top left of the false-color image, with the bright-tipped dust pillars pointing suggestively toward the massive star's position. The Spitzer image spans almost 200 light-years at the distance of Eta Carinae.
M27: The Dumbbell Nebula (2005-06-03)

The first hint of what will become of our Sun was discovered inadvertently in 1764.  At that time, Charles Messier was compiling a list of diffuse objects not to be confused with comets.  The 27th object on Messier's list, now known as M27 or the Dumbbell Nebula, is a planetary nebula, the type of nebula our Sun will produce when nuclear fusion stops in its core.  M27 is one of the brightest planetary nebulae on the sky, and can be seen in the constellation Vulpecula with binoculars.  It takes light about 1000 years to reach us from M27,  shown above, digitally sharpened, in three standard colors. Understanding the physics and significance of M27 was well beyond 18th century science.  Even today, many things remain mysterious about bipolar planetary nebula like M27, including the physical mechanism that expels a low-mass star's gaseous outer-envelope, leaving an X-ray hot white dwarf.

Image credit: Adam Block
First U.S. Spacewalk (2005-06-04)

In 1965, forty years ago on June 3rd, astronaut Edward White made the first U.S. spacewalk. Tethered to his Gemini IV capsule, White is pictured above holding a compressed gas "zip gun" for maneuvers in his right hand. His spacewalk began over the Pacific Ocean near Hawaii and ended 23 minutes later above the Gulf of Mexico. Of course, the term spacewalk is a bit deceiving as White was falling freely in low earth orbit alongside his capsule manned by fellow astronaut James McDivitt. In free-fall, White was able to control his motions by firing bursts from his gun until its supply of compressed gas ran out. He ultimately returned, exhausted, to the two-man Gemini capsule.
A Milky Way Band (2005-06-05)

Most bright stars in our Milky Way Galaxy reside in a disk.  Since our Sun also resides in this disk, these stars appear to us as a diffuse band that circles the sky.  The above panorama of a northern band of the Milky Way's disk covers 90 degrees and is a digitally created mosaic of several independent exposures.   Scrolling right will display the rest of this spectacular picture. Visible are many bright stars, dark dust lanes, red emission nebulae, blue reflection nebulae, and clusters of stars.  In addition to all this matter that we can see, astronomers suspect there exists even more dark matter that we cannot see.

Image credit: John P. Gleason
Saturn: Dirty Rings and a Clean Moon (2005-06-06)

g surface ice from Enceladus might be healthier than eating ice from Saturn's rings -- it certainly appears cleaner.  From their apparent densities and reflectance properties, both the rings of Saturn and its shiniest moon, Enceladus, are thought to be composed predominantly of water ice.  For reasons that are not yet understood, however, many of Saturn's ring particles have become partly coated with some sort of relatively dark dust, while the surface of Enceladus appears comparatively bright and clean.  The contrast between the two can be seen in the above image taken last month by the robot Cassini spacecraft now in orbit around Saturn.  Bright Enceladus shines in the background in contrast to the darker foreground rings.  The reason why Enceladus is so bright is currently unknown but might involve bringing fresh water to its surface with water volcanoes.
Galaxies in View (2005-06-07)

Galaxies abound in this cosmic scene, a well chosen telescopic view toward the northern constellation of Ursa Major. Most noticeable are the striking pair of spiral galaxies - NGC 3718 (above, right) and NGC 3729 (below center) - a mere 52 million light-years distant. In particular, NGC 3718 has dramatic dust lanes sweeping through its bright central region and extensive but faint spiral arms. Seen about 150 thousand light-years apart, these two galaxies are likely interacting gravitationally, accounting for the warped and peculiar appearance of NGC 3718. While a careful study of the deep image reveals a number of fainter and more distant background galaxies, another remarkable galaxy grouping known as Hickson Group 56 can be found just to the right of NGC 3718. Hickson Group 56 contains five interacting galaxies and lies over 400 million light-years away.

Image credit: Johannes Schedler
(Panther Observatory)
Rampaging Supernova Remnant N63A (2005-06-08)

What has this supernova left behind?  As little as 2,000 years ago, light from a massive stellar explosion in the Large Magellanic Cloud (LMC) first reached planet Earth.  The LMC is a close galactic neighbor of our Milky Way Galaxy and the rampaging explosion front is now seen moving out - destroying or displacing ambient gas clouds while leaving behind relatively dense knots of gas and dust.  What remains is one of the largest supernova remnants in the LMC: N63A.  Many of the surviving dense knots have been themselves compressed and may further contract to form new stars.  Some of the resulting stars may then explode in a supernova, continuing the cycle.  Pictured above is a close-up of one of the largest remaining knots of dust and gas in N63A taken by the orbiting Hubble Space Telescope.  N63A spans over 25 light years and lies about 150,000 light years away toward the southern constellation of Dorado.
Venus Returns to the Evening Sky (2005-06-09)

This serene image of boats moored in the harbor of l'�le-Tudy, Bretagne, France was taken on June 1st, about an hour after sunset. It also features Venus, third brightest celestial object after the Sun and Moon. For casual skygazers, this month marks Venus' return to the evening sky as the brilliant 'star', shining low in the west-northwest shortly after sunset. In the picture, astrophotographer and APOD translator Laurent Laveder notes that Venus is easily mistaken for a light atop a sailboat's tall mast, giving the otherwise stunning celestial beacon an unremarkable appearance. Of course, a year ago Venus' appearance was quite remarkable. On June 8, 2004, Venus crossed the Sun's disk, the first transit of Venus since 1882. Late this week Venus shares the evening sky with the young crescent Moon, and will next transit the Sun on June 6, 2012.

Image credit: Laurent Laveder
(Version
fran�aise officielle de l'APOD)
Titan's Cryovolcano (2005-06-10)

Investigators suspect the domed feature detailed above is an ice volcano, or cryovolcano, seen in infrared light through the hazy atmosphere on Saturn's moon Titan. Since Titan's surface temperature is around minus 180 degrees Celsius, lava welling up to form the volcanic mound would be icy indeed - possibly a slurry of methane, ammonia, and water ice combined with other ices and hydrocarbons. The circular feature is roughly thirty kilometers in diameter. If its volcanic nature is confirmed, the discovery of cryovolcanism on Titan could explain the origin of methane in Titan's atmosphere. Before the Cassini-Huygens mission to Saturn, a popular explanation for replenishing Titan's concentration of atmospheric methane was the presence of an extensive, methane-rich, hydrocarbon sea. But Cassini's instruments and the Huygens surface probe have failed to find such a global ocean.
Earth at Twilight (2005-06-11)

No sudden, sharp boundary marks the passage of day into night in this gorgeous view of ocean and clouds over our fair planet Earth. Instead, the shadow line or terminator is diffuse and shows the gradual transition to darkness we experience as twilight. With the Sun illuminating the scene from the right, the cloud tops reflect gently reddened sunlight filtered through the dusty troposphere, the lowest layer of the planet's nurturing atmosphere. A clear high altitude layer, visible along the dayside's upper edge, scatters blue sunlight and fades into the blackness of space. This picture actually is a single digital photograph taken in June of 2001 from the International Space Station orbiting at an altitude of 211 nautical miles.
M2-9: Wings of a Butterfly Nebula (2005-06-12)

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.
Tornado and Rainbow Over Kansas (2005-06-13)

The scene might have been considered serene if it weren't for the tornado. Last June in Kansas, storm chaser Eric Nguyen photographed this budding twister in a different light -- the light of a rainbow.  Pictured above, a white tornado cloud descends from a dark storm cloud.  The Sun, peeking through a clear patch of sky to the left, illuminates some buildings in the foreground. Sunlight reflects off raindrops to form a rainbow.  By coincidence, the tornado appears to end right over the rainbow.  Streaks in the image are hail being swept about by the high swirling winds. Over 1,000 tornadoes, the most violent type of storm known, occur on Earth every year, many in tornado alley.  If you see a tornado while driving, do not try to outrun it -- park your car safely, go to a storm cellar, or crouch under steps in a basement.

Image credit: Eric Nguyen
(Oklahoma U.), 
www.mesoscale.ws
Gliese 876 System Includes Large Terrestrial Planet (2005-06-14)

Is our Earth unique?  In continuing efforts to answer this question, astronomers have now discovered an Earth-like planet orbiting a distant normal star.  Previously over 150 gas-giant planets like Jupiter had been so discovered. Slight, fast, but regular wobbles of nearby small M-dwarf star Gliese 876 showed evidence for a planet with a likely mass slightly higher than a minimum six times the mass of Earth.  The planet's small mass indicates that it is likely terrestrial in nature, similar in composition to the inner planets of our Solar System.   If indeed made predominantly of rock, the planet's surface gravity would not even be able to contain the gasses of a Jupiter-like planet. The newly discovered planet would not make a good vacation spot for humans, however, as it orbits so close that the surface temperature probably tops a searing 200 degrees Celsius.  The system is illustrated in the above drawing as seen from a hypothetical moon orbiting one of the two Jupiter-like planets already known.  The newly discovered terrestrial-like planet is depicted in the insert. Gliese 876 lies only 15 light-years away and is visible with binoculars toward the constellation of Aquarius.

Image credit: Lynette Cook (spaceart.org); 

 Discovery Credit:  
E. Rivera 
(NASA's ARC) 
et al., 
HIRES, 
Keck Obs., 
NASA, 
NSF
Cassiopeia A Light Echoes in Infrared (2005-06-15)

Why is the image of Cassiopeia A changing?  Two images of the nearby supernova remnant taken a year apart in infrared light appear to show outward motions at tremendous speeds.  This was unexpected since the supernova that created the picturesque nebula was seen 325 years ago.  The reason is likely light echoes.  Light from the supernova heated up distant ambient dust that is just beginning to show its glow.  As time goes by, more distant dust lights up, giving the appearance of outward motion.  The above image is a composite of X-ray, optical, and infrared light exposures that have been digitally combined.  The infrared light image was taken by the orbiting Spitzer Space Telescope and was used in the discovery of the light echo.  The portion of Cassiopeia A shown spans about 15 light years and lies 10,000 light years away toward the constellation of Cassiopeia.