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Cassini Flyby Shows Enceladus Venting

APOD — Tue, 24 Nov 2009 09:07:09 GMT

What's happening on the surface of Saturn's moon Enceladus? Enormous ice jets are erupting. Giant plumes of ice have been photographed in dramatic fashion by the robotic Cassini spacecraft during this past weekend's flyby of Saturn's moon Enceladus. Pictured above , numerous plumes are seen rising from long tiger-stripe canyons across Enceladus' craggy surface . Several ice jets are even visible in the shadowed region of crescent Enceladus as they reach high enough to scatter sunlight. Other plumes, near the top of the above image , appear visible just over the moon's sunlit edge. That Enceladus vents fountains of ice was first discovered on Cassini images in 2005, and has been under close study ever since. Continued study of the ice plumes may yield further clues as to whether underground oceans, candidates for containing life , exist on this distant ice world.

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Crescent Earth from the Departing Rosetta Spacecraft

APOD — Tue, 24 Nov 2009 09:07:09 GMT

Goodbye Earth. Earlier this month, ESA 's interplanetary Rosetta spacecraft zoomed past the Earth on its way back across the Solar System. Pictured above , Earth showed a bright crescent phase featuring the South Pole to the passing rocket ship. Launched from Earth in 2004, Rosetta used the gravity of the Earth to help propel it out past Mars and toward a 2014 rendezvous with Comet Churyumov-Gerasimenko . Last year, the robot spacecraft passed asteroid 2867 Steins , and next year it is scheduled to pass enigmatic asteroid 21 Lutetia . If all goes well, Rosetta will release a probe that will land on the 15-km diameter comet in 2014.

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Light Echoes from V838 Mon

APOD — Tue, 24 Nov 2009 09:07:09 GMT

What caused this outburst of V838 Mon? For reasons unknown , star V838 Mon 's outer surface suddenly greatly expanded with the result that it became the brightest star in the entire Milky Way Galaxy in January 2002. Then, just as suddenly, it faded. A stellar flash like this has never been seen before -- supernovas and novas expel matter out into space. Although the V838 Mon flash appears to expel material into space, what is seen in the above image from the Hubble Space Telescope is actually an outwardly moving light echo of the bright flash. In a light echo , light from the flash is reflected by successively more distant rings in the complex array of ambient interstellar dust that already surrounded the star. V838 Mon lies about 20,000 light years away toward the constellation of the unicorn ( Monoceros ), while the light echo above spans about six light years in diameter.

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NGC 253: Dusty Island Universe

APOD — Tue, 24 Nov 2009 09:07:09 GMT

Shiny NGC 253 Galaxy, is one of the brightest spiral galaxies visible, and also one of the dustiest. Some call it the Silver Dollar Galaxy for its appearance in small telescopes, or just the Sculptor Galaxy for its location within the boundaries of the southern constellation Sculptor . First swept up in 1783 by mathematician and astronomer Caroline Herschel , the dusty island universe lies a mere 10 million light-years away. About 70 thousand light-years across, NGC 253 is the largest member of the Sculptor Group of Galaxies , the nearest to our own Local Group of Galaxies . In addition to its spiral dust lanes, striking tendrils of dust seem to be rising from a galactic disk laced with young star clusters and star forming regions in this processed color image . The high dust content accompanies frantic star formation, giving NGC 253 the designation of a starburst galaxy. NGC 253 is also known to be a strong source of high-energy x-rays and gamma rays, likely due to massive black holes near the galaxy's center.

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Meteor between the Clouds

APOD — Tue, 24 Nov 2009 09:07:09 GMT

This bright meteor streaked through dark night skies over Sutherland , South Africa on November 15. Potentially part of the annual Leonid meteor shower, its sudden, brilliant appearance, likened to a camera's flash, was captured by chance as it passed between two clouds. Of course, the two clouds are also visible to the eye in dark southern skies - the Large and Small Magellanic Clouds - satellite galaxies of our own Milky Way. This year's Leonid meteor shower peaked on November 17 as the Earth passed through the stream of dust from periodic comet Tempel-Tuttle .

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Leonid over Mono Lake

APOD — Tue, 24 Nov 2009 09:07:09 GMT

Eerie spires of rock rise from shore of Mono Lake in the foreground of this early morning skyscape. The salty, mineral-laden lake is located in California's eastern Sierra Nevada mountain range and the spindly rock formations are naturally formed limestone towers called tufa . In the scene, recorded near the peak of the annual Leonid meteor shower (now subsiding) on November 17th, a meteor trails through the frigid predawn sky. Arcturus is the brightest star to the right of the meteor streak, while the constellation Leo and the shower's radiant point lie well above the field of view. Reports for this year's Leonids suggest the peak activity briefly exceeded 120 meteors per hour, but rates were typically much lower for many locations .

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Water Discovered in Moon Shadow

APOD — Tue, 24 Nov 2009 09:07:09 GMT

Why is there water on the Moon? Last month, the LCROSS mission crashed a large impactor into a permanently shadowed crater near the Moon's South Pole. A plume of dust rose that was visible to the satellite , although hard to discern from Earth. The plume is shown above in visible light. Last week, the results of a preliminary chemical analysis gave a clear indication that the dust plume contained water . Such water is of importance not only for understanding the history of the Moon , but as a possible reservoir for future astronauts trying to live on the Moon for long periods. The source of the lunar water is now a topic of debate. Possible origins include many small meteorites, a comet, or primordial moon soil .

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Dawn Before Nova

APOD — Tue, 24 Nov 2009 09:07:09 GMT

Will this dawn bring another nova? Such dilemmas might be pondered one day by future humans living on a planet orbiting a cataclysmic variable binary star system. Cataclysmic variables involve gas falling from a large star onto an accretion disk surrounding a massive but compact white dwarf star. Explosive cataclysmic events such as a dwarf nova can occur when a clump of gas in the interior of the accretion disk heats up past a certain temperature. At that point, the clump will fall more quickly onto the white dwarf and land with a bright flash. Such dwarf novas will not destroy either star, and may occur irregularly on time scales from a few days to tens of years. Although a nova is much less energetic than a supernova, if recurrent novas are not violent enough to expel more gas than is falling in, mass will accumulate onto the white dwarf star until it passes its Chandrasekhar limit . At that point, a foreground cave may provide little protection, as the entire white dwarf star will explode in a tremendous supernova .

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M83's Center from Refurbished Hubble

APOD — Tue, 24 Nov 2009 09:07:09 GMT

What's happening at the center of spiral galaxy M83? Just about everything, from the looks of it. M83 is one of the closest spiral galaxies to our own Milky Way Galaxy and from a distance of 15 million light-years , appears to be relatively normal. Zooming in on M83 's nucleus with the latest telescopes, however, shows the center to be an energetic and busy place. Visible in the above image -- from the newly installed Wide Field Camera 3 pointing through the recently refurbished Hubble Space Telescope -- are bright newly formed stars and giant lanes of dark dust . An image with similar perspective from the Chandra X-ray Observatory shows the region is also rich in very hot gas and small bright sources. The remnants of about 60 supernova blast s can be found in the above image .

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M57: The Ring Nebula

APOD — Tue, 24 Nov 2009 09:07:09 GMT

It looks like a ring on the sky. Hundreds of years ago astronomers noticed a nebula with a most unusual shape. Now known as M57 or NGC 6720, the gas cloud became popularly known as the Ring Nebula . It is now known to be a planetary nebula , a gas cloud emitted at the end of a Sun-like star's existence. As one of the brightest planetary nebula on the sky, the Ring Nebula can be seen with a small telescope in the constellation of Lyra . The Ring Nebula lies about 4,000 light years away, and is roughly 500 times the diameter of our Solar System . In this picture by the Hubble Space Telescope in 1998, dust filaments and globules are visible far from the central star. This helps indicate that the Ring Nebula is not spherical, but cylindrical .

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