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Old 15-02-2007, 01:37 PM
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Comet clash kicks up dusty haze

I saw this interesting story on the bbc web page
http://news.bbc.co.uk/1/hi/sci/tech/6357765.stm
Comet clash kicks up dusty haze


Collisions between comets may be kicking up copious amounts of dust observed around a dead star.
This has surprised astronomers, because when the star died and expelled its outer layers, the dust in this system should have been blown away.

A favoured explanation is that the dust is being freshly churned up by comets smashing into each other in the outer fringes of the white dwarf's system.

The dead white dwarf star lies at the centre of the Helix nebula.

Eventually, our own Sun will turn into a white dwarf.

Stars of medium or low mass become white dwarfs after they have exhausted the hydrogen which powers their thermonuclear reactions.

Near the end of the nuclear burning stage, such a star expels most of its outer material, creating a planetary nebula. Only the hot core of the star remains.

Star in the eye

The Helix nebula is a shimmering cloud of gas with an eerie resemblance to a giant eye.

The dusty dead star appears as a dot in the middle of the nebula, like a red pupil in a green monster's eye.

It is located about 700 light-years away in the constellation Aquarius.


Nasa's Spitzer infrared telescope


More details

Radiation from the dead star's hot core heats the expelled material, causing it to fluoresce with vivid colours.

The Spitzer Space Telescope, an infrared space-based observatory, was able to pick up the glow of a dusty disk circling around the stellar corpse at a distance of about 35 to 150 astronomical units.

An astronomical unit is the distance between our sun and Earth, equivalent to 150 million km or 93 million miles.

"We were surprised to see so much dust around this star," said Dr Kate Su of the University of Arizona, Tucson, the lead author of a paper on the results which is due to appear in Astrophysical Journal Letters.

"The dust must be coming from comets that survived the death of their sun."

Cosmic smash-up

A few million years ago, when the star was still lively like our sun, its comets - and possibly planets - would have been in stable orbits, travelling harmoniously around the star.

But when the star died, any inner planets would have been burned up or engulfed as the star expanded.

Outer planets, asteroids and comets would have been thrown into each other's paths.

Our own Solar System is expected to undergo a similar transformation in about five billion years.

Like the Helix nebula, it will sparkle with colours. Our sun, which will have become a white dwarf, will be circled by a band of surviving outer planets and colliding comets.

The Spitzer data might also help explain a mystery surrounding the Helix nebula's white dwarf.

Previous observations indicated that the white dwarf was throwing out highly energetic X-rays. While the white dwarf is hot - about 110,000 Kelvin (109,727C; 200,000F), it is not hot enough to explain the energetic X-rays.

Astronomers thought that the white dwarf was accreting matter onto itself from a hidden companion star.

But the Spitzer observations point to a different answer. According to team member Dr You-Hua Chu of the University of Illinois at Urbana-Champaign, material in the dusty disk surrounding the white dwarf might be falling on to the star and triggering the X-ray outbursts.


Gazz
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Old 15-02-2007, 01:41 PM
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Another interesting story form the same source
http://news.bbc.co.uk/1/hi/sci/tech/6246333.stm

The three iconic columns of gas and dust pictured in space by the Hubble Telescope in 1995 may have met their end, the US space agency says.

Hubble's image, dubbed the "pillars of creation", has featured in countless papers, magazines and posters.

Now, new data shows the pillars being scorched by an exploding star - and a shockwave has probably torn them apart.

But because of the time taken for light to reach Earth, we will not see their final destruction for 1,000 years.

"In the Hubble pictures, the pillars seem to be compact and solid; but in fact they are not," said Nicolas Flagey, from France's Institut d'Astrophysique Spatiale.

"Only the top of the pillars and some parts within are dense enough to resist the shock. But the rest is going to be blown away by the shockwave," he told BBC News.

Hubble inspiration

The pillars are actually dark columns of cool interstellar hydrogen gas and dust that serve as incubators for new stars.

They are part of the Eagle Nebula (also called M16), a star-forming region 6,000 light-years from Earth in the constellation Serpens.


One of the most famous pictures taken by Hubble
Mr Flagey, a PhD student who helped make the latest discovery, presented his findings here at a meeting of the American Astronomical Society.

He says he was inspired to study astrophysics after seeing Hubble's image of the pillars on the cover of a French magazine more than a decade ago.

So he was surprised to be offered the chance to work on the Eagle Nebula during a six-month attachment to the Spitzer Science Center in California.

The centre operates Nasa's Spitzer Space Telescope, which took new infrared pictures of a shell-shaped cloud of hot dust close to the pillars. The shell is being heated by an exploding star.

Using the telescope data, Dr Flagey and colleagues were able to measure the temperature of the dust and match it to a supernova (exploding star) event.

Back in time

"Something else besides starlight is heating this dust," said Dr Alberto Noriega-Crespo, Mr Flagey's advisor at the Spitzer Science Center.

Astronomers had long predicted that a supernova blast would spell the end for the popular pillars.


Nasa's Spitzer infrared telescope


More details

"We know that there are some massive stars inside the Eagle Nebula. The fate of these stars is to explode as supernovas," Mr Flagey told BBC News.

"So it was not completely unexpected that one of these has already exploded and has produced the shockwave that is heating the gas and dust."

The star is thought to have exploded around 8,000-9,000 years ago.

Since light from the Eagle Nebula takes 7,000 years to reach us, the stellar explosion would have appeared as an oddly bright star in our skies about 1,000 to 2,000 years ago.

The astronomers estimate that the blast would have spread outward and toppled the three pillars around 6,000 years ago. This means that we will not be able to see the destruction for another 1,000 years.

When the mighty pillars are seen to crumble, gas and dust will be blown away, exposing newborn stars forming inside. A new generation of stars might spring up from the dusty wreckage.

Despite being inspired to pursue his current career by Hubble's iconic picture of the structures, Mr Flagey said he was not disappointed to discover that they are - in all likelihood - already gone.

"I will not be here to see it destroyed," he said, "there are plenty of other regions like this. So I'm not sad."
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