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Last Updated: Tuesday, 13 February 2007, 16:52 GMT
Comet clash kicks up dusty haze
The picture has been likened to a red pupil at the centre of a green monster's eye

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

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.

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