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Wednesday, April 7, 1999 Published at 19:17 GMT 20:17 UK


Stars join nature's spirals

A spiral of dust emerges from two hot stars

By BBC News Online Science Editor Dr David Whitehouse

Many things in nature are spiral: sea shells, water as it goes down a plug-hole, cyclonic weather systems and distant galaxies. Now stars can be added to that list.

The newly-discovered spiral star is called WR 104. The spiral shape is a consequence of material being swept outward like a "lawnsprinkler" throws water on to a garden.

[ image: Spinning in space]
Spinning in space
WR104 is very distant and so appears very small, requiring the world's largest telescopes to see it in detail.

It is a so-called Wolf-Rayet star, named after the two astronomers who first discovered them. Only about 200 such stars are known in our galaxy. They are rare because they are a short-lived phase in the life of some stars.

They are hot, massive, luminous stars typically three times the size and 25 times heavier than our Sun.

They are incredibly bright, brighter than our sun by a factor of 100,000. This is the same ratio as the brightness of our Sun compared to the Moon.

The pressure of the light from the star's surface drives off the star's outer atmosphere: Wolf-Rayet stars are so bright they literally fly apart.

The image of WR 104 was taken in April 1998 at the Keck telescope in Hawaii. The diameter of the spiral of gas coming from the star is about twice the diameter of the orbit of Pluto around the Sun, about six billion miles.

[ image: Wolf-Rayet stars can eject clouds of hot gas]
Wolf-Rayet stars can eject clouds of hot gas
The image was produced using a technique called aperture masking interferometry. Most of the area of the Keck telescope was masked off, making it equivalent to 36 small circular regions or holes in which light could pass through and interfere.

The method allows astronomers to see much finer details on the sky than conventional methods. Interference patterns produced by the multiple light beams can be processed to produce a picture of the star.

Without this technique we would have seen only a blurry shape about 10 times larger.

Wolf-Rayet 104 spins around once every 220 days, the period being set by a close companion star orbiting WR104.

The work is published in Nature Magazine.

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