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Last Updated: Friday, 13 June, 2003, 10:02 GMT 11:02 UK
Strange star puzzles astronomers
By Dr David Whitehouse
BBC News Online science editor

Achernar is one of the most distorted stars ever observed by astronomers. It is shaped like a child's spinning top.

Achernar model, Eso
Scientists have been able to show the star's equatorial radius is 50% larger than its polar radius.

Distorted stars are known in astronomy. They are generally pulled out of shape by the gravity of a companion.

Achernar, on the other hand, is distorted because it is spinning rapidly - but this does not fully explain its ovoid shape, say researchers.

Achernar, otherwise known as Alpha Eridani, is the 9th brightest star in the sky. It is a hot star, several times more massive than our Sun, and is fairly close to us, being about 145 light-years distant.

Fast spin

It was observed by the European Southern Observatory's interferometer at the Paranal Observatory in Chile.

The interferometer consists of a combination of telescopes that combine their signals to provide more detailed observations than any of individual telescopes can achieve.

When Achernar was observed, it was found to be much flatter than expected.

Analysis of the interferometer data suggests the distinctly ovoid shape of the star is the result of its fast rotation - 225 kilometres per second; a centrifugal effect has caused it to flatten.

However, this high degree of flattening poses a puzzle for theoretical astrophysicists. Their models cannot currently explain the behaviour.

They wonder if the star is spinning faster than their data indicates. Effects such as a "north-south" circulation in the star's atmosphere are also under investigation.




SEE ALSO:
Six telescopes act as one
19 Mar 02  |  Science/Nature
Two become one: Giant telescopes linked
15 Mar 01  |  Science/Nature
Paranal power
25 Mar 99  |  Science/Nature


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