Squashed star. Very precise measurements show that the hot, bright star Achernar is the flattest known.

The Flattest Star

Stars are round, right? Not always. They can be flattened if they rotate fast. Astronomers have now measured what turns out to be the flattest star known: one that's hot, blue, and shaped like an M&M's chocolate candy. The measurements will help astronomers to better understand the insides of rapidly spinning stars, says team member Pierre Kervella of the European Southern Observatory (ESO).

The team, led by Armando Domiciano de Souza of the University Astrophysical Laboratory in Nice, France, used ESO's Very Large Telescope Interferometer in Chile to observe Achernar--a star 10 times larger and six times more massive than the sun--in the fall of 2002. The interferometer combines the light of two small telescopes to achieve the same image resolution as an imaginary 140-meter telescope. This enabled the astronomers to measure the shape of the star, which, at a distance of 145 light-years, looks as small as an M&M in Chicago would appear from New York.

Achernar turned out to be much flatter than expected, Kervella says. In fact, its polar diameter is a mere 64% of its equatorial diameter (the previous record was 85%). That was a surprise, given that the spin rate of 225 kilometers per second suggested a flattening of only 80%. One possible reason is that gas inside a star may spin even faster than the gas on the surface, which would bloat its midriff, says Grant Bazán of Lawrence Livermore National Laboratory (LLNL) in California. Studying that effect would require a three-dimensional model of the star, says Kervella.

Peter Eggleton, also of LLNL, says that shape measurements like the ones of Achernar are "very important" to improve the models and thus our understanding of the interior structure of stars. But Eggleton warns that the team may actually have measured the shape of an equatorial disk of hot gas, ejected by the rapidly spinning star. However, Kervella says simultaneous measurements of the spectrum of light emitted by Achernar show that this is unlikely.

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