Star Out of Round

The Very Large Telescope Interferometer (VLTI), an array of 2 telescopes which combine their light signals to achieve a higher angular resolution than is possible with any one scope, has determined that the star Achernar is the flattest star ever studied. The VLTI, which does not provide an actual image of the star but can provide an accurate estimate of the star's profile, has determined that Achernar's equatorial radius is 50% larger than its polar radius. This is quite oblate compared to most other celestial bodies, such as our Earth, whose equatorial radius is only 0.3% larger than its polar radius. Theorists do not yet know how to explain how a star like this could turn fast enough to adopt with such a shape without flying apart. Achernar is about 145 light years away from Earth in the southern sky and has a mass of about 6 solar masses. The telescopes used to make the interference map were not the giant 8.2-m VLT telescopes, but more modest 40-cm reflectors set at various configurations with separations as large as 140 m. (European Southern Observatory press release, 11 June)