ESO spots 'flattest' star ever seen

The configuration of the telescopes on the VLT site, in this case set up to to simulate the resolution of a single instrument 140 m across. Copyright: European Southern Observatory Astronomers have found the flattest star ever seen. Measurements made using the Very Large Telescope Interferometer (VLTI) at the European Southern Observatory (ESO)’s Paranal Observatory in Chile show that Achernar has an equatorial radius some 50% larger than its polar one.

Achernar or Alpha Eridani is the bright star at the southernmost end of the constellation Eridanus (the river) and is always below the horizon from UK skies. It has a surface temperature of 20000°C, is 6 times as massive as our Sun and 145 light years away.

VLTI's superb resolution

3D models of Achernar, based on the VLTI measurements. In model A, the polar axis is inclined 50 degrees to the line of sight whilst in model B the inclination is 90 degrees. Copyright: European Southern Observatory Scientists have been testing and implementing the VLTI system for the last couple of years. It uses the four large and several smaller telescopes making up the Paranal Observatory to achieve the same resolution as one giant telescope up to 140 metres across. This allows incredibly fine measurements – Achernar subtends the same angle as a minibus on the surface of the Moon.

Achernar's bulging waist

The science team used VLTI to observe Achernar between 11 September and 12 November 2002. It rotates some 10 times more quickly than our Sun, giving it an ellipsoidal shape rather like an exaggerated version of the planet Jupiter. They found that the equatorial radius of the star is around 8.4 million km, whilst the polar radius is only 5.4 million km – surprisingly Achernar has an equatorial diameter 50% larger than a similar cut from pole to pole.

Before long, VLTI should allow ESO astronomers to observe the shapes of many more stars and even obtain direct images of their surfaces.