Many astronomical bodies – including the Earth – are known to be ‘oblate’: in other words, they are larger around the equator than around the poles. As a rigid, rocky planet, the Earth is just 0.3% wider at the equator than at the poles, but astronomers have long expected the effect to be larger for gaseous objects like stars. Fast-rotating bodies with large centrifugal forces at their surfaces are also likely to be more oblate.

In line with these predictions, a previous study suggested that the equatorial radius of Achernar – which is six times more massive than the Sun – was about 14% larger than its polar radius. But the new measurements show that the equatorial radius of Achernar is equivalent to 12 solar radii, compared with about eight at the poles.

Domiciano de Souza and colleagues exploited the rotation of the Earth to make regular observations of the star using two telescopes almost perpendicular to each other. Merging the signals produced interference patterns that allowed the group to measure the angular size of the star in directions almost 90 degrees apart.

The astronomers add that Achernar, which is 145 light years away, could be even more oblate than their data suggest, because they are uncertain of the orientation of the star’s axis of rotation during their observations.