* Astronomy

Thanks to the unique possibilities offered by ESO's Very Large Telescope Interferometer (VLTI), astronomers have solved a 140-year-old mystery concerning active hot stars. They indeed show that the star Alpha Arae is spinning almost on the verge of breaking and that its disc rotates the same way planets do around the Sun.

"This result could only be achieved because of the great details we could observe with the AMBER instrument combining three 8.2-m Unit Telescopes of ESO's VLT" - Philippe Stee, leader of the team that performed the study.

With AMBER on the VLTI, the astronomers were able to see details on the scale of one milli-arcsecond, corresponding to being able to distinguish, from the Earth, the headlights of a car on the Moon.
Lying about 300 light-years away from the Sun, Alpha Arae is the closest member of the class of active stars known as 'Be stars'. Be stars are very luminous, massive and hot stars that rotate rapidly. They are losing mass along the poles through a strong stellar wind and are surrounded at the equator by a disc of matter. Alpha Arae has ten times the mass of the Sun, is three times hotter and 6 000 times as luminous.
The question how the discs around active stars known as Be-stars rotate was posed since the discovery of the first one, Gamma Cassiopeiae, by Italian astronomer Father Angelo Secchi, exactly 140 years ago, on 23 August 1866 in Rome.
With AMBER, the team of astronomers could examine in details the structure of the disc surrounding Alpha Arae. Moreover, because AMBER also provides spectra, the astronomers could study the motion of the gas in the disc and so understand how it rotates.

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