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Telescopes and space missions

Proxima Centauri comes into focus

10 Dec 2002 Belle Dumé

An international team of astronomers has succeeded in accurately measuring the size of a small star for the first time. Damien Ségransan and colleagues at the European Southern Observatory in Chile and the Canada-France-Hawaii Telescope (CFHT) have studied Proxima Centauri -- the nearest star known after the Sun. The star, which is about 4.2 light years away, was found to be just one-seventh the diameter of our Sun (D Ségransan et al. 2002 Astronomy and Astrophysics to appear)

Proxima Centauri is a very low-mass star that — together with the double-star Alpha Centauri — forms a triple stellar system in the constellation of Centaurus. Proxima Centauri is about half as hot as the Sun and about 150 times fainter, which has made it difficult to measure its size — until now.

Ségransen and co-workers used interferometric measurement data – obtained last year — from the Very Large Telescope Interferometer (VLTI) and the VLT Interferometer Commissioning Instrument (VINCI) at the Paranal Observatory in Chile. Optical interferometry combines the light from multiple telescopes so that they behave as a single, much larger telescope. The interference between the different light waves can be used to eliminate glare from bright stars or to measure distances and angles precisely.

The VLTI was used with two large VLT telescopes that provide the image sharpness needed to resolve small stellar disks like that of Proxima Centaur. These telescopes are large enough to observe objects that are fainter than those normally studied by interferometric measurements. The researchers analyzed the observational data using improved software and a novel technique called “wavelet analysis” to overcome the effects of turbulence from the Earth’s atmosphere on their measurements.

The team found that Proxima Centauri’s diameter – and mass — is about one-seventh that of the Sun. It is about one and half times bigger than Jupiter, but is 150 times more massive. Proxima Centauri lies in the critical transition region between stars and brown dwarfs – stars that are not heavy enough to sustain fusion in their cores.

The group now hopes to measure even smaller extra-solar objects with the VLTI.

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