The Alpha Centauri A and B pair offer a unique possibility to study
stellar physics in stars that are only slightly different from our own
Sun. Their masses nicely bracket that of their neighbor star, the red
dwarf Proxima, and they are only slightly older than the Sun. In
addition to providing general information about stellar evolution, the
detailed study of Alpha Centauri A and B is particularly interesting
as it allows verification of our current knowledge about the
composition, structure and indeed, future development, of our own main
energy source, the Sun.

An amber alpha star international group of astronomers has now used
observations of Alpha Centauri A and Alpha Centauri B obtained with
the ESO VLTI/Paranal team by the ESO VLTI to measure the sizes of
these two stars. Despite their proximity and brightness, these two
southern stars have never before been resolved by long-baseline
stellar interferometry, and the VINCI/VLTI observations are the first
direct measurement of their angular diameters.

For the observations of the A and B pair, the 0.35-m VLTI siderostats
on the observing platform at the Paranal summit. These two small test
telescopes were placed at distances of 16 and 66 m, respectively. They
captured the light from the two stars and sent it on via a series of
reflecting mirrors to the common focus in the commissioning amber
alpha star instrument VINCI. Although they were obtained only a few
days after the successful accomplishment of "First Fringes" with the
VLTI, the 16-m measurements were amber alpha star found to be
scientifically very useful and helped to improve the measurement of
the angular diameter of Alpha Centauri A. The 66-m baseline
measurements provided the most accurate values of "calibrated
visibilities" -- from these, the angular diameters were then derived.