Rare quartet of stars aids stellar evolution models
BY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: May 16, 2008

Astronomers have discovered an extremely rare quartet of stars,
disguised as a single speck of light even through some of the world’s most powerful telescopes, orbiting each other within a region smaller than Jupiter’s orbit round the Sun.

Using high resolution spectrograph instruments on the Keck and Canada-France-Hawaii telescopes, which break up the star’s light into a spectrum of different wavelengths, or colours, the astronomers could measure the stars’ speeds and masses to infer the maximum sizes of their orbits. Originally thought to be one star, BD -22º5866 was resolved into four closely orbiting stars, arranged in two pairs. One pair orbits each other in less than 2.5 days with an orbital radius of 0.07 AU, and the second pair orbits with a period just under 55 days and at a maximum radius of 0.26 AU. In turn, the two pairs orbit each other with a maximum radius of just 5.8 AU, about the same as Jupiter’s distance from the Sun.

Artist impression of the gaseous disc that may have once maneuvered the quadruple stellar system into its unusually small orbit. Image: K. Teramur, UH IfA.

"The extraordinarily tight configuration of this stellar system tells us that there may have been a single gaseous disc that forced them into such small orbits within the first 100,000 years of their evolution, as the stars could not have formed so close to one another,” says Dr Shkolnik of the University of Hawaii’s Institute for Astronomy. “This is the first evidence of a disc completely encompassing four stars. It is remarkable how much a single stellar spectrum can tell us about both the present and the past of these stars."

What makes the system even more unique is that the inner pair of stars eclipse each other, and by measuring how much light the eclipses block, the masses and radii of the stars can be inferred. The two stars are found to be almost identical, each being about 60% the size of the Sun.

Since most stars form as part of a binary or multiple star system, this new quadruple system will give astronomers previously unavailable physical information that will help develop models of stellar evolution.