Brightest star has lost its red reputation

Press, 26 December 1985, Page 12

Brightest star has lost its red reputation

By

Robin McKie

in London

Sirius, the brightest star in the night sky, is providing astronomers with a headache. It is simply the wrong colour. Ancient manuscripts — including those of Ptolemy, Cicero, Seneca, Horace, and several Babylonian astronomers — describe the star as bright red. Today it is a very obvious bluish-white. This discrepancy has worried astronomers for decades, for a star’s colour is a crucial guide to its temperature and structure. Red stars are cool, bluish-white ones are very hot, and both take aeons to change.

Now two West German researchers believe they have found a solution to the Sirius mystery, though it may force astronomers to radically re-think their theories about star formation. Dr Wolfhard Schlosser and Dr Werner Bergmann, of Ruhr University, Bochum, believe Sirius’s tiny unseen companion star, Sirius ■B, holds the key.

About 1500 years ago, round the time of the collapse of the Roman

Empire, Sirius B was probably one of the biggest stars in the galaxy. It was of a type called a Red Giant and it was so big it could have swallowed most of our solar system, says Schlosser and Bergmann.

Its bright red light would have completely outshone the main Sirius star that we see today. It may even have been visible in daylight as ancient Babylonian records suggest. This indicates that Sirius may have played an important role in ancient religious life, a fact bom out by Bergmann and Schlosser’s discovery that Romans used to sacrifice red-haired dogs to the star at certain times of the year. Then, about 500 A.D., Sirius B began to collapse into its present tiny dim form, leaving blue-white Sirius to shine on its own, they state in a recent “Nature” report. It is a neat theory. Unfortunately, astronomers have always believed that it takes at least 100,000 years for Red Giants to collapse into small, dim stars.

“It is a lovely idea,” said an astronomer, Dr David Hughes, of Sheffield University. “Unfortunately, Sirius B behaves in a perfectly stable way today. If it collapsed relatively recently the perturbations would still be obvious.”

He and other astronomers say effects in the Earth’s atmosphere could make Sirius’s light appear red at times. Others say ancient Greek records have simply been mistranslated.

Bergmann and Schlosser reject these arguments, however. Atmospheric effects would make all stars appear red at times, but only Sirius was labelled “the rusty star.” They also point out that Babylonian and early medieval texts call Sirius red, not just Greek ones.

Only a complete, radical rethink of star formation and collapse can explain these observations, they say. — Copyright, London Observer Service.