Sirius, the night sky's brightest star, was consistently described by the ancient Babylonians, Greeks and Romans as red. This has long puzzled modern-day astronomers because, although many red stars are known, Sirius today is white with a bluish hue.

Now two West German researchers have come up with new reason to believe that the ancients were right, suggesting that Sirius must have changed color rather suddenly, on the cosmological time scale, from red to white.

It is known that stars go through a life cycle in which, toward the end, they swell thousands of times larger and their outer regions cool to a red color. Stars in this stage are known as red giants. Then, as processes inside the stars run their course, they shrink and become white dwarfs. Billions of years from now the sun is expected to go through this sequence, becoming large enough to touch the Earth and then collapsing to a white dwarf smaller than the by-then-incinerated Earth.

The West German scientists, Wolfhard Schlosser and Werner Bergmann of the Ruhr University at Bochum, suggest in last week's Nature that Sirius made this transition within the last 1,500 years. Since Sirius, like most stars, is actually a binary system of two stars orbiting each other, only one of the two stars is thought to have made the transition.

There is no direct evidence. Instead, the researchers report finding an independent source of information that confirms the accuracy of the ancient observation of a red Sirius. Gregory of Tours, a bishop who lived in the 6th century, wrote of a star called Rubeola, which means red, whose rising time and position matches that of Sirius.

Because Gregory used none of the classical star names, historians believe he probably did not simply carry over the traditional description of Sirius as red. Sirius, the researchers concluded, must have been a red giantt in Gregory's day.