A Star of Another Color

By LEON JAROFF

Every year, when the star rose above the horizon just before dawn, the Romans paid bizarre tribute to it by sacrificing dogs with red fur. Seneca the Younger wrote that "the redness of the dog star is deeper, that of Mars milder." Ptolemy called it "reddish," a description also used by Cicero, Horace and other classical authors. The same hue was attributed to the star in cuneiform texts of Babylonia dating as far back as 1000 B.C.

The object of all this historic attention was the dominant member of the constellation Canis Major--Sirius, the brightest[*]star in the night sky and one of the closest to earth (less than nine light-years away). There is just one problem: as any modern stargazer can testify, Sirius is not red but white. How could the ancients have been so wrong?

Some scientists have attempted to explain away the discrepancy by suggesting that the astronomers of antiquity had observed the star when it was low in the sky; like the setting sun, it appeared red because of particles in the earth's atmosphere. Now two German researchers argue that the ancients did see a red Sirius--and as recently as the 6th century A.D.

Astronomer Wolfhard Schlosser and Historian Werner Bergmann of Ruhr-University Bochum, in West Germany, were led to their conclusion by the discovery of references to Sirius in the chronicles of a Frankish bishop, Gregory of Tours. Written around A.D. 577, Gregory's tome was designed to provide monasteries with clear instructions for setting their predawn prayer schedules; thus it listed for each month the time that certain constellations would rise above the horizon. From the rise times and periods of visibility, the researchers report in the journal Nature, they were able to identify Sirius, which Gregory called Rubeola or Robeola, meaning "red" or "rusty." They point out that because Gregory did not use the classical names of the stars, he was probably unaware of Roman and Greek astronomy. Therefore, the Ruhr team concluded, Sirius looked red no more than 1,400 years ago. Yet only 400 years later, when the Arab astronomer Al Sufi categorized all the stars named by Ptolemy, he did not list Sirius among the red ones. Sometime during that interval, the Ruhr team believes, Sirius changed its hue.

A possible key to that puzzling change is the fact, discovered by 19th century astronomers, that Sirius is part of a binary, or two-star, system. It has a small companion star far too dim to be seen by the naked eye. Sirius B, as the diminutive star was named (the familiar Sirius was renamed Sirius A), is now known to be a white dwarf, the dying ember of a star.

Prior to entering the white-dwarf stage, however, an aging star cools and balloons into a red giant. And that, the Ruhr researchers speculate, is probably what Sirius B was when the Babylonians--and then the Greeks, Romans and Franks--gazed skyward. To the unaided eyes of the ancients, the two closely spaced stars looked like a single pinpoint, with a decided reddish tint imparted by the dominating giant. The combined light of the binary pair would certainly have been brighter than it is today, and indeed Babylonian cuneiforms tell of Sirius' being visible in the daytime sky.

The German researchers have no idea exactly when Sirius B collapsed into the white-dwarf stage and no longer obscured Sirius A's white light. Depending on the original mass of Sirius B, the star's transformation could have ranged from a gradual shrinkage to a sudden collapse that resulted in a gigantic explosion that blew much of its stellar matter into space.

If the change was gradual, the researchers admit, it took a remarkably short time for Sirius B to become a white dwarf. In fact, most astronomers think a red giant takes at least 100,000 years to reach that stage. If the change was violent and abrupt, they say, "no traces of catastrophic effects connected with such an event have been found." Those traces, according to widely accepted astrophysical theory, would include an expanding cloud of glowing gas still visible from the earth. Finally, the brilliance of Sirius B's explosion would certainly have lasted for weeks or months and provided an unforgettable spectacle for those on earth. But there are no known records of a dramatic flare-up of Sirius.

One clue may exist. Spectrographic studies of Sirius A, the German researchers note, show that it has a metallic content higher than normal for stars of its type. The excess metal, they say, could have been showered on Sirius A when its red giant companion collapsed and exploded. The fact that no other evidence of an explosion exists, and that most astronomers say it should, does not disturb Schlosser. "Because of Sirius," he says, "we may have to change our theories about the life and metamorphosis of stars." --By Leon Jaroff. Reported by Andrea Dorfman/New York and William McWhirter/Bonn

With reporting by Reported by Andrea Dorfman/New York, William McWhirter/Bonn