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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