Let me elaborate briefly on why the redness of Sirius in historical times, if true, would be a puzzle for astrophysicists. Sirius, as I mentioned earlier, is in fact two stars. The bright star that we see-and that the Egyptians presumably saw in the sky is Sirius A. It has a magnitude of - 1.5, and is regarded by astronomers as a perfectly normal (A-type) star: nothing much has happened to it, we think, in the last I to I0 billion years. The companion star, Sirius B, is ten magnitudes, or a factor of 10,000 times, dimmer. It cannot be seen without a telescope. Sirius B is not at all a normal star. It is a white dwarf: a star of about the same mass as the sun, but with that mass so condensed that the radius of the dwarf is 100 times smaller than the radius of the sun (about equal to the radius of the earth). The dwarf is supported against further collapse by a pressure that can be accounted for only by quantum mechanics, and not by classical physics. How does such a star arise? Stars in general are thought to begin their lives by the nuclear combustion of hydrogen. They pass from that stage to a stage in which they burn helium, and in doing so they expand into what are called red giants. Eventually the giant's atmosphere of helium is blown off or otherwise consumed. What remains is a white dwarf (figure 4). Now the lifespan of a red giant is thought to be tens of millions of years, and the blow-off time during which a red giant becomes a white dwarf and cools to the presumed temperature of Sirius is also thought to be a truly astronomical time: say a million years. Accordingly, if Sirius looked red to the ancients because Sirius B, a white dwarf now, was a red giant only 2,000 years ago, all theories of stellar evolution would have to change.

How then does an astrophysicist approach the problem of why Sirius was said to be red? Here are the possibilities that come to mind: first of all, the astrophysicist might conclude that it simply wasn't red; it was blue-white. After all, that's the most acceptable astrophysical explanation.

... A third possibility is that Sirius B was a red giant a million years ago-say in pre-Neanderthal times. I think many astro- physicists would reluctantly allow that. If so, one could claim (with every anthropologist in the world convulsed by laughter) that the Neanderthals observed the sky, recorded the fact that Sirius was red, and passed down that fact in a tradition that lasted a million years. It doesn't seem very likely.

A fourth possibility is that Sirius B was indeed a red giant 2,000 years ago. If so, its magnitude would probably have been -3 to -8. It would have been a spectacular star, visible even in the daytime, and that would surely explain why so many different cultures were interested in it. On the other hand, I repeat that one simply cannot get the transformation from red giant to white dwarf to work in 2,000 vears-not even with all the latest possibilities envisaged for subatomic physics by theoretical physicists. In particular, a 2,000-year transformation would seemingly leave the surface temperature of Sirius B at about a million degrees Kelvin or more-far higher than its recently- measured temperature of 30,000 'K.

A fifth possibility is that mass transferred from Sirius A to Sirius B. This happens, for example, in novae-two-star systems, it is now thought, in which a star seemingly explodes over and over again because it keeps receiving mass. To be sure, it is highly unlikely that mass transfer could have occurred in the Sirius system, for those stars are relatively far apart as binary star systems go. Yet if it had in fact occurred, Sirius B might temporarily have become a red giant of sorts, or actually what I would call a slow nova. Examples are known of stars that have made similar detours from normal stellar evolution, such as F G Sagittae or -q Carina.

Brecher 1979:100-102