Close binary stars have been reported to produce red nova at times while they are merging.
Some classical observers reported Sirius as a red star, others reported a white star. Now it is definitely white. Popular explanations try to discount the red claims, not entirely satisfactorily.
Sirius B is reported to be one of the largest known white dwarfs. The possibility for a merger of close binaries in historical times might be considered. Then the claims of red colors could have a scientific basis. It leaves the problem of what a binary pair might have been like while in the main sequence.
Was Sirius B a Binary Pair Of White Dwarfs In Classical Times?
White dwarfs are comprised of electron degenerate matter, and one distinct characteristic of such matter is that the mass of the star varies inversely as the cube-root of its radius, so a more massive star would be smaller, not larger. Good samples of radii and masses of white dwarfs find this relation is a good fit to the theoretical prediction. There is also no evidence for a nebula that might have been produced by either the past evolution of Sirius B, or by any sort of merger. This is attributed to a long time scale for the nebula to disperse, implying short historical-time evolution unlikely.
One reasonable explanation for reported color differences is atmospheric given the star's horizon location from the Northern Hemisphere (see D.C.B. Whittet, 1999, Monthly Notices of the Royal Astronomical Society, vol. 310, Issue 2, p 355).
The merger of two WDs to form the present Sirius B component of the Sirius system is an intriguing and novel suggestion. The question is, however, has it been modeled and is the very short interval since astronomers described Sirius as a red star reproducible in such a modeling? I am not enamored of the atmospheric reddening hypothesis; that is dependent only on Rayleigh scattering and aerosol content of the atmosphere, so one would have to demonstrate that these scattering effects were stronger only for a specific interval of history. I am most persuaded by the "born again" hypothesis in which a white dwarf can for brief intervals become briefly a red giant again. This was put forward by Ed Sion of Vilanova University some years ago.
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