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/Nature/ *318*, 45-46 (7 November 1985) | doi:10.1038/318045a0; Accepted
4 September 1985


    An early-medieval account on the red colour of Sirius and its
    astrophysical implications

Wolfhard Schlosser^* & Werner Bergmann^?

   1. Department of ^*,  Physics and Astronomy and ^? History,
      Ruhr-University Bochum, Postfach 10 21 48, D-4630 Bochum 1, FRG

      Abstract

An unresolved problem regarding ancient astronomical records is that of
the star 'Red Sirius'. While Sirius today shines white with a blueish
hue quite in agreement with its spectral type AIV, many Greek/Roman and
Babylonian sources (although still disputed) definitely assign a red
colour to this star. We now present new and apparently independent
information about Red Sirius from an early-medieval manuscript. This
manuscript is of Lombardic origin (8th century) and contains the
otherwise lost '/De cursu stellarum ratio/' by Gregory of Tours (about
AD 538?593). It is preserved in the library of Bamberg^1 . Red stars in
ancient records are those with colour index B-V = 1.0 or greater.
Assuming an unchanged Sirius A, this lower limit for the combined colour
of Sirius A and B allows the computation of the region of pre-white
dwarf Sirius B in the Hertzsprung?Russell diagram or colour?magnitude
diagram (Figs 1, 2). Sirius B lies on the giant branch, which fits well
with our observational and theoretical framework of stellar evolution.
However, the timescale of transformation of a red giant to a white dwarf
is surprisingly short.