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

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 Bamberg1. 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.