A physical interpretation of the ‘red Sirius’ anomaly


★ E-mail: doug@whittet.phys.rpi.edu


The controversy over whether the brightest star, Sirius (α CMa; HR 2491), was red in visual appearance some 2000 years ago, as suggested by Ptolemy amongst others, is re-examined from a physical perspective. Objections to hypotheses based on evolutionary events within the Sirius binary system itself are briefly reviewed. Scenarios that invoke reddening caused by foreground extinction in the interstellar medium or in the Earth's atmosphere are examined in detail to determine whether they offer viable alternatives. It is deduced that only atmospheric extinction is capable of producing appropriate changes in the colour and brightness of Sirius. This result concurs with the findings of Ceragioli, who deduced, from a re-evaluation of the historical evidence and the cultural role of Sirius in Greek and Roman society, that ‘red Sirius’ refers to observations made at the heliacal risings and settings of the star. Both physical and historical evidence are thus consistent with an interpretation of the ‘red Sirius’ anomaly based on reddening in the terrestrial atmosphere.