Was Sirius B a Red Giant?

Rens Van Der Sluijs: Siriusly Red.

As has often been pointed out, by definition the uniformitarian creed precludes the very real possibility of rare and radical changes in nature.

Since the late 19th century, most geologists have fondly embraced the adage of the British lawyer and geologist, Sir Charles Lyell (1797-1875): ‘The present is the key to the past.’ Its naïve implication is that all phenomena that ever happened in nature still occur today and can be observed. Historical evidence is valuable precisely because it offers an even better key to the past than present-day analogues: eye-witness accounts.

A prime application of the historical method concerns the colour of Sirius A or α Canis Majoris, the brightest star in the night sky. Sirius appears bright white today, but – as the English amateur astronomer, Thomas Barker (1722-1809), first pointed out in 1760 – was emphatically qualified as red in many classical texts. Poetical passages aside, Seneca commented that Sirius was of a deeper red than Mars, while Ptolemy labeled the star “reddish” and grouped it with five other stars, all of which are indeed of red or orange aspect.

Even as late as the 6th century CE, the Gallo-Roman chronicler, Gregory of Tours, could label the Dog Star rubeola or ‘reddish’. It is claimed that the earliest unambiguous reference to Sirius as a white star is found in the pages of the Persian astronomer, ‘Abd al-Raḥman al-Sufī (903-986 CE).

What to make of all this? The paradox has sparked a prolonged and fairly intense debate, which has led to a fair number of publications, including Noah Brosch’s recent book Sirius Matters (2008). The evasive explanation that Sirius’s red traced to a simple textual error is easily refuted by the eminent authority of Ptolemy and Seneca as well as the observation that the same attribution is attested in a number of other cultures. For example, the Pawnee, of the North American Plains, associated each of the four intercardinal points with a colour, a type of weather, an animal, a tree, and a star.

The southeastern corner was the domain of red, the “Red Star” – which might be the planet Mars – and the wolf, explicitly linked to Sirius. Another suggestion, that an optical illusion accounts for the confusion, seems merely a red herring. It may be so that the star, to the unaided eye, often appears to be flashing with red, white and blue hues when near the horizon, but such scintillations would not have deluded such a skilled observer as Ptolemy. The belief in a red Sirius was clearly genuine. But how can it be reconciled with the white hue seen today?

Two Canadian archaeoastronomers, David Kelley and Eugene Milone, followed a rather more promising direction: “We conclude that the bulk of the evidence supports a literal red Sirius interpretation … Thus, the discovery that the bright star, Sirius, was once described as red, when it is now clearly white, may light up formerly obscure paths of stellar evolution.” The trouble is that, on the current astronomical model of stellar evolution, no shift from red to white is possible over such a short time.