A Red Sirius

The Puzzle of A Red Sirius?

Claudius Ptolemy circa 150 AD

Nineteen hundred years ago the Greco-Roman astronomer, Claudius Ptolemy, left us an enigma. In his famous astronomy textbook the Almagest Ptolemy described the star Sirius as 'reddish'. Exactly what Ptolemy meant by this remark has long puzzled people, and in particular astronomers. Was Sirius actually a red star two millennia ago? Some who have investigated this puzzle have sought, and found, apparent confirmation in ancient Greek and Roman sources. One of the most controversial of these investigators was the flamboyant T. J. J. See (1878 - 1962)

If Sirius actually was red what could have caused it to change into the brilliant bluish-white star we see today? Could this change be explained by modern astrophysics? Some astronomers have offered possible explanations. Perhaps they say, the small white dwarf (Sirius B) that orbits Sirius was in the final phases of its red giant stage in the days of Ptolemy. If so it must have been an impressive sight, a ruddy reddish star several magnitudes brighter than it is today. Others have speculated that Sirius B underwent some from of 'nova' event involving a surface explosion. All of these explanations involve severe problems with astronomical time scales (stars evolve too slowly and nova fade too quickly) too explain an ancient red Sirius. Further, modern observations show that Sirius B sits comfortably in its expected equilibrium state and could not have undergone any catastrophic events in the last two thousand years.

So what is the resolution of this age-old mystery? Perhaps the most historically defensible explanation is that the Greeks and the Romans had a tradition of associating the star Sirius with the color red and redness, in much the same way that they associated the star with dogs, and with heat and fire. It was not the visual appearance of Sirius they were speaking of but the cultural attributes of the star. We now know that there exist other Roman sources that also describe Sirius as 'sea-blue' and likewise contemporary Chinese sources that use the term 'white'.