quality:corruption
Colour controversy

Around the year 150 AD, Claudius Ptolemy of Alexandria, an ethnic Greek Egyptian astronomer of the Roman period, mapped the stars in Books VII and VIII of his Almagest, in which he used Sirius as the location for the globe's central meridian. He described Sirius as reddish, along with five other stars, Betelgeuse, Antares, Aldebaran, Arcturus, and Pollux, all of which are at present observed to be of orange or red hue. The discrepancy was first noted by amateur astronomer Thomas Barker, squire of Lyndon Hall in Rutland, who prepared a paper and spoke at a meeting of the Royal Society in London in 1760. The existence of other stars changing in brightness gave credibility to the idea that some may change in colour too; Sir John Herschel noted this in 1839, possibly influenced by witnessing Eta Carinae two years earlier. Thomas J.J. See resurrected discussion on red Sirius with the publication of several papers in 1892, and a final summary in 1926. He cited not only Ptolemy but also the poet Aratus, the orator Cicero, and general Germanicus all calling the star red, though acknowledging that none of the latter three authors were astronomers, the last two merely translating Aratus's poem Phaenomena. Seneca had described Sirius as being of a deeper red than Mars. It is therefore possible that the description as red is a poetic metaphor for ill fortune. In 1985, German astronomers Wolfhard Schlosser and Werner Bergmann published an account of an 8th-century Lombardic manuscript, which contains De cursu stellarum ratio by St. Gregory of Tours. The Latin text taught readers how to determine the times of nighttime prayers from positions of the stars, and a bright star described as rubeola ("reddish") was claimed to be Sirius. The authors proposed this as evidence that Sirius B had been a red giant at the time of observation. Other scholars replied that it was likely St. Gregory had been referring to Arcturus.

It is notable that not all ancient observers saw Sirius as red. The 1st-century poet Marcus Manilius described it as "sea-blue", as did the 4th-century Avienius. Furthermore, Sirius was consistently reported as a white star in ancient China: a detailed re-evaluation of Chinese texts from the 2nd century BC up to the 7th century AD concluded that all such reliable sources are consistent with Sirius being white.

Nevertheless, historical accounts referring to Sirius as red are sufficiently extensive to lead researchers to seek possible physical explanations. Proposed theories fall into two categories: intrinsic and extrinsic. Intrinsic theories postulate a real change in the Sirius system over the past two millennia, of which the most widely discussed is the proposal that the white dwarf Sirius B was a red giant as recently as 2000 years ago. Extrinsic theories are concerned with the possibility of transient reddening in an intervening medium through which the star is observed, such as might be caused by dust in the interstellar medium, or by particles in the terrestrial atmosphere.

The possibility that stellar evolution of either Sirius A or Sirius B could be responsible for the discrepancy has been rejected on the grounds that the timescale of thousands of years is orders of magnitude too short and that there is no sign of the nebulosity in the system that would be expected had such a change taken place. Similarly, the presence of a third star sufficiently luminous to affect the visible colour of the system in recent millennia is inconsistent with observational evidence. Intrinsic theories may therefore be disregarded. Extrinsic theories based on reddening by interstellar dust are similarly implausible. A transient dust cloud passing between the Sirius system and an observer on Earth would, indeed redden the appearance of the star to some degree, but reddening sufficient to cause it to appear similar in colour to intrinsically red bright stars such as Betelgeuse and Arcturus would also dim the star by several magnitudes, inconsistent with historical accounts: indeed, the dimming would be sufficient to render the colour of the star imperceptible to the human eye without the aid of a telescope.

Extrinsic theories based on optical effects in the Earth's atmosphere are better supported by available evidence. Scintillations caused by atmospheric turbulence result in rapid, transient changes in the apparent colour of the star, especially when observed near the horizon, although with no particular preference for red. However, systematic reddening of the star's light results from absorption and scattering by particles in the atmosphere, exactly analogous to the redness of the Sun at sunrise and sunset. Because the particles that cause reddening in the Earth's atmosphere are different (typically much smaller) than those that cause reddening in the interstellar medium, there is far less dimming of the starlight, and in the case of Sirius the change in colour can be seen without the aid of a telescope. There may be cultural reasons to explain why some ancient observers might have reported the colour of Sirius preferentially when it was situated low in the sky (and therefore apparently red). In several Mediterranean cultures, the local visibility of Sirius at heliacal rising and setting (whether it appeared bright and clear or dimmed) was thought to have astrological significance and was thus subject to systematic observation and intense interest. Thus Sirius, more than any other star, was observed and recorded while close to the horizon. Other contemporary cultures, such as Chinese, lacking this tradition, recorded Sirius only as white.