The stars differ obviously in colour. Three or four among the brightest strike the eye by their ardent glow, others are tinged with yellow, and the white light of several has a bluish gleam like that of polished steel. Reddish tints are, however, in the few cases in which they affect lucid stars, the most noticeable, and were the only ones noticed by the ancients.

Ptolemy designates as ‘fiery red’ (ὑπόκιῤῥοι) the following six stars: Aldebaran, Arcturus, Betelgeux, Antares, Pollux, and—mirabile dictu—Sirius! all the rest being indiscriminately classed as ‘yellow’ (ξάνθοι). Now Pollux at present, though by no means red, is at least yellowish, but Sirius is undeniably white with a cast of blue. A marked change in its colour since the Alexandrian epoch might thus at first sight appear certain, the more so that Seneca makes express mention of the dog-star as being ‘redder than Mars;’ Horace has ‘rubra Canicula’ as typical of the heat of summer; and Cicero, in his translation of Aratus, speaks of its ‘ruddy light.’ Nevertheless the case is doubtful. The questionable epithet, in all probability, crept into the ‘Almagest’ by a transcriber's error, Ptolemy not being responsible for it. In the early Arabic versions of that work it evidently did not occur, for Arab astronomers of the tenth and subsequent centuries ignored the imputation of colour to the dog star, and Albategnius stated the number of Ptolemy's red stars as five.