Castor

Castor is Alpha Geminorum, apparent magnitude about 1.58, at roughly 51 light years, and despite its Alpha designation it is fainter than Pollux beside it, one of several cases where the lettering does not track brightness. What looks like one star is a remarkable sextuple system: two bright components, each itself a spectroscopic binary, orbited at a distance by a third pair of faint red dwarfs, for six stars in all bound together. Castor was among the first stars recognised as a true physical binary rather than a chance line-of-sight pairing.

The two brightest components of Castor each orbit their own centre of mass in a matter of days, both pairs then bound together in a much wider, slower orbit measured in centuries, with the distant red-dwarf pair orbiting furthest out and slowest of all; resolving the full six-star hierarchy took astronomers the better part of two centuries after Castor was first recognised as a true binary. Castor's designation as Alpha Geminorum despite being the fainter of the twins likely reflects Bayer's seventeenth-century ordering by position within the constellation figure rather than by measured brightness, a mismatch that recurs at several other bright stars catalogued in this set of tables.

Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with Almagest (Mathematike Syntaxis)Claudius Ptolemy

Ptolemy gives Castor the nature of Mercury, and the tradition reads it for keen intellect, writing and travel, with a recurring note of sudden misfortune, a cleaner Ptolemaic attribution than most, since here Ptolemy names the star. The pairing with Pollux is thematically neat and probably not accidental in the ancient reading: in the myth the twins are of divided nature, Castor mortal and Pollux immortal, and the tradition duly gives Castor the mild, mercurial character and Pollux the martial one. Robson supplies the modern elaboration, but the underlying Mercury-and-Mars split of the two stars is old.

Reading Castor for a "cleaner" Mercury than most Mercury-attributed stars is possible because Ptolemy names Castor and Pollux directly and individually in the Tetrabiblos rather than leaving a modern compiler to infer a delineation from a general remark about Gemini, a distinction that matters given how much of this catalogue rests on exactly that kind of inference. The Mercury-Mars division between Castor and Pollux is one of relatively few fixed-star delineations that later compilers preserved essentially unchanged from antiquity, most of the catalogue's other personality readings being considerably more recent accretions.