Alpha Geminorum, Which Is Six Stars

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