Mars, the Immortal Twin

Ptolemy gives Pollux the nature of Mars, and the tradition reads it for a spirited, bold and sometimes rash character, contentiousness, and courage under adversity, with Robson adding the usual freight of danger and violence that later writers heaped on the martial stars. The contrast with Castor is the point: the ancient reading assigns the mortal twin the gentle Mercury and the immortal twin the fierce Mars, so that the pair encodes a division of temperament as well as of fate. Pollux is the more dependable of the two to cite, since Ptolemy names it and the attribution has been stable ever since.

The Castor-Pollux division of Mercury and Mars along the line of mortal and immortal twin is neat enough that it is sometimes assumed to be a later systematiser's tidy invention, but the assignment already appears in Ptolemy, which places the pairing squarely within the earliest surviving layer of fixed-star doctrine rather than among the later medieval or Renaissance elaborations that fill out so much of the rest of the catalogue. The martial reading given to the immortal twin sits oddly against Pollux's mild, Sun-like orange-giant nature astronomically, a case where the mythological logic of the pairing, not any observed property of the star, entirely drives the delineation.