Capella

Capella is Alpha Aurigae, apparent magnitude 0.08, at 43 light years, and it is the sixth-brightest star in the sky and the closest first-magnitude star to the north celestial pole, circumpolar from most of Europe, so it never sets.

What appears as one star is four. Capella A and B are two yellow giants of spectral types around G1 III and K0 III, both considerably more massive and far more luminous than the Sun, orbiting each other in about 104 days at a separation of roughly the Earth-Sun distance. They cannot be separated visually and were identified spectroscopically; Capella was among the first spectroscopic binaries discovered. A distant pair of faint red dwarfs, Capella H and L, completes the system.

The two giants are of nearly equal mass and are at different evolutionary stages, which makes the system unusually valuable for testing models of stellar evolution: two stars of known and almost identical mass, formed together, caught at different points. The more evolved of the pair is in a rapid transitional phase.

The Latin Capella means little she-goat, and the star is the goat carried on the shoulder of Auriga, the Charioteer, a constellation figure that makes very little narrative sense and appears to be a conflation of separate traditions. The associated Haedi, the Kids, are the nearby fainter stars, and their rising and setting were weather markers in classical agricultural writing, associated with storms.

The incoherence of the figure is worth dwelling on, because it is common and usually smoothed over. A charioteer has no obvious reason to carry a goat and her kids, and none to be drawn without a chariot, which Auriga is. The likeliest explanation is that two or more older figures were merged when the Greek constellation set was assembled from Mesopotamian material, and the seams were never tidied.

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

Ptolemy gives Capella the nature of Mars and Mercury, and unusually the minor-sounding component dominates the tradition entirely.

The reading that comes down is of Mercury: inquisitiveness, a restless and questioning mind, love of learning and of freedom, eccentricity, honour and wealth obtained through those qualities. The Mars component contributes almost nothing to the standard delineation, which is not martial in tone at all. Compare Betelgeuse, which has the same two planets in the same order in Ptolemy and is read as predominantly martial. Two stars with identical Ptolemaic natures acquire opposite characters in the later tradition.

That divergence is instructive, and it is evidence about the tradition rather than about the stars. If a two-word Ptolemaic assignment can yield a warrior in one case and a scholar in the other, the delineations are not being derived from the assignment. Something else is supplying the content (the constellation figure, the star's colour, its name, accumulated earlier material), and Ptolemy is being cited for authority after the fact.

Capella is one of Agrippa's Behenian fifteen, paired with sapphire and with thyme.

The goat association has a mythological attachment: Capella is frequently identified with Amalthea, the goat that suckled the infant Zeus, whose horn became the cornucopia. That identification is ancient and is a reasonable candidate for the source of the star's association with abundance and good fortune, a more plausible origin for the delineation than the Mars-Mercury pairing, and an illustration of how much of fixed-star tradition is really constellation mythology attached to a bright point within the figure.