CAPELLA (Alpha Aurigae). In early mid-northern winter evenings, Capella shines almost directly overhead, and is one of the three bright stars spread around the northern sky, the others Arcturus of spring and Vega of summer. All are close to the same brightness, Arcturus cool and orange, Vega hot and white, Capella yellow-white and in the middle of the temperature range. Barely the faintest of the three, it distinguishes itself by being the first-magnitude star closest to the north celestial pole (actually zeroth magnitude, 0.08). Capella, meaning "the She-Goat" from old Roman times, is the Alpha star of the constellation Auriga, the Charioteer, a prominent irregular pentagon of stars. Just to the south of Capella is a triangle of fainter stars, an asterism called "The Kids."
Capella and her Kids. North is to the left. Auriga's Capella, near the lower left corner, leads the eye up and to the right to the thin triangle of stars called "The Kids," made (clockwise from the left) of Almaaz (Epsilon Aurigae), Haedus I (Zeta), and Haedus II (Eta). The figure is remarkably similar (though reversed) to "The Little Kids" near Theta Aurigae, made of Nu, Tau, and Upsilon Aurigae.
Capella, at a distance of 43 light years, is one of the sky's most famous double stars, and consists of a pair of similar first magnitude stars so close together that it has long been a challenge to separate them at the telescope, which in recent times has been done both through sophisticated interferometry and with the Hubble Space Telescope. A faint star once thought to be a companion was long ago given the name "Capella B, rendering Capella itself "Capella A." The close components of Capella A are thus known as "Aa" and "Ab." The brighter, Capella Aa, is a class G8 giant with a magnitude of 0.76, while Ab, a class G0 giant, comes in a bit fainter at 0.91. The spectral classes give respective temperature estimates of 4900 and 5700 Kelvin (in agreement with other sources). Luminosities from apparent brightness and distance then come in at 93 and 64 times that of the Sun, leading to radii of 13.6 and 8.3 solar, and from the theory of stellar structure and evolution, masses of 3.0 and 2.5 times that of our Sun. Both stars were once class B dwarfs (albeit on the cooler side of the class). The warmer star, "Ab," is cooling further, and has a dead helium core that is slowly preparing to fire up to fuse to carbon and oxygen, while "Aa" already seems to have reached that point, consistent with the more massive star of a pair being the first to evolve. The two go around each other in a nearly-circular orbit 0.72 Astronomical Units apart with a period of 104.0 days.
Capella Ab is seen to orbit Capella Aa (located at the cross) every 104 days at a distance of 0.72 Astronomical Units. In reality the two orbit each other around a common point between them 46 percent of the way from Aa to Ab. The tick marks on the axes are a mere 0.05 seconds of arc apart. (Taken from the Sixth Catalog of Orbits of Binary Stars, W. I. Hartkopf and B. D. Mason, US Naval Observatory; see R. L. Branham, The Astronomical Journal, vol. 136, p. 963, 2008.)
Analysis through orbital theory gives masses of 3.05 and 2.57 times that of the Sun, remarkably close to those derived from evolutionary theory, showing that all the parameters are in order. As expected, the more evolved (Aa) is the slower rotator, its equatorial velocity 3 kilometers per second (for a rotation period of under 220 days), as opposed to Ab's much higher spin speed of 36 km/s (giving a period of under 11 days). One (probably the faster rotator), or even both, are (like the Sun) magnetically active, the system producing copious X-ray emission. Various other "companions," Capella C, D, E, F, G, and H all seem just lying in the line of sight (though H has been considered as real). It's hard to tell the fate of the pair. Now with an age of around 400 million years, when the more massive ceases helium fusion and expands as with a dead carbon/oxygen core, it will encroach on the other, probably losing mass to it, making it difficult to estimate the final fate of the pair. (Capella is featured in Jim Kaler's The Hundred Greatest Stars.)
Written by Jim Kaler 12/13/98, revised 10/03/08. Return to STARS.