HAEDUS II (Eta Aurigae). Tucked to the south-southwest of Capella in Auriga is a small and beloved triangle of stars, the "Kids" to Capella's "she-goat." The two at the bottom of the triangle, Eta and Zeta, were the original Kids, while the strange eclipsing variable Epsilon (Almaaz) was added later. The original two remain the Roman's "Haedi," the western (Zeta) Haedus I, the eastern our Haedus II, which is far more commonly known just as Eta Aurigae.
The 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.
Though angularly close, the trio have nothing to do with one another. Shining at third magnitude (3.17), Haedus II is in the middle in terms of brightness. At a distance of 220 light years, it is however by far the closest, Zeta (Haedus II) lying 850 light years from us, Almaaz close to 2000 light years. This class B (B3) ordinary (core hydrogen-fusing) dwarf, with a surface temperature measured at 16,600 Kelvin, is also by far the hottest of them. Unlike the other Kids, it is a single star, with no real evidence for a companion. Accounting for considerable ultraviolet light, Haedus II radiates a luminosity 760 times that of the Sun, which yields a radius 3.3 times solar and (from the theory of stellar structure and evolution) a very considerable mass 5.5 times that of the Sun. With an age of about 45 million years, the star is half way through its dwarf lifetime. Spinning with an equatorial velocity of at least 95 kilometers per second, it rotates in under 1.8 days, in contrast to the 25-day period of the Sun. While Eta Aur is considered very stable and so ordinary that it is used for a standard against which to compare others, it is not without some controversy. The temperature is notably low for the spectral class, which implies a temperature closer to 18,500 Kelvin. The luminosity has also been estimated at a higher 955 times that of the Sun. There is unconfirmed evidence for spectral variations with a 24-day period, which if they exist probably originate in the stellar atmosphere. An extremely weak magnetic field has been detected, one only a couple times that of the Earth. Though not orbited by any other star that we know of, Haedus II may still have something of a family, as it has been considered to be part of the vast (as one can see from the name) Cassiopeia-Taurus "association" of hot class O and B stars, an extended group that stretches across 100 degrees of sky from Taurus to Orion, whose members were born more or less at the same time and are now separating forever.
Written by Jim Kaler. Return to STARS.