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HOEDUS I

(Zeta Aurigae). Southwest of Capella (the "she-goat) in Auriga lies a neat triangle of stars, the "Kids." In older tradition, the top star (Almaaz) is excluded, the term belonging to the bottom two, in Latin known as the "Haedi," the eastern (and faintest of the set) one now called Hoedus I, the western Hoedus II. Starting at the north and going clockwise, Bayer named the three in alphabetic order Epsilon, Zeta, and Eta Aurigae. At bright fourth magnitude (3.75), Hoedus I is the fainter of the pair (indeed of the three), though only because it is -- at a distance of 850 light years -- the farther. (Hoedus II is 220 light years away, Almaaz 2000 light years, the three Kids only a line-of-sight coincidence.) Quite the brilliant star, Hoedus I is visually 1700 times more luminous than the Sun. Its spectrum, however, shows that it is not one star but two in mutual orbit, an orange class K (K4) supergiant (or bright giant) and a hot blue class B (B5) star circling each other every 972.183 days (2.66 years). Our delight in the system lies not in the duplicity, but in the orientation of the orbit, which lies within 3 degrees of the line of sight. As a result, the stars of Zeta Aurigae eclipse each other. Every 2.66 years, the smaller but still-bright B star hides completely behind the larger, cooler K star, and the combined visual light drops by 0.15 magnitudes (about 15 percent), not much, but noticeable to the practiced eye. (Coincidences abound in astronomy: though the Kids have nothing to do with one another, two of them are eclipsing binaries, the other Almaaz.) Detailed analysis of the eclipse and of the velocities of the stars tell their stories. Averaging 4.2 Astronomical Units apart, the two go around each other in an elliptical orbit that takes them from 5.9 AU to 2.5 AU. The K star's mass is 5.8 times that of the Sun, the radius 148 solar (about the size of Venus's orbit), the temperature 3950 Kelvin, and the luminosity 4800 solar. The B star's parameters are mass 4.8 solar; radius 4.5 solar; temperature 15,300 Kelvin; luminosity 1000 solar. The total luminosity of the system of 5800 solar is greater than the visual luminosity because the B star produces much of its radiation in the ultraviolet, while the K star radiates significantly in the infrared. From comparison with theory, the pair was born 80 million years ago. Each will ultimately be converted to a massive white dwarf, the K star now well on its way.