LAMBDA AUR (Lambda Aurigae). Auriga, the celestial Charioteer, is so filled with bright and otherwise admirable stars (that include Capella, her three "Kids," the classic "runaway" AE Aur, the list seemingly endless) that it comes as rather a surprise to come across one like the Sun, a class G (G1.5 subgiant-dwarf) "solar analogue" that is just a little more massive and a bit older than our own star. Rather easily found among a string of stars six degrees due south of Capella to the southeast of the Kids, fifth magnitude (4.71) Lambda Aurigae is one of the closer stars to Earth, just 41.2 light years away, give or take a measly 0.1 l-y. With a very well-determined temperature of 5838 Kelvin (60 degrees warmer than our Sun), the star radiates at a rate of 1.75 times that of the Sun, the result of a combination of carrying seven percent more mass than solar and (since hydrogen-fusing stars brighten as they get older) a somewhat advanced age, the best estimate 6.2 billion years (1.7 billion years older than the Sun). Given a theoretical 8 billion-year lifetime, the star, while still a real dwarf, does not have that long to go before giving up core hydrogen-fusion and becoming a real subgiant as expressed by its class. A radius 30 percent larger than solar and a solar type (projected) equatorial rotation speed of 2.5 kilometers per second give a maximum rotation period of 26 days, almost exactly solar. No overt magnetic activity is detected, perhaps the result of advanced age (stars quieting down as they get older and spin slower, the result of magnetic braking). Nor sadly do there seem to be any planets, at least none detected, nor for that matter, any surrounding debris disk, in spite of the fact that Lambda is somewhat metal-rich (planet-holding stars tending to have more metals) with an iron content (relative to hydrogen) 15 percent greater than the Sun. Most other chemical elements are similarly up as well, though nitrogen and carbon are somewhat depressed. Nor does the star appear to have any other companions. Though four are listed (14th magnitude Lambda Aur B 310 seconds of arc away, 13th mag C at 87 seconds, and 10th mag D and E at 203 and 175 seconds), their motions show that all are just line-of-sight "optical" coincidences. Lambda's motion itself, though, shows it to be a visitor from a different part of the Galaxy, the star whipping along at a hefty 83 kilometers per second relative to the Sun, more than five times normal for local stars of the Galactic disk. Nobody seems to have done focused work on Lambda. Instead, it has lent itself to a vast number of statistical studies, the star showing up regularly in examinations of motions, temperatures, chemical compositions, and the relations among them all.
Written by Jim Kaler 2/04/11. Return to STARS.