LAMBDA BOO (Lambda Bootis). The main portion of Bootes, the Herdsman, rather resembles a kite with mighty Arcturus, the luminary, at its end. Less recognized is a long triangle of stars that lies near the tail of Ursa Major (the handle of the Big Dipper) made of Theta and Epsilon Boo (that with much fainter Iota make the outstretched fingers of the Herdsman) and modest fourth magnitude (4.18) Lambda. Modest but mysterious, this class A (A0 peculiar) hydrogen fusing dwarf is still the subject of study and argument. At a distance of 97 light years, Lambda shines to us with the luminosity of 16 Suns from an 8900 Kelvin surface, as expected for a two solar mass star. Spinning with a rapid projected equatorial velocity of 128 kilometers per second and 1.7 times the size of the Sun, the star rotates with a period of less than two thirds of a day. So far, all seems relatively normal. The surprise comes in the Lambda's weird chemical composition. Its outer layers are depleted in metals (chromium, barium, nickel, titanium) by roughly a factor of ten, while the other elements come in as more or less normal. Such stars (the "Lambda Boo stars") are rare: there are only about 50 known. Many class A and B (Elnath and Gamma Corvi for example) stars have odd abundances as a result of separation (gravitational settling and radiative lofting) in quiet atmospheres. But Lambda's spin roils the gasses and hardly makes it quiet! The prevailing idea is that star was at an early age surrounded by a thick cloud of dusty gas. Interstellar dust grains absorb metal atoms from the gas onto themselves, which depletes the gas (a well-recognized phenomenon). The pressure of starlight keeps the dust pushed away while the depleted gas settles inward to become part of the star. Or not. No one really knows. (Lambda Boo was chosen as one of Jim Kaler's Hundred Greatest Stars.)
Written by Jim Kaler. Return to STARS.