10 LAC (10 Lacertae). Surrounded by Cepheus, Cygnus, Pegasus, and Andromeda, we pay poor Lacerta, the Lizard, little heed, all its stars faint and obscure, its brightest star, Alpha Lacertae, just fourth magnitude. Yet treasures it does hold, including an "OB association," a collection of loosely organized, unbound hot class O and B stars called "Lacerta OB I." Among the chief members is one of the few naked-eye class O (09) dwarfs stars (indeed one of the few O stars of any kind) in the sky, Flamsteed's number 10. O stars are the hottest and rarest of the stellar types; of all stars, only 0.00007 percent fall into the class. That we see as many as we do is testimony to their inherent brilliance. And 10 Lac, one of the best, is no exception. The star's apparent fifth magnitude (4.88) faintness is the result of large distance (parallax measures bring it in at between 1000 and 1060 light years) and a bit of absorption by interstellar dust in the Milky Way, without which 10 Lac would climb almost to fourth magnitude (4.53). Being hottest, O stars are also the bluest of the lot. Through the telescope, 10 Lac shines like a blue-white diamond, the star used as a spectral standard against which to compare others. From a surface heated to a quite-amazing 32,000 Kelvin, it radiates a with a luminosity of 26,800 Suns (the majority of the light in the invisible ultraviolet), from which we derive a radius 4.7 times that of the Sun and a great mass of 16 times solar. An equatorial rotation speed of at least 31 kilometers per second gives it a rotation period less than eight days. Typical of such stars, 10 Lacertae is losing mass through a wind that blows at a rate of a tenth of a millionth of a solar mass per year, a million times the rate of the solar wind. The spectrum suggests that the star actually rotates much faster, but is viewed nearly pole-on and is surrounded by a thin gaseous disk, though the contention is argued. The star is somewhat deficient in carbon, nitrogen, and oxygen compared with the Sun, but that is typical, as the Sun actually appears to be a bit rich in heavy elements as compared with its immediate Galactic surroundings. A tenth magnitude class A companion to lies a minute of arc and at least 20,000 Astronomical Units away. If that is the actual distance, the orbital period would be over half a million years. From there, 10 Lac would shine with the light of 4 full Moons. Most likely, however, the fainter star is not attached to 10 Lac proper. Falling close to the "zero-age main sequence" of dwarf stars, 10 Lacertae was recently born. Such stars do not live very long. In only 10 million years, 10 Lac will have used up its internal hydrogen fuel, and will eventually explode as a supernova.
Written by Jim Kaler. Return to STARS.