ALPHA LAC (Alpha Lacertae). Lacerta, the Lizard, one of the dimmest constellations of the sky, is a modern figure ("modern" referring here to the 17th century) invented to fill in the relative blank area between bright Cygnus, Cassiopeia, and Andromeda. Lacerta's brightest star, appropriately given Alpha but otherwise quite un-named, is only fourth magnitude (3.77), which is rather odd for a constellation that lies within the confines of the Milky Way. Alpha Lac is among the more common of naked-eye stars, a white class A (A1) hydrogen- fusing dwarf. Just over 102 light years away, Alpha Lac gives us a chance to see what our brightest star Sirius would look like if it were a dozen times more distant. With a temperature of 9200 Kelvin, Alpha Lac shines with a radiance 27 times that of the Sun, its radius double solar. Unlike many class A stars, which have odd chemical anomalies that are the result of diffusion of the elements in quiet atmospheres (some settling down under gravity, others raised by the pressure of radiation, exemplified by Alpheratz, Elnath, and Mizar), Alpha Lac is "normal," the result of a high rotation speed that keeps things stirred up. Spinning at least 146 kilometers per second at the equator, the star makes a full rotation in under 17 hours. At first glance, Alpha Lac seems to have a companion, a dim twelfth magnitude (11.8) star located 36 seconds of arc away. Alas, the pairing is only line-of-sight. Not only are the two separating from each other much too quickly for the motion to be orbital, but spectroscopy of the "companion" shows it to be a class A5 star. To be that dim, the star must have a distance of 2700 light years, nearly 27 times farther than Alpha Lac proper. The "lack" (no pun intended) of an orbiting companion means we have to calculate Alpha Lac's mass and status from its luminosity and temperature. Just over twice the solar mass, the star is fairly young, and not all that long ago began its billion- year stable hydrogen-fusing lifetime, after which it will become a red giant and then a far- dimmer white dwarf like the true companion to Sirius.
Written by Jim Kaler. Return to STARS.