BETA PAV (Beta Pavonis). Alpha stars are usually, though not always, the brightest of their constellations. Many, however, are the exceptions, which include Cetus (where Beta is luminary), Sagittarius (whose brightest is Epsilon, Alpha way down on the scale), Ursa Major (also dominated by Epsilon), and 26 others! The situation for Beta-stars is even worse (note Sagittarius and Canis Major). The modern far-southern constellation of Pavo, the Peacock, however, is well behaved, at least for the top two. The luminary is bright, second magnitude, Alpha (of the same name as the constellation), while third magnitude (3.42) Beta indeed comes in at number two: at least as long as Lambda Pavonis is not in some kind of outburst as a disk-surrounded B- emission star like Gamma Cassiopeiae. The Bayer scheme then breaks down at Gamma Pav, which falls somewhere around ninth, with Delta Pav coming in at number three. That's nearly all that distinguishes this rather lonely and neglected star. But lack of spectacular qualities does at least give us the chance to go on about the vagaries of the Greek letter system. Yet, classed as an A7 giant (like Gamma Bootis and Theta-2 Tauri), the star is not without some redeeming, indeed instructive, value. At a distance 135 light years (give or take a half) and with a temperature of 8200 Kelvin (as expected for its class), Beta Pavonis glows with the white light of 58 Suns, its radius calculated at 3.8 times solar. A projected equatorial rotation speed of 81 km/s leads to a rotation period of at most 2.3 days, and luminosity and temperature plus theory conspire to give a mass of 2.3 to 2.4 times solar. They also reveal that the star is not a true giant, but a subgiant that is about to give up core hydrogen fusion, if it has not done so already. While "main sequence" stars do not evolve much over their hydrogen-fusing lifetimes, they DO change some as the internal fuel supply dwindles. In its early youth some 60 million years ago, Beta Pav began as a class B9.5 or so dwarf, with a temperature close to 10,700 Kelvin, a luminosity of 32 Suns, and a radius of 1.7 solar. Over its stable lifetime, it has cooled by some 2000 Kelvin, brightened by 80 percent, and doubled its diameter. After becoming a much brighter true giant and losing most of its outer hydrogen envelope (that surrounding the core) through winds, it will expire as a spent cinder -- a white dwarf -- with just 30 percent its present mass. Though currently heating instead of cooling, our lower mass Sun is undergoing similar, though much slower, changes.
Written by Jim Kaler 9/24/10. Return to STARS.