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.