LAMBDA PAV (Lambda Pavonis). Deep in the southern celestial
hemisphere we might pause to admire Pavo,
the Peacock, which is dominated at its northern fringe (57 degrees
south of the celestial equator) by a
second magnitude star of the same name ("Peacock"), better known as Alpha Pavonis.
The rest of the constellation sprawls to
the south and mostly west in a pattern that hardly represents a
beautiful bird: constellations are meant to represent things, not
portray them. The next brightest stars are just fourth magnitude,
including Gamma and Lambda Pav, which
are tied at 4.22, Lambda lying 13 degrees west-southwest of Alpha.
While the same brightness, Gamma (near the eastern edge of the
constellation) and Lambda are very different. Gamma is rather
similar to the Sun and just 30 light years
away, while Lambda is a stunning hot class B (B2) bright giant 1430 light years distant
(give or take 113). Though it lies 24 degrees off the central line
of the Milky Way, Lambda Pav is still dimmed
0.35 magnitudes by interstellar
dust. It is one hot star, half a dozen measures of temperature
averaging 21,900 Kelvin, which means it radiates mostly in the ultraviolet part of the
spectrum. From the star's visual luminosity, temperature (from
which we get the ultraviolet contribution), and distance, we then
find that it radiates at a total rate of 29,600 Suns, which gives
it a radius of 12 times solar, not much for a star called a
"giant." And it isn't. The theory of stellar structure applied to
the temperature and luminosity shows that Lambda Pavonis is a 12.5
to 13 solar mass star that is more of a very mature dwarf or even subgiant and that it is
somewhere around 13 million years old. It appears to be rather
well above the limit at which stars explode as supernovae. Like many of its
kind, it's a fast rotator, spinning at least 167 kilometers per
second at its equator, which gives it a rotation period of under
3.6 days. A possible axial tilt of 27 degrees against the line of
sight would suggest 365 kilometers per second and a rotation period
of just 1.7 days. Like many of its kind (Gamma Cassiopeiae, Zeta Tauri, Delta
Scorpii), Lambda Pav is also a "B-emission" or "Be" star, one
that has strong emission of
hydrogen that originate in a spinning circumstellar disk of
uncertain origin, though its source is clearly related to the fast
rotation. Changes in the disk may cause the star to vary some in
brightness. About a minute of arc away lies Lambda Pavonis B,
which is moving far to fast relative to Lambda Pav proper to be a
real companion and is just a
line of sight coincidence. Looking farther afield, Lambda Pav is
a stepping stone to one of the finer globular clusters of the sky,
NGC 6752, which lies 3.3 degrees to the northeast. Though 15,000
light years away, it's still one of the nearer of such clusters.
Containing perhaps half a million stars, it's actually brighter
visually than much more distant Messier
13, and is potentially visible to the naked eye and an easy
find in binoculars.
Written by Jim Kaler 1/24/14. Return to STARS.