PI LEO (Pi Leonis). Tucked into western Leo just under five degrees south southwest of Regulus and just north of the border with
Sextans lies fifth magnitude (4.70) Pi
Leonis, a giant star of a
somewhat different breed. Most, of class K (usually of the K0 or
K1 subvarieties), are quietly fusing helium into carbon and oxygen
in their deep cores. But they have to get there first. Class M
(M2) Pi Leo may be a good example of a red giant in transition. At
a distance of 406 light years (give or take 15), the star is all by
itself, with no known companions, so we can
concentrate on it alone. With a temperature of 3710 Kelvin, just
right for the class, Pi Leo radiates much of its energy in the infrared part of the spectrum.
Taking that and distance into account, we find the star to shine
with the light of 775 Suns, which with
temperature gives it a radius of 68 times that of the Sun, or 0.31
Astronomical Units, about 80 percent the size of Mercury's orbit.
Pi Leo is just close enough to the ecliptic to be occasionally
occulted by the Moon. The time it takes for the star to disappear
behind the lunar advancing lunar limb gives a radius of 70 solar
radii, while interferometry gives 66, the average of which is right
on the mark. Given the luminosity and temperature (plus the theory
of stellar structure and evolution), Pi Leo's mass should be close
to 1.5 times that of the Sun (though with a fairly large
uncertainty). Pi's claim to any sort of fame is that it is most
likely close to the high-luminosity tip of the "red giant branch"
of evolving stars. When the hydrogen in the core of a star is all
converted to helium, the core contracts, hydrogen fusion expands
outward into a shell, and the star itself swells and brightens.
For stars with masses below about double solar, as the core
squeezes down and its density climbs, the electrons in the ionized
gas begin to fall into a state of "degeneracy" (like that in a white dwarf) in which they
can get no closer. At some point, the internal temperature gets
high enough that the helium nuclei can fuse to carbon and oxygen.
The degeneracy is then relieved, but explosively in a tremendous
"helium flash" whose energy is absorbed by the star and is largely
invisible at the stellar surface. Assuming the stellar parameters
and our premise to be correct, after a hydrogen fusing lifetime of
2.7 billion years, Pi Leo has been ageing and swelling for the last
220 million. It has perhaps another couple million years left
before the helium flash takes place and it dims and shrinks some to
become one of the ordinary class K orange giants that dot the skies
and are so much a part of our familiar constellations. Of course the star might be
in that dimming state as well, or even brightening for the second
time with a dead carbon core to become a more advanced giant, a possibility
supported by a tenth of a magnitude variation. We can only wait
Written by Jim Kaler 5/10/13. Return to STARS.