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 and see.

Written by Jim Kaler 5/10/13. Return to STARS.