RHO LEO (Rho Leonis). Magnificent stars are oft neglected, in part because of their surroundings, and here is a prime example. The constellation Leo is so well known, its figure looking really quite like what it is named for, its bracketing stars Regulus and Denebola so famed, that no one pays much attention to fourth magnitude (3.85) Rho Leonis, which falls off the main pattern to the southeast of Regulus. To the contrary, as a class B (B1) "lesser" supergiant, Rho is one of the hottest, bluest, and most massive stars that you can readily see with the naked eye. But hold that for later. From our perspective on Earth, Rho is also a prime "ecliptic star." Falling only eight minutes of arc off the apparent solar path, it marks the position of the Sun on August 29. A line drawn from Regulus through Rho (on the way to Spica) pretty much shows the ecliptic's locus. Moving just under a degree per day, it takes the Sun eight days to move between the two stars. Rho Leo is one of the few brighter Greek-letter-named naked eye stars that are too far away for parallax measure. We can only estimate its distance on the basis of its spectral class. Class B1 lesser supergiants (in the trade it's called a B1 Ib star, Roman "I" for "supergiant") commonly have "absolute visual" magnitudes of -6 (the absolute magnitude being the magnitude the star would have at a distance of 32.6 light years, where it would far outshine Venus). The difference between the observed and absolute magnitudes then gives distance (just as it does for a much better known star, Deneb). Rho Leo is a bit complicated by an uncertain 0.19 magnitudes of dimming thanks to interstellar dust and the fact that it is a very close double, in which fourth magnitude (4.4) Rho Leo A (the supergiant) is coupled to fifth magnitude (4.8) Rho Leo B. The result is a distance of 3650 light years (with a large uncertainty). A temperature of 24,400 (typical of the class) yields a magnificent luminosity for the primary star of 165,000 times that of the Sun! Blue supergiants like this one are really not all that large (the term referring more to evolutionary status), Rho-A just 23 times larger than the Sun. A rather fast projected equatorial rotation of velocity of 55 kilometers per second gives a rotation period under 21 days. Like many supergiants, it is slightly variable, changing in brightness erratically by about seven percent. Luminosity and temperature combine to give a mass estimate 23 times that of the Sun, well above the limit (about 10 solar masses) at which stars blow up as supernovae. With an age of roughly 7 million years, Rho-A has just given up core hydrogen fusion, and does not have long to wait until the ultimate catastrophe. Nothing much is known about Rho B, not even its class, though being less than half a magnitude fainter than Rho A, it's massive as well. No orbit is known, but from a separation estimate the stars probably take about a year to go around each other. The real importance of Rho Leonis is that it has a very simple spectrum that provides a wonderful background with which to study the intervening interstellar medium. It also provides a lesson in stellar "dynamics." Most stars of this class stick very close to their birthplaces in the Milky Way, the plane of the Galaxy. Quite far off the Milky Way, Rho has somehow "run away" through some kind of gravitational action, and seems to be a lesser version of the more famed "runaway" pair Mu Columbae and AE Aurigae.
Written by Jim Kaler 4/13/07. Return to STARS.