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.