RASALAS (Mu Leonis). Leo's great head,
signified by a backward question mark, more commonly known as the
"Sickle," is famed for announcing northern spring skies. At the
bottom of the Sickle lies first magnitude Regulus, while at the top is Rasalas, to
which Bayer gave the Greek letter Mu. The
name, originally "Rasalasad Borealis," taken from an Arabic phrase
(to which was attached the Latin "Borealis" for "northern"), refers
to the "northern part of Leo's head" (Epsilon, a bit to the
southeast of Rasalas, being the "southern part"). At first, the
star looks like one more class K (K3) giant. But it is one with a
difference. Or maybe even a pair of differences. At the brighter
end of fourth magnitude (3.88), Rasalas shines from a distance of
133 light years. Factoring in its coolish temperature of 4500
Kelvin, which allows for a correction of a bit of infrared
radiation, the star is found to radiate 65 solar luminosities into
space, which combined with temperature yields a radius of 13 times
that of the Sun. Unlike most K giants,
which are quietly fusing helium into carbon and oxygen in their
deep cores, this one, with a mass of about 1.5 to 1.7 times that of
the Sun, appears to be in an earlier stage, one in which the helium
core is still contracting. As such, it is getting ever brighter.
Only 50 million years from now, the star will have increased its
brightness by a factor of 10 to 20, and -- were it at the
same distance as today (which it will not be) -- will glower redly
at first magnitude. Rasalas is also one of the sky's prominent
"super-metal-rich" giants. Though astronomers vigorously argue the
exact numbers, all agree that the star has a significantly higher
metal content than does the Sun, the best estimate leading to an
iron content about 70 percent greater than solar. The star's
birthplace must have been especially enriched. The stars that are
being orbited by newly-discovered planets
also tend quite strongly to be metal-rich. Perhaps Rasalas, which
will soon expand to a radius half that of Earth's orbit (and
eventually, when its helium-fusion dies out, become even bigger),
is in the process of destroying its inner planets.