MIMOSA (Beta Crucis). Few are the first magnitude stars, as their
apparent brightness requires either great luminosity, closeness, or
both. Mimosa satisfies the first requirement. Tied for apparent
brightness at number 19 in the sky with Deneb (apparent magnitude 1.25),
and the second brightest star of Crux, the
Southern Cross, Mimosa is too far south to have a traditional
proper name, one assigned by the ancients. The origin of its name,
which comes from Latin and means "actor,"
is not clear, but may derive from that of the beautiful
southern hemishphere flower. Mimosa the star
is a magnificent blue-white, very hot
class B (B0.5) giant star 350 light years
away with a temperature that soars to 27,600
Kelvin. Such heat causes the star to radiate most of its light in
the invisible ultraviolet. To the eye, Mimosa would appear 3000
times brighter than the Sun; if all the
radiation is taken into account, the luminosity climbs over 11
times higher to 34,000 solar. From these figures we calculate a
radius of 8.1 times that of the Sun, in good agreement with the
value of 8.4 solar found from the small angular radius. Mimosa is
clearly a close double, but one whose components are too near to
each other to resolve separately and that take almost exactly 5
years to orbit. The nature of the second star is unknown, but the
agreement between the two radii above suggests that it does not add
much to the total luminosity. If it does not, Mimosa has a mass of
14 times that of the Sun, and the pair is separated by around 8
astronomical units. Mimosa is also a multiply-periodic "Beta
Cephei" star (named after Alfirk) that
varies between magnitudes 1.23 and 1.31 with periods of 5.68, 3.87,
and 2.91 hours. Though the star is classed a giant, it seems more
to be nearing the end of its hydrogen-fusing stage. Though young
(the Galaxy's metal content increases with time), no more than 10
or so million years old, Mimosa has an iron content only about half
solar, a common occurrence among local young stars. Most likely,
the star will -- after running through its death cycle, in which it
will develop an iron core -- blow up as a supernova.