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-hemisphere flower. Mimosa the star is a magnificent
blue-white, very hot "giant"
star (but see below) 280 light years away (second Hipparcos
reduction) with a temperature that soars to nearly 27,000 Kelvin.
Such heat causes the star to radiate most of its light in the
invisible ultraviolet. To
the eye, Mimosa would appear 2000 times brighter than the Sun; if all the radiation is taken into
account, the luminosity climbs nearly 10 times higher to 19,600
solar. From these figures we calculate a radius of 6.6 times that
of the Sun, in excellent agreement with the value of 7.1 solar
found from the small angular radius. The path to the star seems
clear of dust. An unlikely maximum of 15 percent absorption of
radiation would boost the luminosity to 22,700 Suns but give a
lesser agreement with the direct measure radius. Temperature and
luminosity then lead to a mass of 14 times that of the Sun and
clearly show the star to be not a giant, but a true dwarf that is
roughly halfway through its 11 million year hydrogen-fusing
lifetime (such misclassification not uncommon). A rotation
velocity of at least 40 kilometers per second gives a rotation
period under 8 days. The metal content is about 3/4 solar, typical
of such stars. Mimosa is a multiply- periodic "Beta Cephei" star
(named after Alfirk) that at most varies
by a few thousandths of a magnitudes with periods of 4.588, 4.028,
4.386, 6.805, and 8.618 hours. (There seems to be an additional
tenth or so of a magnitude variation.) The star has a number of companions. A faint 11th
magnitude "B" component lies 42 seconds of arc away. If real, it
would be a low mass class K dwarf separated by at least 3600 AU
with a period of at least 55,000 years. Then the spectrum reveals a closer companion
with a period of 5 years in an eccentric orbit that would have to
lie about 7 AU away from the brilliant primary star. The most
interesting is a low-mass but active "pre-main-sequence" star, a
star that is still in the process of forming. Discovered through its
X-ray radiation, it is quite invisible in the optical spectral
domain. At a separation of at least 350 AU, it would take some
1600 years to make a full orbit. A listed "C" component at 370
seconds of arc separation is clearly just a line-of-sight
coincidence. Mimosa itself is clearly above the cutoff of 8 to 10
solar masses, beyond which stars explode as
supernovae. It has
perhaps only six or so million years left to go. (Thanks to Andrew
James for commentary on the name.)
Written by Jim Kaler 5/25/01; revised
7/10/09. Return to STARS.