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.