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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), among the stars of first magnitude it ranks number 10 in luminosity. 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" (the word also used in botany), is unknown. Mimosa is a magnificent blue-white, very hot class B (B0.5) giant star 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.