ETA SCO (Eta Scorpii). Scorpius's prominence and sparkle are the result of its being made largely of related blue-white stars class B stars that fill that region of the sky and that link as well to the stars of Centaurus and Crux (the Southern Cross) in sets of distant, loosely-bound stellar "OB associations." Here and there, however, are a few interlopers that happen to be in the line of sight, among them no-proper-name Eta Scorpii, which has the distinction of being the most southerly star of the figure's classic outline (barely beating out Girtab and Zeta Sco). It's lost permanently below the horizon to anyone above latitude 47 degrees north. Not blue at all, the star is a white (some would say yellow-white) class F (F3) giant-subgiant that has also been classified as an F2 dwarf (though we will stick with the former classification). From a fairly nearby distance of 72 light years it radiates at a rate only 17.5 times that of the Sun from a surface with an ill-defined temperature of 6700 Kelvin (the quoted range from 6500 to 6900 Kelvin), which lead to a radius just 3.1 times solar. The modest qualities are the result of the star just beginning its trek toward gianthood. With a mass of 1.7 times that of the Sun, born 1.8 billion years ago, Eta Sco has apparently just now ceased its core hydrogen fusion as it prepares to become a genuine and much more luminous red giant. That is, it is really a true subgiant. (If hydrogen fusion still lingers, as suggested by the above dwarf classification, the star is a bit younger and has a somewhat higher mass of 1.8 solar, showing a common uncertainty in our knowledge of stellar lives. It's sometimes hard to tell what is going on inside from surface parameters.) Stars from mid-class F and hotter tend to be fast rotators, and Eta Sco is near the top end. Any sort of evolutionary swelling has not slowed it down, the star spinning with a speed of at least 155 kilometers per second, giving it a rotation period of under a day. The spin-produced magnetic field heats a surrounding hot corona that radiates X-rays. While relative to the Sun Eta Sco is moving about 2.5 times faster than normal, the metal content (the two related) is close to solar. The star's most unusual aspect is that it has been classified as a rare "dwarf barium star." Barium stars (Alphard the classic case) are giants that have been contaminated by the evolution of binary companions that are now white dwarfs. There is no evidence, though, that Eta Sco has any sort of companion. That and the quite-modest barium enhancement suggests mis-classification. Indeed, since the other 11 dwarf barium stars are modest as well, the class as whole may not exist.
Written by Jim Kaler. Return to STARS.