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.