ETA CEN (Eta Centauri). Centaurus, the
Centaur, is filled with bright stars, the best known Rigil Kentaurus, the Alpha star, the
closest to us, just 4.4 light years away. It and Hadar (Beta Cen) so dominate the
constellation that the others become rather neglected, especially
since the constellation is far to the south and much out of sight
for much of the world's population. Few of the stars -- including
Eta Cen -- even have proper names. Lying at a distance of 310
light years (give or take 25 or so), shining at second magnitude
(2.31), Eta is a very hot class B (B1.5) dwarf that is part of the
sprawling Scorpius-Centaurus (Sco-Cen) "OB
association," really a super-association made of several smaller
parts. Eta belongs to the "Upper Centaurus-Lupus" (UCL) group. Such associations are loose
collections of stars born more or less at the same time and marked
with blue high mass class O and B members. Not bound together by
gravity, they are slowly expanding. Eta is assigned a temperature
of 20,000 Kelvin, which is oddly low for its class; 24,000 should
be more like it. At the lower temperature (needed to account for
a lot of ultraviolet radiation), Eta would shine with the light of
5900 Suns, at the higher, 8200 (after
allowing for a 15 percent reduction of light caused by interstellar
dust absorption). The mass of the star thus lies between 8.5 and
10.5 solar, the radius 5 to 6 times solar, the star less than 20
million years old. Eta has several claims to special notice.
First it rotates quickly with an equatorial speed of at least 310
kilometers per second, giving it a rotation period of less than a
day. Second, like many of its class (Zeta
Tauri for example), it is a "Be" star, one with variable
hydrogen emissions in its
spectrum that tell of a thick circumstellar disk that is somehow
related to the rapid rotation. Eta is of an extreme sort called a
"shell star," as the disk is rather edge-on to the line of sight.
Third, it is a subtle, multi-periodic variable that chatters away
by a few hundredths of a magnitude with periods of 18.6, 16.2,
13.5, 6.3, and 5.3 hours, all working in concert with each other.
Most curious is the report of a companion star 6 seconds of arc
away that was seen once in 1897 and never seen again, likely an
error of some sort. The star is near the crossover between those
that become massive white
dwarfs and those that explode as supernovae. Only time will
tell its ultimate fate.