ETA SGR (Eta Sagittarii). There are so many bright stars in Sagittarius, the Archer, which lies within
the stunning star clouds of the Milky
Way, that many get "lost," simply ignored. Eta Sagittarii, which
resides south of the Archer's Bow, is one of these. And too bad,
as this reddish class M (M3.5) giant star stands out in a sea of
hot blue ones. Shining at a nice mid-third magnitude (3.11), Eta
lies 149 light years away. Neglected, there is no measured
temperature. From its M3.5 spectral class, we estimate that its
surface shines at a coolish 3600 Kelvin. Allowing for a fair bit
of infrared radiation gives a total luminosity of 585 times that of
the Sun, and a radius of 62 times solar (0.29 Astronomical Units,
three-quarters the size of Mercury's orbit). Though astronomically
a "red giant," it has been called "brilliant orange," the star
contrasting nicely with the background of the Milky Way. With a
mass estimated around 1.5 times that of the Sun, the three-billion-year-old star is most
likely near the tip of the "red giant branch, either with a dead
helium core or having just fired the helium core up to fuse to
carbon and oxygen. Most of these large stars are unstable, and
vary in brightness, at least to some degree. Eta Sgr is classed as
an "Lb" star, which in the arcane terminology of astronomy means
"irregular giant," and varies erratically between magnitudes 3.08
and 3.12, the total change about four percent (not enough to be
seen with the naked eye). Lying 3.6 seconds of arc away is an
eighth magnitude (7.8) companion. From its visual luminosity, it
must be a class F (probably F7) dwarf with a mass of 1.3 times
solar. At a distance of at least 165 Astronomical Units from Eta
Sgr proper, it takes at least 1270 years to orbit. From the class
M giant, the companion would shine with the brightness of two full
Moons, while from the companion, the M giant would appear as a disk
12 minutes of arc across (20 percent the angular size of the Sun as
seen from Earth), and shine with a brightness of 155 full Moons.