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.
Written by Jim Kaler. Return to STARS.