ETA SER (Eta Serpentis). All constellations are unique, but Serpens (the Serpent), in violation of the rules of English, is even more so. It is the only one that comes in two separate parts. To the west are the snake's foreparts and head (Serpens Caput), to the east the tail (Serpens Cauda). The two are widely separated by Ophiuchus, the (no surprise) Serpent Bearer, who holds the coiled beast. Bayer on the other hand, took them as one figure, Serpens Caput holding Alpha (Unukalhai, the luminary) through Epsilon, Serpens Caput beginning its lettering with fifth magnitude Zeta. Given that third magnitude (3.26) Eta Serpentis is the brightest over on this eastern side and that Theta Ser (Alya) is on top, Bayer rather clearly ordered at least some of his stars by position rather than brightness (seen in a lot of other constellations as well). Eta, one of the many class K (K2) orange giants that flock the sky, lies at a precisely-known distance of 60.5 (give or take just 0.2) light years. With a well-determined temperature of 4890 Kelvin, Eta Ser radiates at a rather paltry rate of just 19 times that of the Sun (including its infrared radiation), from which we derive a radius of 6.1 times solar. Interferometric measures of angular diameter give a satisfyingly close value of 5.9. The rotation, at least the projected spin velocity, is slow, just 0.44 kilometers per second, yielding a rotation period of as much as 1.9 years. Of fairly low mass, about 1.5 Suns, Eta seems most likely to be brightening as a red giant with a dead helium core. By the time it fires its helium to fuse to carbon and oxygen, it will have become some 25 times brighter. With an age of about 2.8 billion years, it gave up hydrogen fusion 150 million years ago. With roughly half the metals of the Sun (compared with hydrogen), and a bit rich in the carbon-nitrogen cyanogen molecule, the star is speeding across the line of sight at nearly a second of arc per year, its velocity relative to the Sun a rather high 79 kilometers per second. Eta Ser thus appears to be visiting from another part of the Galaxy. A seeming double with a dim 11th magnitude companion, it isn't. Eta Serpentis B has moved a whopping 140 seconds of arc over a period of 175 years, so it is just a distant star that lies in the line of sight. As such, it makes a fine marker with which to view the fast motion of nearby Eta Ser A.
Written by Jim Kaler 7/06/12. Return to STARS.