ETA CEP (Eta Cephei). Dim and relatively obscure the constellation Cepheus may be, it is filled with star-forming regions, massive hot stars, and red supergiants. It is, after all, the home of Mu Cephei (Herschel's Garnet Star) and binary VV Cep, two of the largest stars known. Interspersed is usual gang of lower mass stars that includes fairly bright, third magnitude (3.43) Eta Cephei, which lies about four degrees more or less west of Alderamin (the Alpha star). Relatively nearby, at a precisely-known distance of 46.5 light years (give or take just 0.1), Eta Cephei is a class K (K0), not the usual K giant, but a subgiant, which implies that the star is in transition from the dwarf to the giant state. A large number of temperature determinations average out to 4990 Kelvin, from which (to account for infrared radiation) we find a total luminosity of 9.2 times that of the Sun. That and temperature then reveal a radius of 4.07 times the solar value. Direct measure of angular diameter plus distance give a remarkably similar 4.06 solar, showing that all the input parameters are accurately known. Application of theory provides a mass of 1.5 solar masses, an age of 2.8 billion years, and confirms that the star is indeed a subgiant that is just beginning to brighten. In another 150 million years, it will have reached 1000 solar luminosities, and then fire up its internal helium (to fuse to carbon and oxygen), after which it will dim down some to become one of the ever-popular helium-fusing K giants. No companion will witness the event, as the 11th magnitude neighbor Eta Cep B (somewhat under a minute of arc away) is moving far too fast for it to be a real companion, and is clearly just in the line of sight. Actually, it's more that Eta itself is the one doing the moving. From its motion across the sky at nearly a second of arc (0.82) per year, distance, and a radial (line of sight) velocity of 88 kilometers per second, we deduce that Eta is clipping along relative to the Sun at a speedy pace of 104 km/s (some 7 times normal), showing that it is a probably a visitor from more outer parts of the Galaxy. Consistently, the iron content (relative to hydrogen) is a bit low, about two-thirds of that found in the Sun, the star just one of the many hidden treasures of the Kingly constellation.
Written by Jim Kaler 12/16/11. Return to STARS.