ETA PER (Eta Persei). In the twentieth century rather mysteriously given the "Arabic" name "Miram," Eta Persei (of Perseus, the hero who rescued Andromeda from Cetus's jaws) is listed as having no proper name in Allen's vast book, so we go here simply with the Greek letter name given by Bayer, Eta Persei, which lies at the northern tip of the classical outline of the constellation. It also lies within the realm of magnificent massive stars, though with a large distance of 1300 light years, it is unrelated to the Alpha Persei cluster (which is less than half that distance) and other collections of neighboring massive stars. While most everything about the star is uncertain (including the distance), the collection of characteristics seems to hang together with some kind of consistency. This fourth magnitude (3.76) star was originally classified as an M3 supergiant, but most modern listings have it as a class K (K3) supergiant, which is consistent with a determined temperature of 4300 Kelvin. At its large distance within the Milky Way, there has to be some interstellar dust absorption. If class M3 it would be about a magnitude, if K3, then zero, so for want of anything better, we adopt half a magnitude. That, allowance for infrared light, and distance gives us a magnificent luminosity of 13,000 times that of the Sun, which leads then to a very large radius of 210 solar, or nearly as big as the Earth's orbit. Direct measure of angular size through interferometry gives a remarkably consistent value of 220 solar, suggesting that all the parameters are close to correct. A measured equatorial rotation velocity of at least 5.8 kilometers per second gives an amazingly long rotation period of under 5 years (which could, given the axial tilt, be much less). There is no question about the great mass, however. If still expanding with dead helium core, the star carries a mass of 11 times that of the Sun, while if it has already fired its internal helium, it comes in at somewhat less, around 9 solar. The star is right on the cusp. It could created the most massive of white dwarfs, near 1.4 solar (the maximum allowable, beyond which white dwarfs collapse), it could make a rare neon white dwarf (most are balls of carbon and oxygen), or it could be just above the uncertain limit at which stars explode as supernovae. Clumping around the big one are a batch of six lesser so-called "companions" that are all likely to be just line of sight coincidences. The most accepted is Eta-B, a 9th magnitude B9 dwarf that lies 29 seconds of arc away. If a true companion, it would be at least 11,500 Astronomical Units from Eta proper, and take at least 350,000 years to make a circuit, such numbers making the companionship even less unlikely.
Written by Jim Kaler 12/05/08. Return to STARS.