GAMMA PSC (Gamma Piscium). Pisces, the Fishes, the constellation that contains the Vernal Equinox, is oddly nearly devoid of proper names, only Alpha (Alrescha) and Eta (Kullat Nunu) getting them. A bit odder still, the brightest star in the figure is Eta, while Alpha is number three. In between is nameless Gamma, which shines to us from just over the fourth magnitude line (3.69). Outwardly, it is just another yellow-orange giant at the warm end of class K (K0) or the cool end of neighboring class G (G9), the temperature of 4833 Kelvin more supporting the latter. From a distance of 130 light years, the star shines (accounting for some infrared radiation) only with the light of 61 Suns, not all that much for a giant, the radius a mere 11 solar. The reason is that two-solar-mass Gamma Psc is rather at the bottom of its gianthood, resting so to speak, while it fuses the last part of its internal helium core into carbon. Once upon a time a white class A (about A2) star with a current age of 1.4 billion years, Gamma Psc is preparing to make a run toward much greater luminosity and size, when it will eventually slough off its outer envelope and turn into white dwarf. What rather dramatically distinguishes the star is its speed. While the spectrograph shows it to be approaching us at a modest 14 kilometers per second, observation of its motion ACROSS the sky reveals it to be moving to the east at over three-quarters of a second of arc per year, which at 130 light years corresponds to 145 kilometers per second, over 7 times faster than most local stars. Its speed reveals that Gamma Piscium is a visitor from a another part of the Galaxy, from outside the thin disk of stars that makes the Milky Way and that includes the Sun. Such stars are commonly somewhat low in metal content, as they were formed in a region in which exploding stars did not have sufficient time to build up the amounts of iron and other elements that we see in the thin disk. And sure enough, Gamma's metal content is low, only about a quarter that of the Sun, consistent with its detailed spectral class, which appends the note "cyanogen-weak" and which reveals a low carbon-nitrogen content. All alone, with no known companions, the star will quickly -- in astronomical terms -- pass us and leave the vicinity of the Sun.
Written by Jim Kaler. Return to STARS.