HOMAM (Zeta Pegasi). The name really belongs to a pair of stars, Zeta and Xi Pegasi (in Pegasus, the Flying Horse), which together to the ancient Arabs became known (translated from an obscure phrase) as the "lucky stars of the hero," presumably of the hero Perseus. The name then devolved specifically to third magnitude (3.40) Zeta Pegasi, leaving poor fourth magnitude Xi Peg (which lies just to the northeast of Zeta) nameless. Homam, a name never really used (so we'll go with Zeta), is a class B (B8) dwarf (but see below) that lies 209 light years away. From that and an accurate temperature of 11,190 Kelvin (which allows the calculation of ultraviolet light), we find a luminosity 227 times that of the Sun and a radius of 4.0 solar. Like many stars of classes A and B, it is a fast rotator, spinning at an equatorial speed of at least 140 kilometers per second, 70 times that of the Sun, which gives it a rotation period of under 1.4 days (as opposed to the 25 day solar period). Luminosity and temperature then tell of a mass between 3.3 and 3.5 times that of the Sun depending on the exact state of evolution, which in any case is more that of a subgiant rather than a dwarf, a star that is giving up core hydrogen fusion (sort of consistent with an alternative classification as a B8.5 giant). The age is around 260 million years. Another classification has Zeta as a B8 "emission" dwarf, suggestive of an equatorial disk related to the high rotation. Typical of local class B stars, its metal abundance is a bit low relative to that of the Sun, some 40 percent solar compared to hydrogen. There is no X-ray emission. Showing how far we have advanced in precision of measurement, Zeta Peg seems to be a member of the class of "slowly pulsating B stars" (whose prototype is 53 Persei), varying by a mere 0.00049 magnitudes over a 22.95 hour period. (Most stars would be variable if examined with enough accuracy.) Zeta appears at first to have two companions. Zeta Peg B is a 12th magnitude (11.6) star at a current separation of 59 seconds of arc. Given that in 1879 the separation was 64 seconds, "B" is clearly a line-of-sight coincidence (the difference being much to great for an orbiting star). Eleventh magnitude (11.0) Zeta Peg C, however, seems real, as over more than a century the separation has stayed constant at 177 seconds of arc. If it is a real companion at the same distance as Zeta A, Zeta C is a K6 dwarf with a mass around 0.6 solar. At a minimum orbital distance of 11,000 Astronomical Units, the orbital period would be at least 600,000 years, making "A" and "C" a "fragile binary." From Zeta B, Zeta A would shine with the light of rather well under that of the full Moon, while from Zeta A, Zeta B would be about as bright as our Venus.
Written by Jim Kaler 11/16/07. Return to STARS.