XI PEG (Xi Pegasi). Poor fourth magnitude (4.19) Xi Peg, in southwestern Pegasus (to the southwest of the Great Square), used to have a proper name, "Homam," which it shared with brighter third magnitude Zeta Pegasi (and which lies just two degrees to the southwest of it). Meaning "Lucky Stars of the Hero" (probably referring to the flying horse's rider, Perseus), the name ultimately went to Zeta alone. Though seemingly close, the two have nothing to do with each other, Zeta lying 209 light years away, Xi a mere 53.2 (give or take 0.2) light years. Not all that much different from the Sun (as compared with most naked-eye stars), Xi is a bit more advanced, as it is traditionally considered a class F (F6) giant-subgiant. With a temperature of 6290 Kelvin, the star shines with the light of just 4.4 Suns, from which we derive a radius of 1.7 times that of the Sun. A projected equatorial rotation speed of 6.7 kilometers per second (3.4 times that of solar) then gives it a rotation period of under 13 days. Theory reveals a mass of 1.25 to 1.3 times that of the Sun and confirms that the star is indeed a subgiant that has given up, or will shortly give up, core hydrogen fusion. Though born some 5 billion years ago, about the same time as the Sun, its higher mass has caused it to race through its lifetime (the Sun having another 5 billion years to go before it too gives up the fusion of hydrogen to helium in its core). Though. like many of its kind, it has been examined for an orbiting planet, or even a debris disk that might suggest a planetary system, it seems to have neither, which is consistent with its subsolar metal content (the ratio of iron to hydrogen just 55 percent of solar), since planet- holding stars tend to be metal-rich. Xi Peg has two companions, one real, one not. Xi-B, a twelfth magnitude star (probably an M5 dwarf with a mass a quarter that of the Sun) now 11 seconds of arc from Xi proper, is keeping pace with its bigger brother. If it is indeed a real companion, then it lies at least 180 Astronomical Units away from Xi-A and takes at least 2000 years to make a full orbit. From Xi- A, the lesser star would appear about the brightness of a crescent Moon, while from Xi-B, Xi-A would shine with the light of five full Moons. Xi-C, an 11th magnitude star 3 minutes of arc away, is separating fast, and is clearly a line-of-sight coincidence.
Written by Jim Kaler 11/30/07; revised 8/12/16. Return to STARS.