Thanks to James B. Kaler. These contents are the property of the author and are reproduced from original without the author's express consent because of fair use and valid educational purposes.


(Eta Pegasi). Pegasus (the Flying Horse) is so well known for its Great Square that we sometimes give the other stars little thought. Coming off the northwestern star of the Square, Scheat, is a pair of stars that with Scheat make a rather prominent triangle, the northern one Matar, which Bayer called "Eta," and which (at mid third-magnitude, 2.95) actually ranks fifth in brightness rank (ignoring Alpheratz, Delta Pegasi, which is actually Alpha Andromedae). "Matar," from Arabic, has to do not with a horse, but with "rain," though just what is unclear, one source suggesting "lucky rain." At a distance of 215 light years, Matar is double and may well be quadruple, consisting of a very unequal pair of pairs, an unbalanced double-double. The bright naked-eye star is actually a close pair separated on the average by only three astronomical units (a bit over half the size of Jupiter's orbit). The brighter, 262 times the luminosity of the Sun, is an evolving class G (G2) 5100-Kelvin giant with a quiet, contracting helium core, the fainter a hotter (7800 Kelvin) class A (A5) hydrogen-fusing solar type dwarf. The measured orbit (its period 2.24 years) reveals the stars to contain respective masses 3.2 and 2.0 times the mass of the Sun. Ninety seconds of arc away is a much fainter (ninth magnitude) class G (G5, a bit cooler than the Sun) star that separates into another pair only 0.2 seconds of arc (at least 13 astronomical units) apart that take at least 34 years to orbit. That the two doubles are actually related is not fully known, some say yes, others no, that they are a line-of-sight coincidence. The luminosity of the dim pair, however, is close to being right for G stars if assumed to be at Matar's measured distance, so they are probably a true couple (of couples). If so the two doubles are at least 6000 Astronomical Units apart and take a minimum of 170,000 years to orbit. Even at that separation, however, each would be separable into a double from the other (the large pair having the combined brightness of 5 full Moons as seen from the faint pair). The brighter of the bright pair is on its way to becoming a much larger giant, and will eventually expand to a radius of a quarter the distance that now separates the two stars, streams of matter running from the brighter to the dimmer creating quite a sight from the smaller pair. Eventually the bright star of the brighter pair will fade to become a white dwarf, this double perhaps looking something like Sirius does today.