MATAR (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, 2.94)
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.