SCHEAT (Beta Pegasi).
Scheat (she-at), at the northwest corner of the famed Great Square of
Pegasus, rises first to
announce the coming appearance of the constellation of the "Winged
Horse," which in mythology
Perseus rode to rescue
Andromeda. From
Arabic meaning "the shin," the name has nothing to do with the
horse, having been misplaced from another constellation, though it
is sometimes translated as "the foreleg." At the fainter end of
second magnitude (2.42) and the Beta star of Pegasus, variable Scheat
averages the second brightest star in the Great Square. Unusual
for a modestly bright star, Scheat has a fairly low surface
temperature of about 3700 degrees Kelvin, 65% that of the Sun, and
is categorized as a class M (M2.5) red giant or even "bright giant," its
color quite noticeable, especially through binoculars or the
telescope. From its distance of 200 light years, we calculate the
star to be 340 times more luminous to the eye than the
Sun.
However, Scheat radiates most of its light in the invisible
infrared, and when that is taken into account, the true luminosity
climbs to 1500 times the solar energy output. To produce this
much radiation at that temperature requires the star to be 95
times the solar radius. Consistent with its giant status, the
star, if placed at the Sun, would extend 70% of the way to the
orbit of Venus. Scheat is big enough and close enough that its
angular diameter of 0.015 seconds of arc (6-millionths of a degree)
is easily measured, from which we derive the same physical size.
Like many red giants of its class, Scheat is actually an irregular
variable star that slowly changes from middle second magnitude to
bright third, a range of half a magnitude that is easily visible to
the naked eye. There is no particular period of variation, and the
star's changes are unpredictable. Scheat is surrounded by a thin
envelope of gas, produced by its strong wind, in which water vapor
has been found.