MARKAB (Alpha Pegasi). Markab epitomizes what
seems almost to be a celestial joke, a comic opera of sorts in
which stars scramble their names and appear as different characters
on the sky's stage. Four stars make the Great Square of Pegasus, Markab (Alpha) at the
southwestern corner, Scheat (Beta) at
the northwestern, Algenib (Gamma) and the southeastern, and Alpheratz (Alpha Andromedae) at the
northeastern, this last star linking the Winged Horse to Andromeda. "Markab" comes from an
Arabic phrase meaning "the horse's shoulder," but in more recent
times was mistakenly taken from what is now Scheat. But Scheat's
name ("the shin" was mistakenly taken from "Skat," the Delta star
in Aquarius. Continuing the confusion,
Algenib's name was taken from Mirfak
(the Alpha Star in Perseus), whose
alternative name is ALSO Algenib. Not to be outdone, "Alpheratz"
may have come from an original name for Scheat. The Greek letter
system is not much better. As a linking star, Alpheratz -- Alpha
Andromedae -- is also Delta Pegasi. Moreover, Mirfak, the Alpha
star, just barely second magnitude (2.49), is only third brightest
in the constellation, and is exceeded by both the Beta star
(Scheat), by of all things, Epsilon (Enif), as well as by the Delta star
(Alpheratz of Andromeda). There will be a quiz on all this next
Tuesday. Markab itself is much clearer. It is a relatively hot
class B (B9) dwarf (some say giant), appearing not much
different from (though visually fainter than) Regulus in Leo.
Its distance of 140 light years leads to a total luminosity 205
times that of the Sun (including some
invisible ultraviolet), which with the star's temperature (10,500
Kelvin) yields a radius 4.3 times solar. Just a hair over three
solar masses, and still spinning fairly rapidly (its rotation
period under 1.5 days), Markab has just begun to die. If hydrogen
fusion has not already ceased in its core, it is very close. The
star is in a sense clinging to its lifeline on the hydrogen-fusing
normal "main sequence" of stars and is about to leap into the
abyss, in which it will quickly expand, slow its rotation, and
become a much cooler orange giant (perhaps someday looking like Kornephoros in Hercules). It will then brighten to many times its
current luminosity to die finally as a massive white dwarf like Sirius-B. Some stars are remarkable for
their chemical compositions, others for their multiplicity, yet
others for strange variability. Though there is some evidence from
Markab's motion for a small stellar companion, the star is famed
for none of these. It is in fact noted for its very normality,
behaving quite like it should. Such normalcy is extremely
important in stellar astronomy, as it provides a bedrock standard
against which to measure the odder members of the stellar zoo. Of
the 100 publications in which Markab has been mentioned in the past
15 years, nearly all use it as such a base. Go out and enjoy a
perfectly normal, and quite admirable, star.