ALSHAIN (Beta Aquilae). Altair, Tarazed, and Alshain make the prominent
trio of stars in Aquila (the Eagle)
that together seem like a bird, even an airplane, flying across the
sky. They also reminded the Arabs of a scale beam, or balance.
While the Persian name for the "balance" was originally applied to
all three, Alshain eventually got one of the two Persian words,
while Tarazed got the other. Alshain is in such a prominent
position in its constellation that it received the Beta designation
even though, at bright fourth magnitude, it ranks seventh in
brightness in its constellation. Why Bayer did not give "Beta" to
much brighter Tarazed we will never know. Though brighter and
farther along in its evolution, Alshain, a class G subgiant with a
temperature of 5300 Kelvin, bears some similarity to the Sun. From
its nearby distance of only 45 light years, we find a luminosity
six times that of the Sun, and a radius about thrice solar.
Alshain is slightly variable (by about 5% of a magnitude), has a
magnetic field slightly stronger than that of the Sun, and displays
some solar-like activity. Its motions have been examined for the
effects of planets, but none have been found. Neither has any
surrounding dust disk that might indicate a planetary system.
Perhaps that is just as well, as its subgiant status and that it is
overly bright for its temperature show that it has stopped fusing
hydrogen in its core, and with a dead helium core is preparing to
become a much more luminous giant star. The Sun will reach such a
stage in about seven billion years. Alshain, with a mass 30%
greater than the Sun's, has a hydrogen-fusing lifetime only about
80% solar. Alshain has a companion 13 seconds of arc away,
although at 12th magnitude it is so dim that it is hard to see. A
class M red dwarf that is a bit warmer than Proxima Centauri, Alshain-B has a
temperature of only 3400 Kelvin and a very low luminosity a mere
2.5% that of the Sun (yet one still 10 times brighter than
Proxima). If at the projected separation of 175 Astronomical
Units, Alshain-B would shine only about as brightly as our quarter
Moon as seen from Alshain proper (or from some undetected close-in
planet). While such red dwarfs are faint, they are numerous,
making up 70% of the stars in the Galaxy. With low masses
(Alshain-B's only a third that of the Sun), red dwarfs are so long-
lived that none has ever yet had a chance to die.