Beta Aquilae - Alshain

Alshain, or Beta Aquliae (β Aql), makes a prominent trio of stars in Aquila with with Altair and Tarazed. 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 received the Beta designation even though it ranks seventh in brightness in its constellation.

Properties

Though brighter and farther along in its evolution, β Aquilae bears some similarity to the Sun. It is a class G subgiant with a temperature of 5300 K. From its nearby distance - only 45 light years - we find its luminosity to be six times the Sun's, and its radius about three times solar.

Alshain is slightly variable (by about 0.05 magnitudes), has a magnetic field slightly stronger than the Sun's, and displays some solar-like activity. Its subgiant status and overly bright luminosity for its temperature indicate that it has stopped fusing hydrogen in its core. With a dead helium core, it is preparing to become a much more luminous red giant star. The Sun will reach such a stage in about five billion years; Alshain, with a mass 30% greater than the Sun's, has a main-sequence lifetime of about 80% of the Sun's.

Companion

Alshain has a companion 13" away, although at 12th magnitude it is so dim that it is hard to see. It is a class M red dwarf with a temperature of only 3400 K and a very low luminosity, 2.5% the Sun's. If at the projected separation of 175 AU from Alshain, β Aql B would shine only about as brightly as our quarter Moon as seen from Alshain. With a mass only a third the Sun's, β Aql B's hydrogen-fusing lifetime is hundreds of billions of years. Indeed, red dwarfs are so long-lived that none has ever died; such faint red dwarfs make up 70% of the stars in our galaxy.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]