BETA TRI (Beta Trianguli). Ideally, the Alpha star of a constellation should rule, though for a variety of reasons it commonly does not. The brightest star in Orion is Rigel, Beta Orionis, while in Sagittarius, Rukbat (Alpha Sagittarii) is rather far down the brightness list. Triangulum, the Triangle, is another case in point, as our star Beta Trianguli, right at dead-on third magnitude (3.00), well outshines Mothallah, Alpha Trianguli, and moreover has no proper name to glorify it. This warm white class A (A5) star is hardly without interest, however. Originally classified as a giant, it has more recently been placed into the subgiant category, meaning that it has (or will soon) give up core hydrogen fusion and is beginning to make its run to become a true red giant. From a distance of 124 light years its 8020 Kelvin surface shines with a luminosity 71 times that of the Sun, rendering the star 4.4 times larger than the solar diameter. The luminosity and temperature tell of a star 2.5 times the mass of the Sun that -- as indicated by its classification -- is indeed just giving up its hydrogen-fusing dwarf status and is just 580 million years old. Of more interest, Beta Tri is double. A sunlike companion orbits every 31.8 days at a distance of 0.3 Astronomical Units, about three-fourths Mercury's distance from the Sun. A rather large orbital eccentricity takes the secondary star as far away as 0.42 AU and as close as 0.17 AU. Beta Tri is also a "Vega"-type star that is enclosed in a large shell, or more likely a disk, of cool dust that radiates strongly in the infrared. Such dust-disks suggest planets. If such exist -- and there is no evidence at all -- they would have to orbit well outside the companion's orbit, providing any residents with a spectacular double sunset. Alas, when the primary star's evolution really gets underway and it expands to become a giant, its outer envelope will approach if not engulf the solar companion, causing the two to exchange mass (the matter flowing from the primary to the companion) and to draw closer together, paving the way even for a possible future merger.
Written by Jim Kaler. Return to STARS.