6 TRI (6 Trianguli). Fifth magnitude 6 Trianguli seems like an anonymous star with an anonymous name within a small constellation. But wait. It has a bit of a history, as it was once the chief star of a modern subdivision of ancient Triangulum (the Triangle) called "Triangulum Minus" (the "Smaller Triangle") made up in the late 1600s of what we now call 6, 10, and 12 Trianguli. If Triangulum Minus had survived the test of time, 6 Tri would now be "Alpha Trianguli Minoris." But it didn't, and it isn't. So 6 Tri will have to do. Oddly, the star is sometimes also called Iota Tri, probably by mistake as there is no such designation listed in the catalogues. Much more important, 6 Tri is a peculiar quadruple star, a double-double in the mold of Mizar, whose parameters remain somewhat confused. At a distance of 305 light years, it is classed as a combination fifth magnitude G5 giant plus a fainter (magnitude 6.44) F5 dwarf, the two stars separated by 3.8 seconds of arc. It's not clear if the stated magnitude of 6 Tri of 4.94 is for the combination of the two or for the brighter G giant alone. Assuming the latter case here, the combination has a magnitude of 4.7. The G giant (6 Tri A) is itself double, the companion another F5 dwarf (6 Tri Ab) going around the G giant (which becomes 6 Tri Aa, its temperature around 5000 Kelvin) with a period of 14.732 days. Estimated luminosities of 65 and 32 Suns suggest masses of 2.7 and 2.3 solar, leading to an average orbital separation of just 0.2 AU, half Mercury's distance from the Sun. The lesser of the visual pair, 6 Tri B, is also double, consisting of (best guess) a pair of mid class F stars that orbit in only 2.24 days. Luminosities of 18 and 9 solar suggest masses of 1.7 and 1.5 times that of the Sun and a very close separation of 0.05 AU, not much bigger than the stellar radii. The visual pair, at least 355 AU apart, would then orbit with a period of at least 2300 years. The pairs are so close, however, that they affect each other gravitationally. Six Tri Aa and Ab rotate with the same periods with which they revolve, thus (like Moon to Earth) keeping one face toward each other. (Six Tri Ba and Bb certainly do as well). The tidal locking has spun up the rotation of the giant star, which then helps produce a strong magnetic dynamo and activity (including spots) much like those seen on the Sun, only exaggerated. Such doubles are called "RS Canum Venaticorum stars" after the prototype, and include Epsilon Ursae Minoris and Lambda Andromedae. As a result of its spottedness and rotation, 6 Tri varies by about a tenth of a magnitude in its brightness, which gives it the variable star name TZ Tri. Combinations such as these are quite hard to study, as it is difficult to separate the light from the various components, 6 Tri in particular a real challenge.
Written by Jim Kaler. Return to STARS.