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.