STAR OF THE WEEK: DELTA TRA: (Delta Trianguli Australe). If you were in
charge of making up the constellations and you got
stuck for a name or configuration, there is always the Triangle. Indeed, there are enough of them to make
Euclid happy. Within the set of "official" constellations accepted by the
International Union there are two: plain old Triangulum in the northern hemisphere and Triangulum Australe in the southern. The northern one
is a fine, small isosceles triangle between Andromeda to the north and Aries to the south that contains one of the great
spiral galaxies of the sky, M 33. The southern triangle is very far south
indeed, its bright stars falling from 63 to 70 degrees south of the equator.
You have to be below 27 degrees north latitude to see it all, but at least
you get a glimpse of the second magnitude luminary, Alpha TrA (the obviously
made-up "Atria.") Hevelius (1611-1687) invented
"Triangulum Minor" near our accepted Triangulum, making the latterTriangulum Major," but it did not last (at least
officially: it's still there), and so we are back to the northern and southern
triangles. And if you wish you can add larger figures that cut across
constellation boundaries, the "Summer
Triangle of Vega, Deneb, and Altair, and the Winter Triangle of Betelgeuse, Procyon, and Sirius.
Small Triangulum Australe does not have much to offer except three nice second
magnitude stars: Alpha (1.92), Beta (2.85), and Gamma (2.89), the list
properly-ordered by brightness then jumping down to Delta (3.85) and Epsilon
(4.10). The best sight is probably the deep red carbon star and long-period
variable, X TrA, which can make it to naked-eye
The first of fainter set, Delta TrA, is an unusually luminous class G (G2:
the same as the Sun) lesser supergiant at a distance of 607 light
years (give or take 19) with a temperature of 4970 Kelvin, which is lower
than the 5680-Kelvin Sun because of the giant's lower surface density.
Factoring in a fair bit of infrared
radiation, Delta TrA glows with the light of 1064 Suns, which with temperature yields a radius 44 times
that of the Sun, or 0.21 Astronomical Units. With a hefty mass of 6.3 Suns,
it looks as if it has just started fusing its core helium into carbon and
oxygen. Only 80 million years old, when done with helium-burning, it will
swell again, slough off what remains of its outer envelope to produce a planetary nebula, the core evolving into a white dwarf
of 0.88 solar masses, in the meantime becoming a carbon star like X TrA. Delta
has long been thought to have a twelfth magnitude partner some 30 seconds of arc away, but
it is moving too fast for it to belong to Delta, which now seems all alone.
Of greater significance, Delta TrA is in the realm of the "hybrid stars" which
have magnetic fields and produce fast, hot magnetically-driven winds as well
as slower, cooler winds.
Written byJim Kaler 10/10/14. Return to STARS.