X TrA (X Trianguli Australis). Extra, XTrA, read all about it.
Sorry, could not resist. And, while not exactly ALL about it,
perhaps enough to intrigue. Not that all that much is known. The
most obvious is that X Trianguli Australis (in Triangulum Australe, the Southern Triangle) is a deep
red jewel of a rare carbon
star, one of the few visible to the naked eye and a delight to
view with optical aid. Not far over the border of sixth magnitude
(nominally 5.81), in blue light (to the old photographic plate) the
star dims to magnitude 9. Somewhat variable, in blue light by
about a magnitude, X TrA is classed as an irregular giant with no discernable period,
the Roman letter name a
giveaway to the star's variability. Consistent with the algebraic
symbol for the unknown, almost everything about "X" is problematic,
including its distance of 1170 light years, which has an
uncertainty of close to 200 ly. While we are sure that the star is
a giant (given as class C5.5, "C" for "carbon"), the temperature
estimate ranges from a low 2700 Kelvin (comparable to class M7) to
am even lower 2200 Kelvin (comparable to M10). The absolute visual
magnitude of -2.0 (how the star would look at the standard distance
of 32.6 light years) squarely makes it a giant, and places it among
a cohort that includes famed Mira. Most of
the radiation, though, comes out in the infrared. The literature
records very uncertain total luminosities (adjusted for the above
distance) that average around 6000 Suns,
which with temperature give a radius somewhere in the neighborhood
of over 400 solar radii, thirty percent larger than the Martian
orbit. Giant indeed! The birth mass is unknown, but from the
carbon-star nature may be around triple solar. Such stars have
gone completely through central helium fusion and have dead,
shrinking, carbon-oxygen cores. Internal convection currents can
then bring fusion by-products upward, making the stellar atmosphere
carbon-rich. Like most stars of its kind it is losing mass, this
one at a rate of some 10 million times the flow rate of the solar
wind. In the Sun, oxygen is almost twice as abundant as carbon.
Carbon stars reverse the ratio, carbon in X TrA measured at 1.2
times the abundance of oxygen. All sorts of other chemical
elements that were manufactured in the depths of the star must also
have been raised to the surface as well. With its low variability,
X TrA may be in the early stages of brightening as an "advanced giant," and may
eventually pulsate like Mira and other long period variables. Part
of the wind condenses into dust, which like the new elements will
be blown into interstellar space to be used in the next generation
of star formation, while the core becomes freed of its overburden
to become a member of the family of white dwarfs.
Written by Jim Kaler 10/19/12. Return to STARS.