0
Theta Arae
THETA ARA (Theta Arae). Scorpius, the
celestial Scorpion of the Zodiac,
is so overwhelming a figure that one might not think to look below
it. Most northerners in fact can't, as Scorpius's southern curve
is not ever that far above the horizon. But from the south, the
stars of one of the most southern of ancient constellations, Ara,
the Altar, can easily be seen shining within the confines of the Milky Way. The whole region around Scorpius, Centaurus, Crux,
Ara, and beyond is filled with hot, class B stars of high mass that
were born more or less at the same time from the same cold interstellar cloud complex, and that
now form gravitationally unbound OB
associations and their various subassociations. Many, however,
are the massive stars whose relatives are unknown. Their number
includes fourth magnitude (but at 3.66 on the bright side) Theta
Arae, a class B (B2) supergiant (though of the lesser kind; and see
below) 813 light years (plus or minus 30)
away. Were it not for 0.19 magnitudes of dimming by the Milky
Way's interstellar dust, the star
would make it into the realm of third magnitude. Though there is
no physical relation (the three at different distances), Theta Arae
fits in nicely with its constellation mates Alpha Arae (a B2 dwarf) and Gamma Arae (a B1 supergiant). With a
temperature of 17,900 Kelvin (needed to find the amount of
invisible ultraviolet
radiation), Theta radiates at the rate of 9100 times that of the Sun. From the calculated radius of 10 times
solar and a projected equatorial rotation speed of 108 kilometers
per second (apparently not enough to give it a surrounding disk),
the star's rotation period is less than 4.7 days. Theory applied
to temperature and luminosity yields an impressive mass of 8.5
times that of the Sun (comparable to Alpha Arae, but not up to
Gamma's level). Rather than being a full blown supergiant, the
star seems more to be a subgiant that is beginning to
turn itself into one. Theta's mass puts it right on the lower edge
of the fuzzy limit above which stars explode as supernovae. If it escapes
that fate, its core will contract into a massive white dwarf, perhaps one
made of neon and oxygen rather than of the usual mix of carbon and
oxygen. If Theta had a close companion, tidal flows might increase the eventual white
dwarf's mass enough to put it over the white dwarf mass limit of
1.4 times that of the Sun. The white dwarf would then
catastrophically collapse and make an even grander supernova. No neighbor, however,
is evident, our star seemingly quite alone.
Written by Jim Kaler 9/13/13. Return to STARS.