ALPHA ARA (Alpha Arae). Though some of the ancient constellations
extend far to the south (Eridanus and Centaurus cases in point), Ara, the Altar, taken in its entirety, is the
most southerly of them all. Its main portion, due south of Scorpius, is fully visible only south of
about 30 degrees north latitude, and though some of its stars are
reasonably bright, none is named. While fairly bright, the third
magnitude (2.95) Alpha star just barely loses the title of Ara's
brightest to Beta Arae, which is 0.10
magnitude (about 10 percent) brighter. Shining from a distance of
240 light years, Alpha Arae is a hot, blue, luminous class B (B2)
dwarf. The term "dwarf" does not mean "small" (though some are),
but that the star is an ordinary core hydrogen-fuser like the Sun that is in the most stable part of its
life. With a surface temperature of 22,000 Kelvin, it radiates
2900 solar luminosities into space, which together tell of a star
over three times the radius of the Sun, one that carries a mass of
about 8 times solar. Hot class B stars are common in and close to
the Milky Way as (though in actuality rather rare in space) they
are bright and seen over great distances. Alpha Arae, however,
falls into a special subclass known "Be" (B-emission") stars, as it
radiates emission lines from hydrogen from a thick disk that
surrounds it. Moreover, it is in a subclass of Be stars called
"shell stars," as the disk is nearly edge on, obscures part of the
star, and superimposes absorptions on the star's spectrum. Alpha
Arae's disk is related to its huge rotation velocity of 300
kilometers per second (coupled to its wind, which causes the star
to lose mass at a rate of over a tenth of a billionth of a solar
mass per year), though the actual process that makes the disk is
still not known. Though the rotation speed is only a lower limit
(the tilt of the axis not known), it cannot be much higher since
the disk's thickness makes it appear to be nearly edge-on. There
is a suggestion of a close-in companion that has never been
confirmed. Alpha Arae does, however, seem to have a distant class
K companion at least 4100 Astronomical Units away that -- if it is
really a companion -- takes
at least 94,000 years to make an orbit. The star has the
reputation of being "eruptive," meaning that it varies its
brightness, which is typical of such shell stars. Perhaps at times
it indeed is the brightest of its constellation. Just under the
limit at which stars explode, it will die as a massive white dwarf.
Updated by Jim Kaler 5/11/07. Return to STARS.