ZETA ARA (Zeta Arae). Directly to the south of Scorpius, within the thick star fields of the Milky Way, lies the deep southern, yet ancient,
constellation of Ara, the Altar, its mythology surprisingly rich. Ara's
most northerly named and second brightest star (mid-third magnitude
Alpha Arae), 50 degrees to the south of
the celestial equator, is invisible to
anyone living to the north of the 40th parallel. Beta (just a bit brighter and the luminary)
and Alpha Arae are followed in brightness not by Gamma, but by Zeta
Arae (not surprising as the brightnesses of the top four stars are
not all that dissimilar from one another). Another class K (K3) giant, Zeta would not be of much
mention were it not also third magnitude (3.13) and readily visible
(if one is far enough south). For all its obscurity, it does have
a couple other things going for it. Fairly distant, 486 light
years away (give or take 14), the star seems fairly well obscured
and a bit reddened by intervening interstellar dust, from the
calibrations of color against spectral class by a full magnitude.
(Interstellar dust absorbs and scatters light better at shorter
bluer wavelengths than at longer redder ones. Obscured stars thus
look redder than they ought for a given spectral class, from which
one can evaluate the degree of total dimming.) That is, were no
dust present, we would see a mid-second magnitude (2.11) star. But
that presents a puzzle, since Zeta Arae does not seem far enough
away for such a significant obscuration. Given it is real,
however, the star shines at a hefty 4920 Suns from a 4350 Kelvin surface, which leads to
a radius of 123 times solar, Zeta quite luminous for its class.
Assuming that there is no dust at all drops the luminosity and
radius to 1920 and 77 times that of the Sun. Interferometry
apparently decides the issue. The measured angular diameter of the
star combined with the accurate distance gives a radius of 128
solar, closely in line with the dust-obscured value. Luminosity
and temperature combined with theory then tell of a massive star
carrying between 6 and 7 solar masses, most likely six, one quietly
fusing its core helium into carbon ad oxygen. Too light to explode
as a supernova (which
takes at least 8 or 9 solar masses), the star will end its life as
a white dwarf (the old
nuclear burning core, which will have lost its outer envelope) with
a mass close to that of the Sun and similar to Sirius B.
Written by Jim Kaler 7/13/12. Return to STARS.