BETA CRA (Beta Coronae Australis). Constellations are commonly
characterized by their luminaries, their brightest stars, Bootes by Arcturus, Lyra
by Vega. While a few figures are famed for
pairs of stars, Orion by Betelgeuse and Rigel, Centaurus by
Alpha and Beta Cen, one of them usually stands out. Corona Australis, the Southern Crown, is
unusual in having not one but two actual luminaries, two stars --
Alpha CrA (Alfecca Meridiana) and Beta
CrA -- that are so close in brightness that the human eye cannot
tell which is on top (not that either is exactly bright, both mid-
fourth magnitude). Which one is "brightest" depends on the source
of data. The "Bright Star Catalogue" has them both at magnitude
4.11, while the Hipparcos satellite places Alpha at 4.10, Beta at
4.12, a big 0.02 magnitudes (about 2 percent). Other sources nail
them at the same to the thousandth of a unit. The result is a
constellation that effectively has two luminaries, one near (Alpha
at 130 light years), one far (Beta at 510). Though a common kind
of star, an orange class K (K0) giant, Beta CrA stands out a bit as
a "bright giant" with a temperature of 4570, a high luminosity of
730 times that of the Sun, a radius of 43
solar (0.20 Astronomical Units, half the size of Mercury's orbit),
and a mass that is either 4.5 solar (if the star is just beginning
to brighten as a red giant
with a quiet helium core) or 5 solar (if Beta CrA is past that
stage and is a stable helium-fusing "clump star," which is much
more likely). All alone, with no binary companion, some 100
million years ago it began life as a hot class B star. Dreadfully
neglected, it has been mentioned in only 16 professional research
papers in the last 50 years! The star's most interesting
characteristic perhaps is its proximity to the Corona Australis
Molecular Cloud, a dusty, dark star-forming region of 7000 solar
masses that lies near the northern arc of the Crown's curve and is
known for a complex set of bright "reflection nebulae" (their dust
reflecting the light of embedded stars). Beta CrA, while not a
part of the cloud, is a mere 20 light years off the end of the
elongated dusty blob, which from the star's perspective would block
a very large part of its local Milky Way from view.