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.
Written by Jim Kaler. Return to STARS.