DELTA CEN (Delta Centauri). Northerners have little chance to admire one the grandest, brightest, constellations of the sky, Centaurus, the Centaur. Passing overhead in temperate southern hemisphere latitudes, only the northern tier of this huge and amazing figure is visible from the temperate north. And what a shame for a huge population that cannot know it. Though nearly second magnitude (bright third, 2.60) and ranking number 100 in the entire sky, Delta Cen (of no proper name) still ranks only eighth in the constellation's brightness parade, beaten out by first magnitude Rigil Kentaurus (Alpha) and Hadar (Beta), and then by Menkent (Theta), Muhlifain (Gamma), Epsilon, Eta, and Zeta. And though a magnificent hot class B (B2) subgiant, it is not even the hottest and bluest, beaten out here by Beta, Epsilon, and Eta (Centaurus filled with such stars). In Delta's favor, however, were it not for some intervening, dimming, interstellar dust, the star would shine at magnitude 2.22 and rank fourth. The temperature is hard to gauge. Delta is a rapidly rotating (at least 263 kilometers per second) "B-emission" ("Be") star that is surrounded by a circumstellar disk of its own making (rather like Gamma Cas), one so thick (most likely nearly edge-on) that Delta is called a "shell star." Such stars are often unstable, Delta varying by 15 percent or so; multiple periods the order of a day may be present. Rapid rotation flattens it, which causes the poles to heat and brighten. Though the apparent temperature is 22,400 Kelvin, a more realistic one might be 1000 K higher. At the higher end, the star shines with a radiance 12,000 times that of the Sun, which gives it a radius of 6.7 solar, a rotation period of under 1.3 days, and a mass 10 to 11 solar, depending on just where it is in its evolutionary path, the star (less than 20 million years old) very close to giving up core hydrogen fusion ("subgiant" right on the mark). Delta's apparent relationships seem to have soured. While generally listed as part of the Lower-Centaurus-Crux subgroup of the giant Scorpius-Centaurus association of hot blue stars, recent observations have rejected it. Two "companions" that seem to be moving along with it, the fourth magnitude B6 giant HR 4618 (3.7 minutes of arc to the north) and the sixth magnitude B9 dwarf HR 4619 (2.5 minutes south) are more likely coincidences, the latter star perhaps twice as far away. Delta's fate is uncertain. Right at the limit at which stars explode as supernovae, it could blow up. If not, it will turn into a massive white dwarf, perhaps one made of neon and oxygen rather the ordinary carbon-oxygen mix.
Written by Jim Kaler 5/04/07. Return to STARS.