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.