MU CEN (Mu Centauri). Think "Centaurus," the celestial Centaur, and the first thing that comes to mind is Alpha Centauri (Rigil Kentaurus), the nearest star, or at the opposite end of the alphabet, Omega Centauri, not a star but a vast collection of them, a globular cluster containing more than a million stars. Most of the rest tend to be ignored, including a fine pair toward the constellation's northern portion, Mu and Nu Centauri, which lie to the northeast of Omega. Just 3/4 of a degree apart, they may look like some sort of naked-eye, similar-class double (Nu to the north), but they are not. At a distance of 505 light years, third magnitude (3.04) Mu is 68 light years farther away than Nu (from each, the other appearing as a minus first magnitude star). Nevertheless, they are indeed related, as both belong to the huge, gravitationally unbound and expanding "Upper-Centaurus-Lupus" association of hot blue stars. And this hot (22,700 Kelvin) class B (B2) subgiant-dwarf (but as in so many cases, see below) fits right in. After a 0.22 magnitude correction for dimming by interstellar dust (common in stars near the Milky Way) and a large correction for ultraviolet radiation, we find a large luminosity of 7180 times that of the Sun, consistent with the star's class. Luminosity and temperature then give a radius 5.5 times solar and, with theory, a mass of 9.1 times that of the Sun. A projected equatorial rotation velocity of 156 kilometers per second (again consistent with class) yields a rotation period under 1.75 days. Rather than being a subgiant, a star that has aged to the point that it has ceased (or will shortly cease) core hydrogen fusion, Mu Cen (like Nu Cen) is really a fairly young hydrogen-fusing true dwarf about midway through its 25 million-year dwarf-life. Other studies suggest it may be even more luminous and younger. Such confusion within the class of hot B stars is common and has more to do with stellar taxonomy than the stars' physical natures, the two quite independent of each other. There seems to be a small companion separated by 5 seconds of arc, but little if anything is known about it. Another (13th magnitude Mu Cen B) some 45 seconds of arc distant, is clearly just a line-of-sight coincidence. More important, Mu Centauri is an "emission-line" ("Be") star with a circumstellar disk presented more or less edge- on, which makes it more opaque and a so-called "shell star." Like many such (Gamma Cassiopeiae comes to mind), Mu Cen is variable, changing between magnitudes 2.9 and 3.4 with no discernable period.
Written by Jim Kaler 8/21/09. Return to STARS.