MU NOR (Mu Normae). The stellar world was electrified last week (July 21, 2010) by the likely discovery of supermassive stars in distant dense southern star clusters NGC 3603 and R136, the latter in the Large Magellanic Cloud, a nearby companion galaxy to ours). One, with an immense luminosity of close to nine million Suns, seems to have been born with a mass of some 300 solar masses, far beyond the long-thought maximum of 120-150 Suns. We will see, as astronomers rush to confirm or discount the claim, which is the essence of science. That discovery only serves to highlight our star, fifth magnitude (4.94) Mu Normae, which lies within the obscure modern constellation of Norma, the Square (originally Norma et Regula, the Level and Square), which lies to the southwest of Scorpius. As a blue class B (B0, some say O9.7) supergiant, it is still among the most luminous stars of the Galaxy. And measures agree. Obscured by nearly a magnitude of dust absorption, the result of a grand distance within the Milky Way of 4240 light years (give or take a whopping 1540), if in clear space it would shine at magnitude 4 (4.01). From that and a temperature of some 30,500 Kelvin (to account for a LOT of ultraviolet light), we get a luminosity of half a million times that of the Sun. The uncertainty in distance could halve it, but also allows for a luminosity twice as great, a million Suns. Luminosity and temperature, plus theory, then give a mass somewhere between 25 and 60 times that of the Sun, centering on 40 solar. The latter scenario suggests that the star is not yet a true supergiant but is closing in on the end of its four-plus million year hydrogen-fusing dwarf lifetime. With a radius of 25 or so times that of the Sun, plus a projected equatorial rotation speed of 72 kilometers per second, the rotation period must be under 17 days. Like most such stars, it possesses a powerful wind, one that blows with a surface velocity of 1750 km/s. With a current mass-loss rate that could be as high as a few millionths of a solar mass a year (nearly 100 million times the solar mass loss rate), it has already lost perhaps ten percent of itself back into space. While seemingly single, with no orbiting partner, Mu Normae still has plenty of local companions, as it appears to be by far the brightest of a poorish cluster called NGC 6169, about which almost nothing is known. Indeed, such systems are known as "Mu Normae clusters." Looking farther afield, Mu Nor is also touted as belonging to the Ara (the Altar) OB1 association of hot class O and B stars, such OB associations holding so many of Mu Nor's kind.
Written by Jim Kaler 7/30/10. Return to STARS.