MU PER (Mu Persei). Perseus is so filled with hot blue stars that it's a bit jarring to come across one that is not, especially one that's a rather rare supergiant that falls in or near the so-called "Hertzsprung Gap" of the HR diagram (that famed plot of absolute magnitude -- what the apparent magnitude would be at 32.6 light years -- against spectral class) where there are not so many stars, especially class G (G0) supergiants. Shining at a modest fourth magnitude (4.14), Mu Persei is dimmed by 0.64 magnitudes by interstellar dust, giving us a true magnitude of 3.50, on the border of third, the relative faintness the product of a large distance of 900 light years (the absolute magnitude then coming in at -3.70, rather faint for the class). A measured temperature of 5337 Kelvin, consistent with spectral class, tells of little infrared radiation, and thus with distance gives a substantial luminosity of 2030 lsun.html">Suns and a radius of 53 times solar. A projected equatorial rotation speed of 9 kilometers per second then leads to a rotation period that could be as long as 300 days. The mass depends on the state of evolution. If the star is "crossing the HR diagram," cooling with a dead helium core, it weighs in at a hefty 6 solar masses. If it has already begun stable helium fusion, then the mass drops to 5.7. In either case, it began life as a hot blue (around class B3) dwarf some 60 or 70 million years ago. Not massive enough to explode, Mu Per will die as a heavy white dwarf with a mass between 0.9 and 0.95 times that of the Sun, not far off that of Sirius B. Mu Per falls into a regions of temperature and luminosity where we find Cepheid variables (regular substantial pulsators like Delta Cephei). While slightly variable (between magnitudes 4.1 and 4.3), as expected for a supergiant, there is no evidence that the star is of the Cepheid variety. Tucked in close to it is a spectroscopically-detected companion, most likely a class B9 or so dwarf, that orbits with a period of 0.78 years. Given the stars' masses, Kepler's laws then yield an average orbital size of 1.7 Astronomical Units. Mu Persei is also surrounded by a trio of so- called visual companions, 10th magnitude Mu Per B at a separation of 16 seconds of arc, 10th magnitude Mu Per C at 84 seconds, and 13th magnitude Ab at 49 seconds. If B were a real companion, it would be an A-F dwarf that orbits at a distance of at least 4500 AU with a period of at least 95,000 years. However, from their motions, all three appear to be just line-of-sight coincidences. The star is, after all, in the heart of the Milky Way, so there are lots of faint stars around to fool us.
Written by Jim Kaler 10/16/09. Return to STARS.