HR 8752 Cas (HR 8752 Cassiopeiae). And now for something completely different, a huge massive star, one of the brightest in the Galaxy. Or not. The thing seems almost unknowable. The name comes from the Yale Bright Star Catalogue, though the "HR" is from "Harvard Revised," curious to start with. "Bright Star" is relative, as it shines at a fairly dim fifth magnitude (5.1) in, of course, Cassiopeia. An unstable irregular variable also known as V509 Cas, it changes between magnitudes 5.0 and 5.2. A monster class G (G4) hypergiant (or G0 supergiant), it seems to be a part of a huge collection of hot, luminous stars called Cepheus OB1 (never mind that the star is in Cassiopeia, as the expanding group sprawls all over). As such, it would lie 11,000 light years away. Given 1.7 magnitudes of interstellar dust absorption and a temperature-guess of 6000 Kelvin, it then appears to radiate at a rate of 415,000 times that of the Sun, making it one of the most luminous stars of the Galaxy. Those figures translate into a star with a radius of three Astronomical Units -- as big as the main asteroid belt -- and an initial mass of 40 times that of the Sun. After ceasing core hydrogen fusion, it seems first to have evolved to the red supergiant state. A fierce wind then exposed the innards as it heated to become a "yellow hypergiant" rather like Rho Cassiopeiae, the mass being cut in half, the star bouncing against the "yellow evolutionary void" and unable to get much hotter. Consistently, HR 8752's temperature has been increasing, starting at 4300 Kelvin in 1953 and going to 7200 K in the mid-nineties. During the same time, the spectral class has flopped around from G4 to F8. Which gives us as bit of a problem, since if you can't rely on temperature then you can't trust the results that depend on it, so precision is lost. The mass loss, well over a millionth of a solar mass per year (100 million times the flow rate of the solar wind), has produced a huge shell that surrounds the star. A rotation velocity of 35 km/s (which is probably wrong) gives HR 8752 a rotation period of 2.5 years or under. That's the standard. However, all's not well, as we have a problem with distance. The original Hipparcos satellite parallax reduction was unable to measure it. Recent re-evaluation of the data puts the distance of the star at 4500 light years, far closer, which gives a luminosity of "just" 35,000 Suns and a mass of 15 solar, vastly less than earlier thought. On the other hand, the stated error is large. Plus, small parallaxes tend statistically to be too large, the star estimated to be too close. Moreover, stars in this mass range do not return from being red supergiants to become yellow supergiants. From all the information, the larger distance seems the more likely. But whatever the case, HR 8752 will almost certainly blow up as a huge supernova. And if nothing else, it humbles our attempts to learn about it and for that matter about others of its kind.
Written by Jim Kaler 9/18/09. Return to STARS.