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.