RHO CAS (Rho Cassiopiae). Cassiopeia is
full of bright stars, yet precious few have proper names. Even
very bright Gamma Cassiopeiae has none,
at least in western lore. Pity then the seemingly lesser stars,
which have no chance at all. At least in one spectacular case,
however, the "lesser" tag is totally wrong. Look to fifth
magnitude (4.54, just over the line from fourth) Rho Cas, way down
on the Bayer Greek Letter list. From its
membership in the vast Cas OB5 association of O and B stars (which
it shares with the bright supergiant 6
Cas), Rho is estimated to be an amazing 8100 light years away.
Greater even than a supergiant, Rho is a class G (G2, some say F8)
"hypergiant." Dimmed by two magnitudes by interstellar dust, it
still shines at near-fourth magnitude, radiating 550,000 times more
light than the Sun from a surface measured
at 7300 Kelvin, the star's energy mostly pouring out in the visible
part of the spectrum. The temperature and luminosity tell of a
distended surface 450 times larger than the Sun, one 4.3
Astronomical Units across, 40 percent larger than the Martian
orbit. Rotating at least at 29 kilometers per second, Rho Cas
could take up to two years to make a full spin. Though it has no
companion from which to gauge its mass, the immense luminosity
suggests roughly 40 times solar. Theory shows that hydrogen-fusing
dwarf stars from 10 to about 60 solar masses evolve from blue class
O first to become blue supergiants and then into red class M
supergiants. From around 40 to 60, however, they loop back,
turning from red supergiants back into much hotter and smaller blue
supergiants. Higher than 60, they bump into a wall and stay as
blue supergiants. Rho Cas now seems to be on its way back from
being a red supergiant, when it may have been some five times
larger. If so, it is bouncing against the "yellow evolutionary
void," in which stars become unstable and do not like to linger.
And Rho Cas certainly is unstable. It is an irregular variable, or
at best a semi-regular, and seems to have multiple periods of 820,
350, 510, and 645 days. But these change, so the star may really
be quite unpredictable. In the summer of 1946, Rho took a dive
from 4th to 6th magnitude and, more remarkably, altered its
spectral class. Pumping a huge amount of gas into an expanding
thick atmosphere, it seemed to chill to become a cool M star. A
year later, it was returning to normal. The star did not so much
dim as cool, the lower temperature causing it to place much of its
radiation in the invisible infrared. In its more stable state, Rho
Cas still blows a 10 kilometer per second wind at a loss-rate of a
hundred thousandth of a solar mass a year, a hundred million times
the flow rate of the solar wind. It does not have much time left
before it grows its iron core and blossoms into the sky as a
stunning supernova. Thanks
to Brian Heard, who suggested this star.