6 CAS (6 Cassiopeiae). The Star of the Week celebrates its 300th
star with a magnificent
white class A (A3) supergiant, one of the most luminous stars of
the Galaxy. Don't let its apparent fifth magnitude (at 5.43, near
sixth) dimness fool you. At a distance of roughly 8100 light
years, it is hugely far away, and partly hidden within Cassiopeia by the Milky Way's interstellar
dust that dims it by 1.9 magnitudes (a factor of 5.75). Were the
dust not there, the star would shine at an easily-visible near-
third magnitude (3.57). The star is too dim, however, to have
obtained a Greek letter name (much less a
proper name), but was bright enough for John Flamsteed to have recorded
it as star 6 in the constellation of the Queen. 6 Cas is much too
far away for parallax measure, so we have to use indirect means.
It is a member of the huge loose "Cas OB5" association of hot O and
B stars (which were born at roughly the same time) that includes Rho Cas. From estimates of the distances of
the members based on their spectra and thus assumed luminosities,
we find the association's distance, which for any specific star
becomes rather problematic. Nevertheless, it is all we have.
Assuming 8100 light years for our "6," and correcting for
interstellar dimming and a small bit of ultraviolet radiation from
the roughly 9000 Kelvin surface, we find a magnificent luminosity
of 195,000 Suns!, which yields a radius 170
times that of the Sun (0.8 Astronomical Units, bigger than the
orbit of Venus), and from the theory of stellar evolution a mass of
25 times solar, all these statistics placing 6 Cas in league with
such stars as first magnitude Deneb.
Rotating with a minimum speed of 50 kilometers per second, the star
may take as long as 170 days to make a full spin. Like most such
immense stars, 6 Cas is slightly variable (by 0.09 magnitudes), and
is classed as an "irregular," with no apparent period. It is also
possessed of a variable strong wind that ejects matter in blobs at
speeds of up to 180 kilometers per second. Such huge stars evolve
fast, 6 Cas a mere 6.4 million years old. Its only fate is to
develop an iron core and then to collapse and explode as a superb
supernova, creating a
spinning neutron star in
the process, one of immense density and the size of a small town.
If that is not enough, 6 Cas seems to have a companion 1.4 seconds
of arc away, an eighth magnitude class A2 star, and if really a
companion and not a line-of-sight coincidence, also a supergiant,
but of lesser quality, one that shines with a brilliance of "only"
19,000 Suns, rendering it 10 solar masses, and on the border
between producing a supernova or a rare neon/magnesium white dwarf.
(Most white dwarfs are
made of carbon and oxygen; the massive end of the white dwarfs have
fused their stuff to heavier elements). If the companionship is
real, the two are separated by at least 3500 Astronomical Units and
take at least 35,000 years to orbit. Given this distance apart,
from 6 Cas proper the companion would appear with the brightness of
1600 full Moons, while from the companion, 6 would shine 1/100 as
bright as the Sun in our Earthly sky.