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.
Written by Jim Kaler. Return to STARS.