PHI CAS (Phi Cassiopeiae). The stars of Cassiopeia, the celestial Queen, are so prominent that we tend to overlook the many others of considerable interest. Such is the case with fifth magnitude (4.98) Phi Cassiopeiae, whose Greek letter ranks way down the alphabet, number 21 out of 24. Situated between Ruchbah (Delta Cas) and the two Marfaks (Theta and Mu), the star is the subject of some contention. Right on the edge of a prominent open cluster called NGC 457 (popularly called the "ET" or "Owl" Cluster), Phi has long been thought to be the brightest member (and even called NGC 457 no. 136). Or not. As seen in a great many apparent double stars, just because two things line up does not mean that they are related. (As another example, look at Aldebaran and the Hyades.)
Phi Cas Phi Cassiopeia, in the heart of Cassiopeia's Milky Way, lies at the center of the picture between Ruchbah (Delta Cas) down and a bit to the right of Phi and the two Marfaks up and to the left (Theta and Mu, the former the brighter of the two). Gamma Cas shines at lower left, Achird (Eta) near the left center edge. Between the two are Upsilon-1 (the upper) and Upsilon-2. Immediately down and to the left of Phi is the cluster NGC 457, which was thought to be host to the star, but apparently is not, the star lying in the foreground. North is down and a bit to the left.
And the consideration is important, as Phi Cas is of a rare breed, a class F (F0) bright supergiant that has even been tossed into an ethereal bin of stars called "yellow hypergiants." The relative motions between the star and cluster are not sufficiently the same or different to allow a decision. Parallax would seem to settle the case. The cluster is at a distance of 7900 light years, while the star comes in at 2300 light years, making it a foreground object. The statistical error of measurement, however, is perilously high, and there is a significant chance of the star lying as far as 4500 light years and perhaps even as far as the cluster. Using the actual parallax distance and an allowance of 1.7 magnitudes of absorption by interstellar dust gives an absolute magnitude for the star (what the magnitude would be at 32.6 light years) of -6.0, not bright enough to make the ranks of bright supergiants (which is around -8.5). Only if we put the star at the cluster's distance does it reach the required brightness. However, at the greater distance, the luminosity would be 215,000 Suns, the mass 25 Suns, and the age a mere six million years, as opposed to the cluster's age of 20 million, so that does not work. If we take the upper parallax limit of 4500 light years, the stellar parameters are a luminosity of 70,000 Suns, a radius of 0.75 Astronomical Units, a rotation period of under 155 days (from a projected rotation velocity of 27 kilometers per second), a mass of 17 solar, and an age of 10 million years (with the star evolving with a dead helium core). The deciding factor seems to be the several companions. Phi Cas B (magnitude 12.3, 49 seconds of arc away) is (from its motion) clearly a line of sight coincidence. Phi Cas C (class B1 Ia, magnitude 7.0, 134"), D (10.2, 179"), and E (10.6, 170 seconds) are all co-moving with Phi Cas proper, and have the most consistent relations among color, class, and absolute magnitude at the 4500 light-year distance, which we accept here. Thus the star is lies in the foreground between us and the more distant cluster. The companions, which were probably born with Phi Cas A, are just sailing along with it. With separations the order of a light year, they would be very fragilly bound, and will most likely escape the more massive star's gravitational grip. Things astronomical are not always straightforward, and can be difficult to unravel. We do know, though, that whatever the case or distance, Phi Cas is bound at some point to blow up as a grand supernova.
Written by Jim Kaler 12/7/07. Return to STARS.