By Jim Kaler

Gliding nearly overhead in northern autumn nights, Cassiopeia rules the most northerly reaches of the Milky Way and is the counterpart to the Southern Cross, which marks the galactic disk's most southerly stars. Cassiopeia, the mythical Queen (married to Cepheus), is instantly recognizable by the upside-down "W" made of her brightest stars, which from west to east are Beta, Alpha, Gamma, Delta, and Epsilon. Add a couple more (Kappa and Iota) and you get her Chair, from which she ruled and made the gaff that got Andromeda in trouble. Oddly, one of the brightest of them, Gamma, has no proper name (also true of Kappa and Iota). She can be an angry queen. Of the 10 naked-eye supernovae humanity has witnessed over the past two millennia, three have taken place within her heavenly embrace, the rest scattered across the sky. And it's not just number, as Cassiopeia has the honor of highlighting the two fundamentally-different kinds of supernova. Moreover, there are a remarkable number of stars waiting within her boundaries to add to the list.

The best known among of Cassiopeia's supernovae, perhaps the most famed of them all (though the "Guest Star" of 1054 that produced the Crab Nebula in Taurus can certainly make a claim), is Tycho's Star of 1572. While Tycho Brahe, the last and greatest of the pre-Galilean astronomers, did not discover it (the brilliant star being as bright as Venus and obvious to anyone), he studied and followed its progress. Such was the renown of the "new star" that it was included in Bayer's great Atlas, the Uranometria of 1603, even though it had long since faded away. The most recent of the trio was discovered not by visual observation, but as the brightest Galactic radio source, Cassiopeia A. (In the early days of radio astronomy there were so few known discrete sources that they were named after their constellations of residence. The scheme did not last long.) Cas A is a supernova remnant, or SNR, an expanding shell that consists of stellar debris mixed in with the hot shock wave produced by the explosion. The event itself may have been seen by JohmFlamsteed in 1680 and later recorded as 3 Cassiopeiae. If so, and the sighting is vigorously contended, it must have hit around sixth magnitude.

The two have different origins. Cas A is a Type II supernova that was produced by the explosion of a massive star. A star initially runs on, and is supported by, the fusion of hydrogen into helium in its deep core. When the fuel runs out, the core contracts, while the outer envelope expands to create a giant or, in the case of a massive star, a supergiant. When the core gets hot enough, the helium fuses into carbon and oxygen. In the case of lower mass stars like the Sun, the envelope is ejected and briefly forms a planetary nebula. The exposed core becomes a white dwarf with an average density of a ton per cubic centimeter, the exemplar the faint companion to Sirius. As birth mass increases, so does the mass of the remnant white dwarf. A white dwarf can have a mass not larger than 1.4 times that of the Sun, else it will collapse. That limit is hit with a birth mass of about 8 to 10 times that of the Sun. However, above eight or so solar masses, gravitational compression makes stars hot enough inside to fuse their carbon/oxygen cores into heavier elements (neon, magnesium, sulfur, silicon) until the sequence arrives at iron. At that point no further energy can be extracted. The iron core catastrophically collapses to form a neutron star, while the resulting shock wave blasts the rest of the star apart. Temperatures climb high enough to create all the chemical elements, including a tenth of a solar mass of iron, which are then launched into the interstellar gases to be incorporated into the next stellar generation. If the birth mass is high enough (40 Suns?), the neutron star turns into a black hole. Cassiopeia's other supernova, that of 1181, was also a Type II event, as was the 1054 explosion that made the Crab.

Tycho's Star was most likely a Type I (now Type Ia), for which there are two routes to grandeur. In the first, an ordinary star (most likely a red dwarf) has been driven close enough to a compact white dwarf that it's subject to tides so severe that it sends matter over to the white dwarf. When the fresh hydrogen erupts in a flash fusion reaction, we see a fairly common nova. But if the white dwarf is so massive that it can be pushed over limit before it blows off steam as a nova, the white dwarf collapses and annihilates itself, leaving nothing but a violently expanding SNR behind filled with freshly-made chemical elements and even more iron than is made in a Type II explosion. Alternatively, two white dwarfs in a binary system draw together and merge. If the resulting mass is more than 1.4 Suns, it again goes bang in the night. We don't know which kind is dominant.

Which of Cassiopeia'a bright stars is destined for glory? At the center of the "W," just 550 light years away, Gamma Cas is probably the most interesting. A classic "Be" star (B0.5, almost class O), one with a surrounding orbiting disk that radiates emissions of hydrogen (hence the appended "e"), Gamma spins with an equatorial speed of 400 kilometers per second. Although Alpha Cas (Shedar) is the accepted luminary, Gamma is erratically variable (a characteristic of Be stars), has reached close to first magnitude (topping Alpha), and can dip as low as third. With a luminosity of around 65,000 solar luminosities and a mass of 20 Suns, it has little choice but to explode; the resulting supernova might be as bright as a quarter moon. Fifth magnitude Rho Cas, a yellow hypergiant with a luminosity of perhaps half a million suns and fourth magnitude Kappa Cas (a B1 supergiant in the Chair), may reach 40 solar masses and (who knows?) could collapse into black holes. Without a Greek letter, fifth magnitude HR 8752, well to the west of the "W," might be in that exalted rank as well. In front of the Chair, Flamsteed's 6 Cas, with a luminosity of nearly 200,000 suns, follows at 25 solar masses, while at just slightly lower mass we meet up with Phi Cas, a class F0 supergiant (even hypergiant) with a luminosity of 200,000 suns. Near 8-10 Suns we find Zeta and Sigma, which if they don't blow, will make massive white dwarfs near the theoretical limit of 1.4 suns.

The astronomers' problem with many of these massive stars is their distances. Most are too far for parallax measures to be even close to accurate. We then have to rely on memberships in groups. Phi Cas, for example, is often accepted as the brightest member of NGC 457, the "Owl Cluster," an open cluster that lies some 8000 light years away. But the association is tenuous at best, and the alignment is probably accidental; note Aldebaran's positon in front of the Hyades. Others are assumed to be members of large OB associations of hot massive stars, HR 8752 belonging to Cepheus OB1, 6 Cas to Cas OB5, and so on. Such distances not very reliable. Nevertheless, Cassiopeia, as history well shows, is a phenomenal breeding ground for core-collapse supernovae. And that does not even count the possibility of the white-dwarf variety of supernova, whose progenitors are much fainter and hard (if not impossible) to locate prior to explosion.

Excluding the progenitor of Cas A and the recent supernova SN 1987a in the Large Magellanic Cloud, bright supernovae blast the sky roughly every couple of centuries. We have not seen one in our Galaxy since 1604, when Kepler's Star illuminated Ophiuchus. We are obviously due for another. But since such events are random, we might go hundreds of years before next big one. The best odds, however, seem to lie within the arms of the Queen.

Copyright © James B. Kaler, all rights reserved. These contents are the property of the author and may not be reproduced in whole or in part without the author's consent except in fair use for educational purposes. First published in the August 2015 Newsletter of the Lowestoft and Great Yarmouth Regional Astronomers, who are gratefully acknowledged.