QUEEN OF THE NIGHT
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.