1. Supernovae
A supernova is an explosion of a massive supergiant
star. It may shine with the brightness of 10 billion
suns! The total energy output may be 1044 joules, as
much as the total output of the sun during its 10
billion year lifetime. The likely scenario is that
fusion proceeds to build up a core of iron. The quot;iron
groupquot; of elements around mass number A=60 are
the most tightly bound nuclei, so no more energy
can be gotten from nuclear fusion.
Crab supernova remnant
In fact, either the fission or fusion of iron group elements
will absorb a dramatic amount of energy - like the film of a
nuclear explosion run in reverse. If the temperature increase
from gravitational collapse rises high enough to fuse iron,
the almost instantaneous absorption of energy will cause a
rapid collapse to reheat and restart the process. Out of
control, the process can apparently occur on the order of
seconds after a star lifetime of millions of years. Electrons
and protons fuse into neutrons, sending out huge numbers
of neutrinos. The outer layers will be opaque to neutrinos,
so the neutrino shock wave will carry matter with it in a
Cassiopeia A supernova
cataclysmic explosion.
remnant
Supernovae are classified as Type I or Type II depending upon the shape of their light
curves and the nature of their spectra
Type I and Type II Supernovae
Supernovae are classified as Type I if their light curves exhibit sharp maxima and then die
away gradually. The maxima may be about 10 billion solar luminosities. Type II
supernovae have less sharp peaks at maxima and peak at about 1 billion solar
luminosities. They die away more sharply than the Type I. Type II supernovae are not
observed to occur in elliptical galaxies, and are thought to occur in Population I type stars
in the spiral arms of galaxies. Type I supernovae occur typically in elliptical galaxies, so
they are probably Population II stars.

2. With the observation of a number of supernova in other galaxies, a more refined
classification of supernovae has been developed based on the observed spectra. They are
classified as Type I if they have no hydrogen lines in their spectra. The subclass type Ia
refers to those which have a strong silicon line at 615 nm. They are classified as Ib if they
have strong helium lines, and Ic if they do not. Type II supernovae have strong hydrogen
lines. These spectral features are illustrated below for specific supernovae.

3. Supernovae are classified as Type I if their light curves exhibit sharp maxima and then die
away smoothly and gradually. The model for the initiation of a Type I supernova is the
detonation of a carbon white dwarf when it collapses under the pressure of electron
degeneracy. It is assumed that the white dwarf accretes enough mass to exceed the
Chandrasekhar limit of 1.4 solar masses for a white dwarf. The fact that the spectra of
Type I supernovae are hydrogen poor is consistent with this model, since the white dwarf
has almost no hydrogen. The smooth decay of the light is also consistent with this model
since most of the energy output would be from the radioactive decay of the unstable
heavy elements produced in the explosion.
Type II supernovae are modeled as implosion-explosion events of a massive star. They
show a characteristic plateau in their light curves a few months after initiation. This
plateau is reproduced by computer models which assume that the energy comes from the
expansion and cooling of the star's outer envelope as it is blown away into space. This
model is corroborated by the observation of strong hydrogen and helium spectra for the
Type II supernovae, in contrast to the Type I. There should be a lot of these gases in the
extreme outer regions of the massive star involved.
Type II supernovae are not observed to occur in elliptical galaxies, and are thought to
occur in Population I type stars in the spiral arms of galaxies. Type Ia supernovae occur in
all kinds of galaxies, whereas Type Ib and Type Ic have been seen only in spiral galaxies
near sites of recent star formation (H II regions). This suggests that Types Ib and Ic are
associated with short-lived massive stars, but Type Ia is significantly different. .
.
The synthesis of the heavy elements is thought to occur in supernovae, that being the only
mechanism which presents itself to explain the observed abundances of heavy elements.

4. Type Ia Supernovae
Type Ia supernovae have become very important as the most reliable distance
measurement at cosmological distances, useful at distances in excess of 1000 Mpc.
One model for how a Type Ia supernova is produced involves the accretion of material to
a white dwarf from an evolving star as a binary partner. If the accreted mass causes the
white dwarf mass to exceed the Chandrasekhar limit of 1.44 solar masses, it will
catastrophically collapse to produce the supernova. Another model envisions a binary
system with a white dwarf and another white dwarf or a neutron star, a so-called quot;doubly
degeneratequot; model. As one of the partners accretes mass, it follows what Perlmutter calls
a quot;slow, relentless approach to a cataclysmic conclusionquot; at 1.44 solar masses. A white
dwarf involves electron degeneracy and a neutron star involves neutron degeneracy.
A critical aspect of these models is that they imply that a Type Ia supernova happens
when the mass passes the Chandrasekhar threshold of 1.44 solar masses, and therefore all
start at essentially the same mass. One would expect that the energy output of the
resulting detonation would always be the same. It is not quite that simple, but they seem
to have light curves that are closely related, and can be related to a common template.
Carroll and Ostlie summarize the character of a Type Ia supernova with the statement that
at maximum light they reach an average maximum magnitude in the blue and visible
wavelength bands of
with a typical spread of less than about 0.3 magnitudes. Their light curves vary in a
systematic way: the peak brightnesses and their subsequent rate of decay are inversely
proportional.

5. The above illustration is a qualitative sketch of the data reported by Perlmutter, Physics
Today 56, No.4, 53, 2003. It illustrates the results of careful study of supernova Type Ia
light curves which has led to two approaches for standardizing those curves. The above
curves illustrate the quot;stretch methodquot; in which the curves have been stretched or
compressed in time, and the standardized peak magnitude determined by the stretch
factor. With such a stretch, all the observed curves on the left converge to the template
curve on the right with very little scatter. Another method for standardizing the curves is
called the multicolor light curve shapes (MCLS) method. It compares the light curves to a
family of parameterized light curves to give the absolute magnitude of the supernova at
maximum brightness. The MCLS method allows the reddening and dimming effect of
interstellar dust to be detected and removed.
Carroll and Ostlie give as an example of distance determination the Type Ia supernova
SN 1963p in the galaxy NGC 1084 which had a measured apparent blue magnitude of B
= m = 14.0 at peak brilliance. There was a measured extinction of A = 0.49 magnitude.
Using the template maximum of M=19.6 gives a distance to the supernova
Distance uncertainties for Type Ia supernovae are thought to approach 5% or an
uncertainty of just 0.1 magnitude in the distance modulus, m-M.