It is found that typical there is an energy release of about 1051
ergs during a supernova. Not all nuclear reactions that could take place
within a star can account for such an enormuous amount of energy. Finding suitable
reactions puts very strong constraints on the type of star that explodes. And
in this case not anly the amount of energy that is released is important. Also
the timescale is of importance. Take for example a star like our sun. When the
sun reaches the evolutionary stage where he ascends the giant branch for the
first time, he has released an amount of energy that is many, many factors of
ten larger then the required SN-energy, but this release took place in a very
long time. So, not only the amount of energy is important; for a SN this energy
is released in a timescale of 1-100 s and this makes it for example impossible
to assume that suddenly all hydrogen in the star burns through p + p -> d +
(beta)+ + (neutrino). This would yield the large supply of 6*1018
ergs g -1, but this energy cannot be released quickly enough
because the reaction is a slow one, even at high temperatures. Quick reactions
involving hydrogen require other light nuclei, like 12C, 14N,
16O or 20Ne. At temperatures of about 108 K
addition of hydrogen to this element results in processes that take place at
a sufficient high rate. The problem in this case however is the limited amount
of available light nuclei. For example, if you calculate the energy release
for a star like our sun (with an effective surface temperature of Tsurface
~ 5700 K) under the assumption that for some reason the temperature has
risen to a temperature of 108 K, you find an energy release of about
1049 erg. (calculation)
In this calculation it is assumed that the whole sun explodes at once, which
is off course not a very realistic situation. So, apparently solarmaterial is
not potentially very explosive. Only under the assumption that the number of
light nuclei is comparable to the number of hydrogen-atoms, it is possible to
obtain supernova-energies.
The light nuclei themself however are potentially very explosive, no matter
whether hydrogen is present or not. The only thing you need is temperature that
is high enough. At a temperature of about 1.5*109 K, pure
carbon is very explosive through:
12C => 20Ne + (alfa), and
212C => 23Na + p
This reactions take place at a timescale as short as 1 s and the energie yield
is ~5*1017 erg g -1. Also 16O, 20Ne
are unstable at a bit higher temperature, as are magnesium and silicium and
elements of similar atomic weight at about 2.5*109 K. In all
cases the energie yield is in the order of 1017 erg g -1,
such that in the temperature range of ~2±0.5*109 K a
total energy of about 1051 erg is released if only one-tenth
of a solar mass explodes.
This short considerations are not intended to give a full proof of WDs being the progenitors of SN type IA explosions. Even more, I never mentioned something like a distinction between different types of SNe and indeed the above consideration are constaints that count for every single SN, no matter the type. It shows only that you need heavier elements than just hydrogen to explain any SN. But, already familiar with the fact that the progenitor of a type IA SN must be a CO WD, at least it is now clear that indeed such an object can give rise to SN energies. And more that that, it eliminates some other possibilities. For example in star that was just born there isn't enough of this light nuclei present, so such a star can never explode as a SN. Also some other possibilities like brown dwarfs and main sequence stars in their stable phase are ruled out.
In the next parts I'll take an extensive look at the spectra of type IA SNe in different stages of the explosion. These spectra will tell a lot about the processes that are going on, but also about the progenitors that must be involved.