Physics of a Type II Supernova
Stars of low mass live very long lives
because they burn their fuel relatively slowly compared to massive stars
that expend their fuel quickly. This is due to the higher gravity inside
a massive star that the thermonuclear reactions must keep up with to maintain
the mechanical equilibrium of the star. All stars go through a phase of
stellar evolution when they are converting hydrogen to helium called the
main sequence. This period is the longest, most stable time in a stars
life. Current models show that after the main sequence stars of masses
lower than eight solar masses go through a series of expansions, and contractions
as they start to fuse heavier and heavier elements (Kaufmann & Freedman,
550). But, these stars do not have enough gravity to contract and reach
the temperature needed to create iron. Eventually they shed their outer
layers leaving a mostly carbon core, a white dwarf. These stars can remain
in gravitational equilibrium indefinitely (Begelman & Rees, 29).
As a massive star runs out of hydrogen
to convert to helium it moves off the main-sequence in a series of contractions
and expansions similar to a smaller mass star. The star contracts when
it runs out of the fuel it is burning in the thermonuclear reaction at
its core. The contraction causes the temperature to rise in the core until
it reaches the necessary temperature to accelerate the particles in its
core to speeds high enough to overcome the electric repulsion between the
protons and start the next reaction in the chain (Begelman & Rees,
31). During this process of contraction and expansion the star can eject
much of its mass. If the star remains massive enough it will move through
the chain of reactions up until iron.
No further fusion reactions are possible
beyond iron without an input of energy. The core collapses under the weight
of the star’s outer layers. The outer layers fall inward as the pressure
in the core suddenly drops. As the core reaches the density of an
atomic nucleus the electrons and protons are forced together creating neutrons
and releasing on the order of 10^57 neutrinos (Begelman & Rees, 33).
Neutrinos are particles that react with other particles very infrequently,
but the 10% that do interact with another particle on their way out of
the core may push the pressure wave through the star’s outer layers (Begelman
& Rees, 34).
At this point the core becomes stiff
because the neutrons cannot be compressed any further. The outer layers
of the star are reflected back outward. The pressure wave that is created
compresses and carries the outer layers of the star out into space. This
enormous pressure and energy input from the neutrinos that interact with
other particles, heats the material traveling in the pressure wave to temperatures
high enough to fuse elements heavier than iron (Kaufmann & Freedman,
551). This is the only place in any natural system that elements heavier
than iron can be created (Kaufmann & Freedman, 551). When the pressure
wave reaches the surface of the star it blows off the outer layers into
space in a supernova explosion. At this point the supernova becomes visible.
The energy released in a supernova explosion is about 10 times the amount
of energy released over the whole life the star (Begelman & Rees, 32).
Gravity is the force that releases the majority of a stars energy over
its whole life, not its thermonuclear reactions (Begelman & Rees, 32).
It does so within the last seconds of its life creating a supernova.