Expert: Tom Whiting - 1/5/2007

Question
what r the steps leading to a supernova?

Answer
Hi Dylan.....
Supernovae status is directly related to the starting mass of
the star.  (We're talking only the very, very  massive stars that end their life as a supernova...that one in a thousand, because the most common stars are the little red dwarfs).

Virtually all stars of 10 solar masses will eventually
go supernova.  And almost virtually all star of 5-10 solar
masses will also end up as supernovae.  On the other hand,
virtually all stars of less than 3 solar masses (including the
sun of 1 solar mass) will not go supernova -- they go the
gentle Red Giant/white dwarf/black dwarf route to death.
Currently the big "problem" is those stars between 3 and
5 solar masses....do they, or don't they?  Well, since it's so
close, it will depend upon the star's composition, rotational
rate, companions influencing, etc...other factors, so we just
don't know ahead of time.

Ok, for the steps...the mechanism...

As you probably know, all stars start their life (and spend
95% of their life) fusing hydrogen nuclei to helium nuclei
in their cores, to produce energy.  In low mass stars, this
reaction is called the "proton-proton" chain (sound familar
from your notes and lectures?).  In more massive stars,
from slightly bigger than the sun up to the massive, hot
blue-white giants, this process is called the CNO cycle.
So all stars (that are on the main-sequence of the H-R diagram
are producing energy by fusing H to He) are creating a
"Helium nuclei ash" in the center of their cores.

Eventually the hydrogen nuclei in the cores runs low, the
core contracts (causing the outer layers to expand into a
red giant), driving the core temp up to well over 100 million
degrees and this is now hot enough to start a new nuclear
reaction called the "triple alpha process" whereby the
core quickly fuses  3 helium nuclei to a carbon nuclei to produce energy. (Why the word "alpha"? - because in nuclear physics, a helium nuclei is also called an alpha particle.)

Ok the next round of nuclear reactions is the key here....once
the He is fused to C nuclei, the core once again contracts
to drive the temperature up, trying to start another round
of reactions, up to 600 million degrees; but the low mass
stars like the sun can't reach that temperature, so they end
their life with a very hot core (white dwarf) embedded in
neblosity...called a planetary nebula....eventually the outer
gas dissipates, and we are left with a "dead" inactive white
dwarf, which eventually cools into a black dwarf...a dead
carbon cinder.
But the more massive stars, 5 solar masses or higher, can
heat up their core to the required temperature and start a
new round of reactions, called alpha capture reactions.
Now in the last few % of the life of a massive star, carbon
nuclei joins with a helium nuclei to form an oxygen nuclei
and so on up to the iron nuclei, forming all the elements
up to, and including iron.  All these reactions are exothermic,
producing energy (to keep gravity from collapsing the star).
But.....once you have an iron nuclei core....there's a problem.
Any reactions involving iron nuclei...be it fission or fusion...
are endothermic, REQUIRING energy.  The star's in big
trouble, as elements merge with iron nuclei taking energy
from the core.  The core rapidly contracts,driving the
temperature still higher, which actually makes the iron
reactions proceed even faster. (Note-- the rolling snowball
effect here)!!   With no energy to support
the outer layers, they all come crashing down (by gravity) onto the super-hot (I mean 100-900 billion degrees hot by then) core, and basically the star implodes.  All that incoming
plasma onto the superhot core immediately fuses, the core
rebounds, and we see a supernova as the implosion becomes
an explosion.  (So production of iron nuclei in the core are the key to the supernova process).
There can be 3 outcomes to the core in a supernova....the
core can be completely disrupted by the implosion, leaving
nothing but hot gases and dust.
Or, the core can collapse down to a neutron star, or in some
cases, the neutron star can even be crushed down to
a black hole....but the death of a massive star is another story.
Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA