Expert: Tom Whiting - 2/20/2004

Question
I know that nuclear fusion reactions takes place in the core of the sun where the temperature and density are highest.
But why are high temperatures and high density needed?

Thanks

Answer
James,
The high temperature and density are required for hydrogen
fusion to occur because you are combining two or more
positively charged particles, namely protons, which under normal circumstances, will try to repel each other under the laws of 'electrostatic force'. (like charges repel).

So for fusion reactions to be successful, the electrostatic
force must be overcome....the positively charged protons have to be violently forced together so very close, that a new force, the 'strong nuclear force', which works over a very short distance,  binds the two or more protons together....actually, eventually four protons, to create one helium nucleus. (Of course, two of those protons lose their
positive charge and become neutrons).
Thus the need for high temps....read that as very fast atomic motion and high collision velocities...
and densities (lots of atoms to work with in a confined
volume-the core).

This is an exothermic process, thus releasing energy, mainly
in the form of gamma radiation, which eventually degrades
to infrared (heat), and this heat creates gas pressure within the sun, which is what keeps gravitational forces from collapsing the sun (or any other main sequence star) down to a much smaller size.  Thus the fusion reaction inside the core balances off the gravitational collapse,
and thus the sun is in 'balance' or 'equilibrium' and
maintains pretty much it's current size (diameter) and
relatively steady luminosity.

So basically, a main sequence star is nothing more than
a well controlled nuclear fusion furnace, in near-perfect
balance.

 It is interesting to note that above, we were only looking at fusion forming only one helium nucleus, but in reality (and the numbers may have changed slightly since I taught astronomy) but we learned that the sun's core fuses 400 million tons of hydrogen nuclei to 396 million tons of helium nuclei, every SECOND...the resultant 4 million tons loss of mass being converted directly to energy by Einstein's famous equation, E = mc squared.
But there is enough available hydrogen present in, or near,
the core, to give the sun a total lifespan of about 10
billion years, of which it has used up approximately half of the amount originally present.

If a protostar does not eventually achieve, by new matter infall and gravitational collapse, a certain high
temperature (and corresponding high density) in its core,
then nuclear fusion of hydrogen to helium nuclei does not
commence, and the body does not become a "star"...it stops
its evolution as a brown dwarf, or maybe even a Jupiter-sized body. In fact, that is how Jupiter formed....
it ran out of time and material during the accretion phase of our Solar System....no more infalling material available back 4.5 billion years ago, and thus fell just short of being a main sequence little red dwarf star....in which case, we would now be living within a binary system.

Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA