[AGarvin]

Hi Pete,

No problems with the questions. If I can answer I'll will, if not .... I'll stay out of it.

The onset of the CNO cycle is related to the level of impurities in the star (ie, heavy elements) and temperature. I think (?) it's at around 2 solar masses that the CNO cycle begins to dominate in main sequence stars. It plays no role in a white dwarf as a white dwarf is a dead star. It's the left over core of a star after it's red giant stage. There is no fusion going on in the star and they're held up from collapse by what is call electron degeneracy pressure (a quantum thing based on the Pauli exclusion principle). It's generally believed that stars below 8 solar masses will end up as white dwarfs. Basically they're not big enough to produce enough pressure and heat to allow fusion beyond carbon. The core ends up a white dwarf and the the outer layers just slowly blow away and leave behind a wonderful planetary nebula for us to observe. The Sun will end up a white dwarf at the end of its life. Stars above 8 solar mass are believed to end up going supernova leaving behind a neutron star or black hole. They are large enough to allow fusion beyond carbon. When they hit iron fusion stops as energy has to be supplied to fuse beyond iron. Splitting atoms (fission) beyond iron is what produces energy, eg, atomic bomb.

Cheers,
Andrew.