[PeteMo (Pete)]

Hi Andrew
Thanks for the detailed CNO explanation. It was only briefly mentioned on my course as the dominant process in stars with greater than 8 solar masses. I'm more familiar with the PPI and PPII (Proton - Proton) reactions as my course focused more on our Solar System.

The larger (more massive) the star the higher the core temperatures, the faster the nuclear processes convert lighter elements into heavier elements. I was quite surprised that some of the Super Giants might only be around for a few million years before boom day. Like the old saying goes "A candle that burns twice as bright, burns for half as long".

I was quite intrigued by the times for some of these processes take. Like Alex, I certainly appreciate you taking the time to explain this. Here's some of my notes about the Proton-Proton (PP) chains, so so we have a more comprehensive outline of the nuclear processes that take pace in stars.

The Proton-Proton PPI, PPII and PPIII chains are how Hydrogen is convereted to Helium in the the core of a star. A protostar must reach a mass of at least 0.08 Solar Masses before there will be enough matter and heat to kick off the first PPI chain reaction whereby 2 Hydrogen atoms fuse to create a Deuterium atom. If the mass is less than 0.08 Solar Masses, there is insufficient mass and heat for nuclear reactions to start, so it is a 'failed star' or Brown Dwarf. Makes you wonder about Jupiter being a potential Brown Dwarf.

Deuterium is an isotope of Hydrogen (like Carbon 12 and Carbon 14, or Uranium 235 and Uranium 238) having the same number of protons but a different number of neutrons. Whereas Hydrogen has an atomic weight of 1 (single proton in the nucleus), Deuterium has an atomic weight of 2 (single Proton and single Neutron in the nucleus). You can also get Tritium, another Hydrogen isotope with an atomic weight of 3 (1 Proton and 2 Neutrons in the nucleus).

My notes are a bit simplistic, but give the general outline of the PPI, PPII and PPIII chains. There are several permutations of how Hydrogen can be converted into Helium, hence the 3 PP chains. The PPI chain is the most important of these Proton-Proton chains.

e+ = positron
e- = electron
v = neutrino
y = gamma ray

H = Hydrogen
D = Deuterium
He = Helium (AKA Alpha particle)
Li = Lithium
Be = Beryllium
B = Boron

PPI Chain
H + H ==> D + e+ + v * takes 14,000,000,000 years
D + H ==> 3He + y * takes 6 seconds
3He + 3He ==> 4He + H + H * takes 1,000,000 years

PPII Chain
3He + 4He ==> 7Be + y
7Be + e- ==> 7Li + v (decays back to Lithium)
7Li + H ==> 4He + 4He

PPIII Chain
7Be + H ==> 8B + y
8B ==> 8Be + e+ + v (decays back to Beryllium)
8Be ==> 24He (decays back to Helium)

You will notice that after the second reaction in the PPI chain, the 3He created can either join with another 3He to form 4He (last step in the PPI chain), or, join with 4He (created in the last PPI step) to form 7Be (become the first step in PPII chain). If the temperature is above 14,000,000K there is enough heat for 3He to fuse with the 4He.