Expert: Philip Stahl - 5/5/2009

Question
Hi, I just had some astronomy questions I was wondering if you could clarify:

I know that the dominant process in solar energy production in the sun is the P-P process but in that process is it deuterium that is formed or nitrogen?
Also one solar mass star is on the main sequence for 4.5 billion years correct?

Thank you =)

Answer
Hello,

The specific sequence for the p-p chain fusion is as follows:

1)  e(-) + H1 + H1 ->  D2  +(neutrino) + 1.44 MeV

2) D2 + H1 - >  He3  + (gamma particle) + 5.49 MeV

3) He 3  + He 3 ->  He 4 + H1 + H1 + 12.85 MeV


e(-) = positron

As you can see, no Nitrogen emerges in any of the links. Deuterium emerges and is fused to a proton in (3) to sustain the chain of reactions. (The numbers at the end of each reaction stage denote the energies given off in millions of electron volts, where 1eV = 1.6 x 10^-19 J)

Note: there is an alternative sequence to step (3) which begins:

He 3 + He 4 -> Be 7 + (gamma)

Thus, Beryllium 7 appears (Beryllium 8 in a following step). This is the highest atomic weight component. However, the alternative reaction sequence only occurs about 0.015% of the time.

As to how long the Sun will remain on the Main Sequence, as usual the answer is not engraved in stone. Perhaps the optimal response is 4.5 billion years plus or minus a half billion.