Expert: Philip Stahl - 4/10/2009

Question
Hello Philip, I find it surprising that stars are so variable in size. I would have thought that as as gas and dust condenses into a protostar that there would be a certain critical mass at which nuclear fusion would begin, after which any further gas would be blown away from the star. Put another way, how can a star get to say 100 solar masses before "igniting" if our own sun ignited at a much smaller mass?

Answer
Hello,

The key answer to your question resides in the *type* of nuclear fusion the star undergoes. For stars of mass less than about 1.5 solar, the proton-proton cycle is the fusion reaction, with the net effect written:

H1 + H1 + H1 + H1 ->   He4 + energy

For stars more massive, the C-N-O or carbon -nitrogen -oxygen cycle is key and this doesn't kick in until about 1.3 x 10^7 K, contrasted to the p-p cycle that kicks in much lower and has lower mass thresholds, etc. operative.

You can read much more about the C-N-O cycles and how they apply to the more massive stars here:

http://en.wikipedia.org/wiki/CNO_cycle

Along with the respective energies given off at each reaction step.

Bear in mind too, that though some stars may attain whopping masses, e.g. 100 solar, the price they pay is in a much shorter lifetime. Thus, the more massive star always consumes its nuclear energy stores a lot faster than its much smaller counterparts. It is allotted much higher luminosity (according to the mass-luminosity law) but at a cost in duration.

Think of it as an energy spendthrift, which squanders its energy at a rapid rate, leading to a much earlier death.