i know that you can't see the fusion directly but you can see the energy it gives off after it is reemitted at the surface. octane numbers are usless when refering to fusion. fusion is a nuclear process while octane numbers refer to the combustion of alkanes. young stars do have a fusion limit but it is because of the density(or lack thereof) and temperature that constrains it, not heavier atoms although these do play a role in the last stages of the stars life.