Astronomy Stack Exchange is a question and answer site for astronomers and astrophysicists. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

How can a pre-main sequence star radiate more energy by gravitational contraction than a main-sequence star can by hydrogen fusion?

share|improve this question
up vote 3 down vote accepted

pre main sequence evolution

Although pre-main sequence stars have lower temperature, they are essentially huge clouds of gas, often as large as 1 pc wide. The luminousity being proportional to square of the radius is essentially large for pre-main sequence stars.

Plus, the problem with gravitational contraction is not the amount of energy that can be generated per second. That can actually be larger than that produced from fusion if you insist on really massive objects. The logic, when we ruled out gravitational contraction as a source of energy for Sun, was that gravitational contraction cannot sustain it for long. Geologists knew from their dating science that Sun had to be older than grav. contraction accounts for.

Thus, admit it. Gravitational contraction makes pre-main sequence stars radiate more energy than main sequence starts, but that's just luminousity and not really energy. Luminousity is energy per second.

share|improve this answer
This explains where the luminosity comes from but does not explain why the rate of release of gravitational potential energy is larger than the rate at which energy is released by main sequence nuclear burning. – Rob Jeffries Oct 9 '15 at 8:19

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.