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Why do proto stars on the Hayashi track get dimmer as they contract?

My expectation is that they would get hotter and brighter the more they contact. Why is this wrong?

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The Hayashi track has an almost constant effective temperature. The pre main sequence star (not protostar) is powered by the release of gravitational potential energy caused by its contraction.

If the star is getting smaller at constant effective temperature, then of course it's luminosity ($\propto R^2 T^4$) decreases.

So perhaps your question is why do PMS stars contract at constant temperature? In other words, why is the Hayashi track almost vertical in the HR diagram? The answer to this is derived in some detail on the relevant Wikipedia page and arises from the fact that low mass PMS stars are fully convective and have atmospheric opacity dominated by H- ions.

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  • $\begingroup$ Thanks! But yeah, why don't they heat up? $\endgroup$
    – user10250
    Jan 20, 2016 at 0:09
  • $\begingroup$ @Dawn Read the Wikipedia page and then ask a question about which part is unclear. The derivation is quite involved and I see no purpose in cutting and pasting it here. $\endgroup$
    – ProfRob
    Jan 20, 2016 at 0:11
  • $\begingroup$ Nevermind then :( $\endgroup$
    – user10250
    Jan 20, 2016 at 0:19

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