Stellar Evolution: After Hydrogen Exhaustion

[vertices]

why after hydrogen exhaustion (of low mass stars) does the the core become isothermal. (according to my notes)

I mean, after exhaustion, its all helium, and it keeps on contracting right? So by the virial therem it must heat up (so by definition, it is NOT isothermal)! Indeed it must do so so as to trigger hydrogen shell burning.

Thanks:)

 

[astrorob]

I think you're right, but missing a stage between the exhaustion and rapid contraction of the core. I always thought there was an intermediate stage where the core released energy gravitationally until a critical mass was reached (there's a name to the limit, probably to do with Chandrasekhar!) and rapid core contraction happened. It is this stage that's isothermal, not the rapid contraction.

EDIT:

http://en.wikipedia.org/wiki/Schönberg-Chandrasekhar_limit

 

[vertices]

thanks astrorob...

Yes you are right.

1)hydrogen burns in shell around inert core (shell-burning phase)

2)*The core grows until Mass of core DIVIDED by Mass of star ~ 0.1. (Schoneberg - Chandresekar limit)*

3)At this stage, the core the thermal pressure is not sufficient to balance gravitational pressure and it rapidly contracts...

questions:

a)am i wrong about the fact that the inner core must heat up to trigger hydrogen shell burning? Or does this just occur spontaneously when the core hydrogen is depleted?

b)why is the core isothermal between stages (1) and (2) anyway? What's the physical explanation for this?

thanks:)

 

[astrorob]

i) The only reason the core becomes inert is because it uses its fuel up much faster than the outermost layers. Normally hydrogen continues burning in a shell which causes the accumulation of matter onto the inert core until the SC limit is reached.

ii) It arises from the fact that there is no nuclear burning and the temperature gradient is radiative, so its thermal stratification is essentially isothermal.

 

[vertices]

i) The only reason the core becomes inert is because it uses its fuel up much faster than the outermost layers. Normally hydrogen continues burning in a shell which causes the accumulation of matter onto the inert core until the SC limit is reached.

ii) It arises from the fact that there is no nuclear burning and the temperature gradient is radiative, so its thermal stratification is essentially isothermal.

thanks rob.

 

[astrorob]

no problem buddy, glad to be of help.