[mesa-users] A common stellar-structure myth

RICHARD H D TOWNSEND townsend at astro.wisc.edu
Tue Aug 30 23:56:46 EDT 2016


But it *did* involve the luminosities of the two stellar profiles, and the virial theorem is insufficient to calculate those luminosities. 

cheers,

Rich

> On Aug 30, 2016, at 9:47 PM, Tomasz Plewa <tomekplewa at gmail.com> wrote:
> 
> The original problem did not involve time, but two stellar profiles.
> 
> Tomek
> --
> 
> 
>> On Aug 30, 2016, at 10:27 PM, RICHARD H D TOWNSEND <townsend at astro.wisc.edu> wrote:
>> 
>> But the virial theorem is insufficient, since it only gives the instantaneous relationship between thermal and gravitational energies. It does not indicate how these quantities vary with time — i.e., the star’s luminosity.
>> 
>> cheers,
>> 
>> Rich
>> 
>>> On Aug 30, 2016, at 9:14 PM, Tomasz Plewa <tomekplewa at gmail.com> wrote:
>>> 
>>> The problem does not involve time. Then we may not need anything more than the virial theorem to solve it.
>>> 
>>> Tomek
>>> --
>>> 
>>> 
>>>> On Aug 30, 2016, at 6:17 PM, RICHARD H D TOWNSEND <townsend at astro.wisc.edu> wrote:
>>>> 
>>>> That’s as maybe, but the gravity is not stronger.
>>>> 
>>>> In any case, what really drives the large energy generation rate in massive stars is the energy loss rate through the stellar surface, which scale as L ~ M^3 (independent of nuclear reactions). The pre-MS star continues to contract until the core energy generation rate matches this loss rate.
>>>> 
>>>> cheers,
>>>> 
>>>> Rich
>>>> 
>>>>> On Aug 30, 2016, at 5:05 PM, Tomasz Plewa <tomekplewa at gmail.com> wrote:
>>>>> 
>>>>> It's a deeper potential well.
>>>>> 
>>>>> Tomek
>>>>> --
>>>>> 
>>>>> 
>>>>>> On Aug 30, 2016, at 5:52 PM, RICHARD H D TOWNSEND <townsend at astro.wisc.edu> wrote:
>>>>>> 
>>>>>> Hi folks —
>>>>>> 
>>>>>> While reading the textbook I’m going to be teaching from this semester (“Pathways to Astronomy”, by Schneider & Arny), I came across this common myth about stellar structure:
>>>>>> 
>>>>>> “The stronger gravity of a more massive star drives up the rate of nuclear fusion dramatically, while smaller stars consume their fuel at a more leisurely pace”.
>>>>>> 
>>>>>> The attached figure, courtesy of MESA, shows the internal gravity for 1Msun and 10Msun stars at the ZAMS — and nicely busts the myth.
>>>>>> 
>>>>>> cheers,
>>>>>> 
>>>>>> Rich
>>>>>> 
>>>>>> <gravity.pdf>
>>>>>> ------------------------------------------------------------------------------
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>> 



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