[Mesa-users] Energy conservation on the main sequence

[RICHARD H D TOWNSEND]

Hi folks --

I'm writing to ask about energy conservation on the main sequence, for a solar-mass star. I'm not yet sure whether this is more of a question about stellar evolution, or about MESA.

In the attached figure, I plot two quantities as a function of time:

tot_E_change -- the total energy change across each timestep
total_energy_sources_and_sinks -- the total energy sources/sinks across each timestep

For this evolutionary stage, total_energy_sources_and_sinks is practically identical to (L_nuc - L)*dt, where L_nuc is the nuclear luminosity, L the surface luminosity, and dt the timestep.

My understanding is that the two quantities should align perfectly -- the net energy gain/loss is matched by a change in the star's total energy (which, according to the virial theorem, will result in an expansion/contraction). But that's clearly not what we see; while total_energy_sources_and_sinks is always positive (with the exception  of the ZAMS), the tot_E_change is initially negative and only becomes positive later on in the evolution.

I'm clearly missing something important, but I'm not sure what. So, can anyone help clarify what's going on?

cheers,

Rich

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