Can we speak of a slight linear increase of the core density of sun-like stars during their stay in the main sequence ? As this is a period where gravitational effects are steadily balanced by the radiative pressure resulting from the progressive conversion of H into He. And the same with a bigger slope for the further period of conversion of He into C ?
Over a limited time the density may increase in a linear fashion with time, it is not because of radiation pressure. Sun-like stars depend on gas pressure for support. Although the heat is released through fusion reactions, the thermal reservoir is the gas. The point when X=0 at the center is when the innermost regions begin to slowly collapse to release gravitational energy and support the rest of the star. The conversion of He to C is a far more complicated process.
Thanks for your answer ! Could you give a figure for the "limited time", as the process is expected to last 10 billions years ... or did I miss anything ? Concerning the conversion of He to C (to last 1 billion years ?) can one anyway have a rough idea of the evolution trend of the core density ?
Dear Guibert,
The core densities of MS stars are checked observationally by apsidal motion theory. Long term period variation observations of binaries in eccentric orbits reveal the observational structure constants of the component stars which can be compared with theoretical values obtained from stellar models. As I remember the present stellar models predict always the less central condensations for the MS stars. Recently we are also preparing a paper on this topic. You can read papers by Kopal 1965, Claret, Gimenez 2010. It seems the core density already increased before the MS stage, and continues increasing through the evolution.
Best regards
Dear Dr Demircan,
Thanks for this clear answer. I'll try to have a look on the mentioned papers and to your's when it is published.
Best regards
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