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Schönberg-Chandrasekhar limit

(maximum stellar isothermal helium core mass)

The Schönberg-Chandrasekhar limit is the maximum stable isothermal mass of the helium stellar core formed from fusion in a main sequence star. A core is isothermal (same temperature throughout) if the energy source (fusion) surrounds it (i.e., forming a shell) and everything is sufficiently stable that the core falls into and essentially remains at equilibrium. Only stars in a certain mass range grow such a core large enough to reach the limit. For those that do, helium fusion begins and the star enters the red-giant stage. The limit depends upon factors in the star:

Mcore/M ≈ 0.37(μenvcore

Further reading: