Question

when a star runs out of protons in its core, why does it becomes more luminous? why does the surface temperature of the star cool down?

Answer 1

Dear Student,

When the hydrogen fuel of a star runs out,i.e. when stars run out of protons in its core, then fusion of hydrogen into helium stops, thus shutting off the outward radiation pressure. The helium core begins to contract due to gravitational attraction. This contraction results in the conversion of gravitational potential energy into thermal energy. Even though the fusion is not occuring, still the thermal heat generated heats up the shell of hydrogen surrounding the core until it is hot enough to start hydrogen fusion again, producing more energy than its previous state.

The new increased radiation causes the external layer of the star to expand in order to maintain the pressure gradient. As the gas expands it cools, this leads to drop in temperature in the outer layer. Convection transfers huge amount of heat from the core to the outer surface. This increases the luminosity of the star by a factor of 1000 or so. Thus, we observe that when a star runs out of proton, it becomes more luminous while its surface temperature cools down.

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