Which two energy sources can help a star maintain its internal thermal pressure? nuclear fission and gravitational contraction nuclear fusion and chemical reactions chemical reactions and gravitational contraction nuclear fusion and gravitational contraction nuclear fusion and nuclear fission

Nuclear fusion and gravitational contraction

Explanation:

In stars, there is an equilibrium between two forces, the force of gravity in the inward direction due to their own mass, and the radiation pressure in the upward direction as a consequence of the nuclear reaction in their core, that is known as hydrostatic equilibrium.

The radiation pressure is gotten from the nuclear reactions at the core (when lighter elements fuse into heavier elements), but if the nuclear reactions stop, hence, the radiation pressure will also do it and the force of gravity will overcome and break the equilibrium.

Both of that energy sources help to maintain a star's internal thermal pressure, since the contractions of the superficial layers will increase the density at the core.

Fusion and Gravitational attraction

Explanation:

- Fusion reaction:-

Nuclear fusion is a process that combines nuclei in order to release energy. It begins when the temperature in a star reaches about 10,000,000 Kelvin. This temperature occurs as a result of the gravitational contraction of a forming star, causing its gases to heat up and pressure to rise.

- Gravitational attraction:

gravity is the force that creates the pressure to fuse atoms, which makes the stars shine. ... Eventually the temperature is high enough that the star starts fusing hydrogen into helium. When the outward pressure produced by the heating of the gas by fusion energy balances gravity, a stable star is formed.

the closer the planet is to the sun, the larger the gravity