What process allows stars to generate the power necessary for their existence?

The process that allows stars to generate the power necessary for their existence is nuclear fusion. This is the phenomenon by which lighter atomic nuclei combine under extreme temperatures and pressures to form heavier nuclei, releasing a significant amount of energy in the process. In the case of stars like our Sun, hydrogen nuclei fuse to create helium, and this fusion releases energy in the form of light and heat, which is what we perceive as the star's brightness and warmth.

Nuclear fusion occurs in the core of stars where conditions are ideal: incredibly high temperatures (millions of degrees) and immense pressures due to the stellar mass. The energy produced by fusion reactions is what balances the inward gravitational force trying to collapse the star, thus maintaining a stable structure over millions to billions of years.

In contrast, fission involves the splitting of heavy atomic nuclei into lighter ones and is not the primary process occurring in stars. Gravitational collapse refers to the contracting of gas and dust under its own gravity, which can lead to star formation but is not the process by which they generate energy once they have formed. Condensation pertains to the transition from gas to liquid and does not apply to stellar energy generation either.