What primarily drives the process of stellar evolution?

The primary driver of stellar evolution is gravitational collapse and nuclear fusion. When a gas cloud in space, which is primarily composed of hydrogen and helium, begins to collapse under its own gravity, it leads to an increase in density and temperature at its core. This process initiates the formation of a star.

As the temperature and pressure rise sufficiently in the core, nuclear fusion reactions start to occur, primarily converting hydrogen into helium. This fusion process releases vast amounts of energy, which counteracts the gravitational forces attempting to collapse the star further. This balance between the outward pressure from fusion energy and the inward pull of gravity is crucial for the star's stability and determines its lifecycle.

The evolution of a star will depend on its initial mass, with different processes occurring over time, such as the fusion of heavier elements in more massive stars. This sequence of events continues throughout the star's life, leading to various stages such as the main sequence, red giant, and eventually, its death.

While planetary interactions, cosmic radiation, and orbital mechanics can influence certain aspects of a star's environment or system, they do not drive the fundamental processes of stellar evolution as gravitational collapse and nuclear fusion do.