No, the fusion reaction in stars is not directly caused by gravity. Gravity plays a crucial role in star formation and in maintaining the conditions necessary for fusion to occur, but it is not the direct cause of the fusion reaction itself.
Stars are formed from clouds of gas and dust called nebulae. Gravity pulls these particles together, causing the cloud to collapse under its own weight. As the cloud collapses, it heats up and becomes denser, eventually reaching a point where the core temperature and pressure are high enough to initiate nuclear fusion.
Nuclear fusion occurs when atoms in the stellar core collide with enough energy to overcome their electrostatic repulsion and merge together, forming a heavier nucleus. This process releases an enormous amount of energy in the form of light and heat.
The primary fusion reaction that powers stars is the conversion of hydrogen nuclei (protons) into helium nuclei. This process, known as the proton-proton chain, requires extremely high temperatures and pressures to overcome the electrostatic repulsion between protons. It is the release of this fusion energy that counterbalances the force of gravity, allowing the star to maintain a stable equilibrium.
So while gravity provides the conditions necessary for fusion to occur, it is the fusion reaction itself that generates the energy that keeps a star shining.