Gravity and nuclear fusion reactions drive the formation and development of stars. Stars with different masses grow and change throughout the different stages of their lives.
For most of its lifetime, a star is a main sequenceA stable stage in the life cycle of a star. Nuclear fusion occurs, fusing hydrogen nuclei into helium nuclei. There is a balance between the outwards radiation and the force of gravity pulling inwards. star. It is stable, with balanced forces keeping it the same size all the time. During this period:
gravitational attraction tends to collapse the star
radiation pressure from the fusion reactions tends to expand the star
forces caused by gravitational attraction and fusion energy are balanced
The Sun is expected to be a main sequence star for billions of years.
Fusion reactions
In a main sequence star, hydrogen nuclei fuse together to form helium nuclei. This happens in several steps, but one way to simplify the overall change is: