What causes a star to move off the main sequence?

A star moves off the main sequence when it exhausts its hydrogen fuel in the core.

When a star is on the main sequence, it is in a state of equilibrium where the energy produced by nuclear fusion in the core is balanced by the energy radiated from the surface. This fusion process converts hydrogen into helium, releasing energy in the form of light and heat. As the star consumes its hydrogen fuel, the core contracts and heats up, causing the outer layers to expand and cool. This expansion causes the star to move off the main sequence and enter a new phase of its life cycle.

The exact path a star takes after leaving the main sequence depends on its mass. Low-mass stars, like our Sun, will become red giants as they burn helium in their cores. High-mass stars, on the other hand, will undergo a series of fusion reactions that produce heavier elements, eventually leading to a supernova explosion.

The movement of a star off the main sequence can be observed through changes in its brightness and colour. As the star expands and cools, it becomes redder and brighter, making it easier to detect with telescopes. Studying the life cycle of stars is important for understanding the evolution of the universe and the formation of elements necessary for life.