Star Evolution and Life Cycles

Stars spend most of their lives on the main sequence, in stable equilibrium where the outward pressure from fusion balances the inward pull of gravity. Our Sun has spent about 4.6 billion years here and will remain for another 5 billion.

During this phase, hydrogen fuses to helium in the core, releasing energy that keeps the star hot and luminous. The more massive the star, the faster it burns through its fuel—giant stars may only last millions of years.

When core hydrogen is exhausted, the core contracts and heats up while outer layers expand dramatically. The star becomes a red giant—cooler at the surface but much larger and more luminous.

The core becomes hot enough to fuse helium into carbon. In massive stars, this process continues with heavier elements until iron forms—beyond which fusion releases no energy.

Neutron stars are incredibly dense—a teaspoon would weigh billions of tonnes. Black holes are even more extreme, with gravity so strong that not even light can escape from within the event horizon.