Stars spend most of their lives on the main sequence. It is the long, stable phase when a star fuses hydrogen into helium in its core. The outward pressure from fusion balances the inward pull of gravity, and the star shines steadily for millions to billions of years. The Sun has been on the main sequence for about 4.6 billion years and will stay there for roughly 5 billion more.

The main sequence is not a single state. It is a band on the Hertzsprung-Russell diagram, running from hot, bright blue stars at the top left to cool, dim red dwarfs at the bottom right. A star's position on the band depends almost entirely on its mass. Massive stars burn hot and fast, living only a few million years. Low-mass red dwarfs sip their fuel and can last trillions of years.

When a star exhausts the hydrogen in its core, it leaves the main sequence. What happens next depends on its mass. A star like the Sun expands into a red giant, then sheds its outer layers and becomes a white dwarf. A massive star swells into a supergiant and ends in a supernova, leaving behind a neutron star or black hole.

The main sequence is where the action is. Most of the stars you can see with your eyes are there.