Most of the stars in the Universe are in the main sequence stage of their lives, a point in their stellar evolution where they’re converting hydrogen into helium in their cores and releasing a tremendous amount of energy. Let’s example the main sequence phase of a star’s life and see what role it plays in a star’s evolution.
A star first forms out of a cold cloud of molecular hydrogen and helium. Mutual gravity pulls the stellar material together, and this gravitational energy heats it up. The star first goes through a protostar phase for about 100,000 years, and then a T Tauri phase, where it shines only with the energy released from its ongoing gravitational collapse. This second T Tauri phase lasts a further 100 million years or so.
Eventually temperatures and pressures in the core of the star are sufficient that it can ignite nuclear fusion, converting hydrogen atoms into helium. When this process gets going, a star is said to be in the main sequence phase of its life.
In a star like our Sun, the core accounts for about 20% of its radius. It’s inside this region where all the energy of the Sun is released. The energy released in the core must then travel slowly through a radiative zone, where photons of energy are absorbed and then re-emitted. Energy is then carried through a convective zone, where columns of hot plasma carry bubbles of heated gas to the surface of the Sun where it’s released. The material cools down and falls back down inside the Sun where it’s heated up again. This journey can take more than 100,000 years for a single photon to get from the core of a star out to its surface.
Over time, a star slowly uses up the supply of hydrogen in its core, and leftover helium builds up. But the main sequence phase can last a long time. Our Sun has already been in its main sequence for 4.5 billion years, and will probably last another 7.5 billion years before it runs out of fuel.
The smallest red dwarf stars can smolder in the main sequence phase for an estimated 10 trillion years! The largest supergiant stars might only last a few million. It all comes down to mass.
And mass defines how a star comes out of the main sequence phase of its life. For the smallest red dwarf stars, astronomers think they’ll just shut off once they’ve used up all their hydrogen, becoming white dwarfs. More massive stars, with up to 10 solar masses, will go through a red giant phase where they expand many times their original size before collapsing down to the white dwarf. And the most massive stars will just explode as supernovae.