Stellar Evolution

Nebula — a vast cloud of gas, mostly hydrogen, and dust. Gravitational attraction causes denser regions to collapse.

Protostar — as the cloud collapses, gravitational potential energy is converted to kinetic energy, so the core heats up. It is not yet a star, because fusion has not begun.

Main sequence — when the core reaches roughly 10⁷ K, hydrogen fusion begins. The outward pressure from radiation and the hot gas balances the inward gravitational collapse, and the star becomes stable. This equilibrium lasts for most of the star's life — about 90% of it.

What happens when the hydrogen runs out — with fusion no longer supporting the core, it collapses and heats further, while the outer layers expand and cool. The star becomes a red giant, or a red supergiant if it is massive enough.

The route after that depends entirely on mass.

Low-mass stars, up to about 1.4 solar masses at the end — the outer layers drift away as a planetary nebula, leaving the hot dense core as a white dwarf. No further fusion occurs; it simply cools over billions of years.

The Chandrasekhar limit — a white dwarf can only exist below about 1.44 solar masses. Below it, electron degeneracy pressure — a quantum effect resisting the compression of electrons — supports the star against gravity. Above it, that pressure is insufficient and collapse continues.

High-mass stars — fusion proceeds through successively heavier elements until iron, at which point fusion no longer releases energy. The core collapses catastrophically and rebounds as a supernova, briefly outshining an entire galaxy.

What remains — a neutron star for cores up to around 3 solar masses, supported by neutron degeneracy pressure and so dense that a teaspoon would weigh hundreds of millions of tonnes. Above that, nothing halts the collapse and a black hole forms.

Example: elements heavier than iron cannot be made by ordinary stellar fusion, because fusing them absorbs energy rather than releasing it. They are forged in supernovae. The gold in a ring and the iodine in your thyroid were made in a dying star, which is the literal sense in which we are made of stardust.