The Life Cycle of Stars

Every star begins the same way, and what happens at the end depends entirely on its mass.

The common beginning:

Nebula — a cloud of dust and gas, mostly hydrogen, drifting in space.

Protostar — gravity pulls the cloud together. As it contracts, the particles gain kinetic energy and the temperature rises.

Main sequence star — when the core becomes hot and dense enough, nuclear fusion of hydrogen into helium begins. The star is now stable and remains so for most of its life.

Why a main sequence star is stable. Two effects balance: gravity pulling inwards, and the outward pressure from the energy released by fusion. While fusion continues at a steady rate the star neither collapses nor expands. This balance is the definition of the main sequence, and it is what questions mean by "stable".

A star about the size of the Sun then follows:

Main sequence → red giant → white dwarf → (eventually) black dwarf

The hydrogen in the core runs out, fusion of heavier elements begins, and the outer layers swell and cool, turning the star red and enormous. Eventually the outer layers drift away and the hot, dense core is left behind as a white dwarf, which slowly cools over billions of years into a cold black dwarf.

A star much more massive than the Sun follows:

Main sequence → red supergiant → supernova → neutron star or black hole

The supergiant's core collapses and the star explodes as a supernova, briefly outshining an entire galaxy. What remains depends on mass again: a neutron star for a heavy star, or, for the most massive, a black hole — an object so dense that not even light can escape it.

Where the elements came from. Fusion in stars builds elements up to iron. Anything heavier is made in the extreme conditions of a supernova, and scattered into space by the explosion. The carbon in your body and the iron in your blood were made inside stars that died before the Sun existed.

Example: mass decides everything. A star of the Sun’s mass ends quietly as a white dwarf. One twenty times heavier ends as a supernova, and leaves behind an object with the mass of a star packed into the size of a city.