Every star starts the same way.

1. a nebula, a huge cloud of dust and gas, is pulled together by gravity
2. it becomes a protostar: squeezed tighter, it gets hotter and hotter
3. when it is hot and dense enough, hydrogen nuclei fuse into helium nuclei, releasing energy
4. it is now a main sequence star, and stays stable for millions to billions of years

On the main sequence two effects are in equilibrium: gravity pulls the star's material inwards, and the energy from fusion produces a pressure pushing outwards. They balance, so the star keeps a steady size. The Sun has been like this for about 4.6 billion years.

When the hydrogen in its core runs out, what happens next depends on the star's mass.

• About the Sun's mass: it swells into a red giant, then shrinks to a white dwarf, a small, hot, dense core that slowly cools.
• Much more massive than the Sun: it swells into a red supergiant, then collapses and explodes as a supernova. The core left behind is a neutron star, or, for the most massive stars, a black hole, whose gravity is so strong that not even light escapes.

The early universe was almost all hydrogen and helium. Everything else was made in stars.

• Fusion in stars makes elements up to iron (helium, then carbon, oxygen and so on in giant stars).
• Elements heavier than iron, such as gold and uranium, are made in supernovae.
• A supernova scatters these elements through space, where they end up in new stars and planets, and in you.

Stars do not burn. There is no oxygen and no chemical reaction: the energy comes from nuclear fusion.