From nebula to protostar

1. The Sun formed about 4.6 billion years ago from a nebula, a vast cloud of dust and gas made mainly of hydrogen.
2. Every particle in the nebula has mass, so every particle attracts every other particle by gravity.
3. In a slightly denser region of the cloud, gravity pulled the dust and gas inwards, so that part of the cloud began to collapse.
4. As the material fell inwards, energy was transferred from the gravitational store of the particles to their kinetic store, so they moved faster.
5. The fast-moving particles collided more often and more violently, so energy was transferred to the thermal store of the gas and its temperature rose.
6. The collapse also squeezed the particles closer together, so the density and pressure at the centre increased.
7. The hot, dense, glowing ball of gas at the centre of the collapsing cloud is a protostar.
8. Leftover dust and gas spinning in a flat disc around the young Sun later clumped together to form the planets.

Fusion begins

1. Hydrogen nuclei are positively charged, so they repel each other strongly.
2. Two nuclei can only get close enough to fuse if they collide at extremely high speeds, which needs an extremely high temperature.
3. The collisions must also happen very often, which needs a very high density and pressure.
4. Gravity kept squeezing the protostar until its core reached a temperature of around 10 million ${}^{\circ }C$, which was hot and dense enough for hydrogen nuclei to fuse.
5. In the core, hydrogen nuclei fuse to form helium nuclei, and each fusion releases energy from the nuclear store.
6. This energy heats the core further and travels outwards, finally leaving the surface as light and other electromagnetic radiation.
7. Once fusion is under way, the protostar has become a star.

Stellar equilibrium

1. Two effects act on the material of a star at the same time.
2. Gravity pulls all of the star's material inwards, trying to make the star collapse further.
3. The energy released by fusion heats the gas, and the pressure of the hot gas pushes outwards, trying to make the star expand.
4. When the inward pull of gravity exactly balances the outward push from the energy released by fusion, the star stops changing size.
5. This balance is stellar equilibrium, and a star held in this stable state is a main sequence star.
6. The balance corrects itself, which is why a main sequence star stays stable for so long.
7. If fusion slows slightly, the outward push weakens, so gravity squeezes the core a little.
8. Squeezing heats and compresses the core, which speeds fusion back up until the outward push again balances gravity.
9. The Sun has been a stable main sequence star for about 4.6 billion years and has enough hydrogen in its core to stay this way for around another 5 billion years.

The life cycle of a star like the Sun

1. A star stays on the main sequence until the hydrogen in its core starts to run out.
2. With less fusion in the core, gravity squeezes the core, which heats it up.
3. The extra heat starts fusion in a shell of hydrogen around the core and makes the outer layers swell enormously.
4. The outer layers cool as they expand, so they glow red, and the star becomes a red giant.
5. In the very hot core of the red giant, helium nuclei fuse to form heavier elements such as carbon and oxygen.
6. Eventually the red giant throws off its outer layers into space as a glowing shell of gas.
7. The hot, dense core left behind is a white dwarf, which is roughly the size of the Earth.
8. No fusion happens in a white dwarf, so it slowly cools and fades over billions of years.

The life cycle of a massive star

1. A star with at least eight times the mass of the Sun follows a different path.
2. Its core is hotter, so it uses up its hydrogen much faster and then swells into a red supergiant, which is even larger than a red giant.
3. Its core is hot enough for fusion to build up heavier and heavier elements, as far as iron.
4. Fusing iron does not release energy, so once the core is mainly iron, fusion can no longer support the star against gravity.
5. The core collapses in less than a second, and the outer layers are blasted into space in a supernova explosion.
6. Some elements heavier than iron, such as gold and uranium, are made in supernovae and are scattered through space.
7. The dust and gas thrown out can later collapse to form new stars and planets, which is how the Earth came to contain these heavy elements.
8. If the collapsed core is up to about three times the mass of the Sun, it becomes a neutron star.
9. If the core is even more massive, gravity crushes it further into a black hole.