Stars are born when gravity squeezes a cloud until it ignites

1. A star is formed from an interstellar cloud of gas and dust that contains hydrogen.
2. The cloud's own internal gravitational attraction pulls it inwards, and once it starts collapsing it is called a protostar.
3. As it collapses, the protostar's temperature rises, because the falling material is squeezed into a smaller volume and its particles end up moving far faster.
4. When the centre is hot enough, hydrogen nuclei begin to fuse into helium, and the object starts producing its own energy.
5. A protostar becomes a stable star when the inward force of gravitational attraction is balanced by an outward force caused by the high temperature at its centre.
6. That balance is what stops the collapse, so the star holds a steady size for as long as the fusion keeps running.

Every star runs out of hydrogen and swells up

1. All stars eventually run out of hydrogen as fuel, because each fusion reaction uses some up and none is replaced.
2. Once most of the hydrogen in the centre has been converted to helium, the balance that held the star steady is lost and the star expands.
3. What it expands into depends entirely on how massive the star is, and this is the fork that decides the whole rest of the story.
   1. Most stars, including the Sun, expand to form a red giant.
   2. More massive stars expand to form a red supergiant, larger again.
4. These stars look red because the swollen outer layers are spread over a huge area and are therefore cooler than the surface of a stable star.

Two endings: a quiet fade or a violent explosion

1. The less massive route, ending in a white dwarf.
   1. A red giant from a less massive star throws off its outer layers, which form a glowing shell of gas called a planetary nebula.
   2. The hot, dense core left behind at its centre is a white dwarf, which slowly cools with no fusion left to power it.
   3. This is the ending the Sun is heading for, in about five billion years.
2. The massive route, ending in a neutron star or a black hole.
   1. A red supergiant explodes as a supernova, briefly outshining an entire galaxy.
   2. The explosion forms a nebula containing hydrogen and new heavier elements made during the star's life and in the explosion itself.
   3. At the centre it leaves behind either a neutron star or, for the most massive stars, a black hole.
3. The nebula from a supernova may form new stars with orbiting planets, which closes the cycle back to where the first section started.
4. That last step is why the Earth is rocky at all: the heavier elements in our planet, and in you, were made inside earlier stars and spread out by supernovae.