The life cycle of stars

1. A star begins as a nebula: a huge cloud of dust and gas, mostly hydrogen.
2. Gravity pulls the dust and gas together. As it gets denser, it heats up, forming a protostar.
3. When it is hot enough, hydrogen nuclei fuse to make helium, releasing huge amounts of energy. It is now a main sequence star, like the Sun.

A main sequence star is stable for billions of years. The inward pull of gravity is balanced by the outward pressure from the energy released by fusion.

When a star runs out of hydrogen in its core, what happens next depends on its mass.

A star about the size of the Sun:

main sequence → red giant → white dwarf

It swells into a red giant, then shrinks to a small, hot, dense white dwarf, which slowly cools and fades.

A star much bigger than the Sun:

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

It swells into a red supergiant, then explodes in a supernova. What's left becomes a very dense neutron star. The biggest stars leave a black hole, whose gravity is so strong that not even light can escape.

Fusion in stars makes all the naturally occurring elements.

• During its life, a star fuses lighter nuclei into heavier ones. This makes elements up to iron.
• Elements heavier than iron, such as gold and uranium, are made in a supernova.

The supernova explosion spreads these elements through the universe. They end up in new stars, planets and even people: the atoms in your body were made inside stars.