The life cycle of a star
How a star is born
Every star starts the same way.
- a nebula, a huge cloud of dust and gas, is pulled together by gravity
- it becomes a protostar: squeezed tighter, it gets hotter and hotter
- when it is hot and dense enough, hydrogen nuclei fuse into helium nuclei, releasing energy
- 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.
Two ways to end
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.
Where the elements come from
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.
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