P8.2 Life cycle of a star

How stars are born

Stars form from nebulae — vast clouds of hydrogen gas and dust. Gravity pulls the cloud together, raising the temperature and pressure. When the core becomes hot enough (≈ 10 million K), nuclear fusion begins: hydrogen nuclei fuse to form helium, releasing enormous energy. This is the birth of a main sequence star.

The main sequence

During the main sequence (the longest stage), the outward pressure from fusion exactly balances the inward force of gravity. The Sun has been in this stage for about 4.6 billion years and will remain for another 5 billion.

End of a star like the Sun (small/medium mass)

1. Hydrogen runs out in the core.
2. The outer layers expand (gravity still pulls in; less radiation pushing out from fusion).
3. The star becomes a red giant (cooler surface, much larger).
4. Outer layers are ejected gently as a planetary nebula.
5. The remaining core — mostly carbon and oxygen — is left as a white dwarf (hot but no longer fusing; slowly cools over billions of years).

End of a massive star

1. Becomes a red supergiant.
2. Fusion continues to produce heavier elements (up to iron; iron fusion absorbs energy rather than releasing it).
3. Core collapses catastrophically → supernova explosion (briefly outshines an entire galaxy; scatters heavy elements into space).
4. Remnant core becomes either:
   • Neutron star (if moderate remnant mass)
   • Black hole (if very massive remnant — gravity so strong not even light escapes)

Why does this matter?

All elements heavier than helium (and most heavier than hydrogen) were formed inside stars and scattered by supernovae. Every atom of iron in your blood was once inside a massive star. "We are made of star stuff."

Exam tips

• Learn both pathways — the split at "red giant vs red supergiant" is key.
• The Sun will become a red giant, then a white dwarf (NOT a supernova).