The Life Cycle of Stars

Stars form from clouds of gas and dust and go through a sequence of stages determined mainly by their initial mass.

Life cycle of a star like the Sun (average mass)

1. Nebula: a cloud of gas (mostly hydrogen) and dust.
2. Protostar: gravity pulls the gas together. The cloud contracts and heats up.
3. Main sequence star: nuclear fusion begins (hydrogen fuses to helium). The outward radiation pressure balances the inward gravitational force. The star is stable and can remain in this stage for billions of years.
4. Red giant: hydrogen fuel in the core runs out. The outer layers expand and cool (the star becomes red and much larger). Helium and heavier elements fuse in the core.
5. Planetary nebula: the outer layers are ejected into space.
6. White dwarf: the remaining core is hot and dense but no longer undergoes fusion. It gradually cools over billions of years.

Life cycle of a massive star

1. Nebula and protostar (same as above).
2. Main sequence star: burns hotter and faster than the Sun.
3. Red supergiant: expands even more than a red giant. Fuses heavier elements up to iron.
4. Supernova: the core collapses violently and the outer layers explode outward. Elements heavier than iron are formed and scattered into space.
5. Neutron star (if the remaining core is moderately massive): an incredibly dense remnant. Or black hole (if the remaining core is very massive): gravity is so strong that nothing, not even light, can escape.

The Sun as a source of energy

The Sun produces energy by nuclear fusion: hydrogen nuclei fuse to form helium, releasing enormous amounts of energy. This is the energy source for almost all life on Earth.

Common errors and how to correct them

Saying the Sun will become a black hole. Only very massive stars (much larger than the Sun) can form black holes. The Sun will end as a white dwarf.

Confusing red giant and red supergiant. Average-mass stars become red giants. Very massive stars become red supergiants. The subsequent stages are different.

How to apply this in an exam

Draw or describe the life cycle as a flow diagram with clear stages. State the correct sequence for both average-mass and massive stars. Explain that the balance between gravity and radiation pressure keeps main sequence stars stable.