7.16P-7.18P - Star life cycles

From nebula to main sequence

Stars begin in a nebula, a vast cloud of gas and dust. The particles in a nebula attract one another by gravity. In a region containing enough material, gravitational attraction pulls the gas and dust inward, so the cloud contracts.

As the cloud contracts, its particles move faster and its centre becomes hotter and denser. If the centre becomes hot enough, hydrogen nuclei begin to fuse into helium nuclei. Nuclear fusion releases energy, heating the star's material.

Heating a gas gives its particles more kinetic energy. Their motion and collisions create an outward pressure, so the hot material tends to expand. This is the thermal expansion effect named in the specification. It acts against gravity:

• gravitational attraction pulls the star's material inward;
• the pressure of the hot material creates an outward tendency to expand.

When the outward thermal effect balances the inward gravitational effect, the star stops undergoing an overall collapse or expansion. It has reached a stable main-sequence stage. Stable does not mean that nothing is happening: fusion continues in the core, and the balance is maintained for most of the star's lifetime.

The route of a Sun-like star

A star with a mass similar to the Sun spends most of its lifetime on the main sequence. Its core fuses hydrogen into helium, and the energy released maintains the outward thermal effect that balances gravity.

Eventually, the supply of hydrogen available for fusion in the core decreases. The balance changes: gravity causes the core to contract and heat, while hydrogen fusion continues in a shell around the contracting core. Energy from shell fusion helps the outer layers expand greatly and cool. The star becomes a red giant. Red giant describes the expanded stage of a Sun-like star; it is not a supernova.