Star Life Cycle Balance

Compression makes the centre hotter and denser. At sufficiently high temperatures, nuclear fusion can take place. Fusion releases energy, heating the stellar material. The particles then move more rapidly, producing pressure and a tendency for the material to expand. This outward effect is often described at GCSE as thermal expansion.

Thermal expansion is not a separate fundamental force. It describes the response of hot stellar material: heating maintains a high pressure, and differences in pressure through the star support the layers above against gravity.

How a stable star forms

A star begins as material that contracts under gravity. As it contracts, gravitational energy is transferred to the thermal energy store of the material, so its centre becomes hotter. Eventually the core becomes hot and dense enough for hydrogen nuclei to fuse into helium.

The energy released by fusion keeps the stellar material hot and maintains its outward pressure. When this outward effect balances the inward pull of gravity, the star stops undergoing overall contraction and settles onto the main sequence.

Balance during the main sequence

A main-sequence star is in hydrostatic equilibrium: gravity continues to pull inwards, but it is balanced by the outward pressure of the hot material. The star is not inactive. Hydrogen is continually being fused into helium and energy continually moves outwards, but its overall size remains comparatively stable because neither effect dominates.

This balance can help regulate the star. If gravity compresses the core, its temperature and density rise, increasing fusion and energy output. The resulting heating increases pressure and opposes further compression. Conversely, expansion can cool the material and reduce fusion, allowing gravity to become relatively more important again.

A star spends most of its life on the main sequence because its hydrogen supply can maintain this balance for a long time. Mass affects how long this lasts: a more massive star has a stronger gravitational effect from its overlying layers, so its core must reach a higher pressure and temperature. It therefore uses its nuclear fuel more rapidly and has a shorter main-sequence lifetime, despite beginning with more fuel.

When the balance changes

Eventually, hydrogen in the core is depleted. Hydrogen fusion in the core can no longer provide enough energy to maintain the previous balance, so gravity becomes dominant there and the helium-rich core contracts.

Contraction heats the core and the region around it. Hydrogen in a shell just outside the core can then become hot enough to fuse. Energy released by this shell fusion heats the outer layers, causing them to expand greatly. A lower-mass star such as the Sun becomes a red giant, while a more massive star can become a red supergiant.

The core and outer layers therefore need not behave in the same way: the core may contract under gravity while the envelope expands because it is being heated. Expansion also cools the surface, giving a giant star its redder appearance.

The balance drives stellar evolution

The life cycle of a star is a sequence of changes in this competition:

• gravity causes initially diffuse material to contract and heat up;
• fusion begins and supplies energy that supports a stable main-sequence star;
• depletion of a core fuel disrupts that balance, so the core contracts again;
• contraction raises the temperature and may allow fusion in a new region or of a new fuel, producing renewed expansion;
• when no available fusion process can provide adequate support, gravity again dominates.

Thus, changes in the balance between inward gravity and the outward effect of heating determine when a star is stable and when it moves to another stage of its life cycle.