E.5.1—Stellar equilibrium

Balance inward and outward effects

A stable main-sequence star is in hydrostatic equilibrium: inward gravitational force is balanced by outward thermal or radiation pressure. The star’s radius remains approximately stable while the balance holds.

Link the balance to fusion

Fusion in the core releases energy. The energy transported outward produces thermal and radiation pressure that resists gravitational collapse.

Predict imbalance

If the outward pressure falls, gravity compresses the star and raises core temperature; if pressure grows, the star expands until a new balance is reached.

Common trap

Do not write only “gravity balances pressure” without directions. State that gravity acts inward and thermal/radiation pressure acts outward, with fusion supplying the energy.

How it is assessed

Questions state how main-sequence stability is maintained or explain fusion’s role in the Sun’s stable radius.

Command terms

State / Outline

What earns marks

Name both forces or pressures and their directions, then link outward pressure to energy released by fusion.

Watch for

Mentioning fusion without the force balance or saying gravity acts outward.

Retrieve stellar balance

Fusion releases energy, outward thermal/radiation pressure balances inward gravity, and high temperature and density allow fusion in the core.

Retrieve stellar inference

Mass controls evolution; HR regions classify stars; parallax gives distance; and $L=4\pi R^2\sigma T^4$ gives stellar radius from luminosity and temperature.

E.5.1—Stellar equilibrium

Model Stellar Equilibrium

E.5.1 Exam Analysis

Assessment in practice

Retrieve the Stellar Model