A star remains stable because inward gravitational attraction is balanced by outward pressure from its hot interior. This balance is called hydrostatic equilibrium and is required at both SL and HL in IB Physics subtopic E.5, Fusion and stars.
How the balance works
- The star's mass produces a strong gravitational force, pulling its material inward toward the centre.
- Nuclear fusion in the core transfers energy to the surrounding plasma, maintaining an extremely high temperature.
- Rapidly moving particles produce thermal pressure, while photons moving outward produce radiation pressure.
- The outward pressure decreases with distance from the centre. This pressure gradient produces an outward force that balances gravity.
| Inward effect | Outward effect |
|---|---|
| Gravity pulls stellar material toward the centre. | Thermal and radiation pressure push the stellar layers outward. |
| Compression tends to decrease the star's radius. | Expansion tends to increase the star's radius. |
| Stronger gravity increases core compression and temperature. | Higher core temperature can increase fusion and pressure. |
The equilibrium is self-regulating during the main-sequence stage. If gravity causes slight contraction, the core becomes hotter and denser, increasing the fusion rate and outward pressure. The star then expands back toward equilibrium.
If the star expands slightly, its core cools, reducing the fusion rate and pressure. Gravity then causes contraction toward equilibrium. Stability does not mean that no forces act; it means that the opposing effects are balanced.
A common misconception is that fusion directly holds the star up. More precisely, fusion supplies the energy needed to maintain the high temperature and radiation field that generate outward pressure. When core fuel becomes depleted, this balance changes and the star leaves the main sequence.
Exam Technique
For an IB Explain question, link the causal steps: fusion releases energy, energy maintains high temperature, thermal and radiation pressure act outward, and this pressure balances inward gravity. Use the term hydrostatic equilibrium and avoid saying that there is “no gravity” or “no force.”
