Topic 14 – Stellar evolution — Edexcel GCSE Astronomy
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Topic 14 – Stellar evolution explained
A neutron star forms when a massive star's core collapses after a supernova, leaving a compact remnant about 20 km across with roughly 1.4 to 3 solar masses.
Read the full explanation
Gravity pulls every particle inward, but neutrons resist further compression through neutron degeneracy pressure, a quantum effect that prevents identical neutrons from occupying the same state. In a stable neutron star these two forces balance: gravity's inward pull equals the outward pressure support. If the remnant mass exceeds the limit that neutron pressure can support, collapse continues to a black hole. In an MCQ, identify that neutron pressure pushes outward and gravity pulls inward, and that stability requires them to be equal in magnitude and opposite in direction.
Your focus
- Describe the inward pull of gravity on a neutron star.
- Explain how neutron degeneracy pressure opposes gravity.
- State that a stable neutron star exists when these two forces balance.
Topic 14 – Stellar evolution exam tips
Marking Points
- Neutron pressure acts outward and supports the star against gravitational collapse.
- Gravity acts inward and tries to compress the neutron star.
- In a stable neutron star, the inward gravitational force and outward neutron pressure are balanced.
- If the mass is too large, neutron pressure cannot balance gravity and the star collapses further, possibly to a black hole.
- Neutron degeneracy pressure arises because neutrons cannot occupy the same quantum state.
Examiner Tips
- 💡Read the question carefully to identify whether it asks about the direction of forces or the condition for stability.
- 💡Remember that balance means equal magnitude and opposite direction, not zero force.
- 💡Link the concept to the mass limit: if mass is too high, neutron pressure cannot win and collapse continues.
Common Mistakes
- Thinking neutron pressure is the same as thermal gas pressure: correct it by explaining that neutron degeneracy pressure is a quantum effect, not heat-driven.
- Believing gravity disappears in a neutron star: correct it by stating gravity still acts inward and is balanced by pressure.
- Assuming the balance is static with no forces: correct it by saying the forces are equal and opposite, so the star is in equilibrium.
- Confusing neutron stars with white dwarfs: correct it by noting white dwarfs are supported by electron degeneracy pressure, while neutron stars are supported by neutron degeneracy pressure.