The Life of a Star
3 marks · NormalExplain why gravitational forces become dominant over outward pressure at the end of the main sequence stage of a star similar to our Sun.
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Also accept: hydrogen runs out, so fusion stops, pressure drops, gravity pulls star inwards
Common mistakes: saying gravity increases (incorrect, pressure decreases)
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Marking points
M1 · 1 mark · States that hydrogen fusion stops at the end of the main sequence.
A1 · 1 mark · Explains outward pressure decreases due to reduced energy from fusion.
A2 · 1 mark · Concludes gravitational forces become dominant, causing the star to collapse.
Worked solution
At the end of the main sequence, the hydrogen fuel is used up and fusion reactions stop. With no fusion, the outward pressure falls. Gravity is now greater than the pressure and so the star begins to collapse under its own gravity.