Stellar Evolution

IGCSE Edexcel Physics
8.9–8.10 Life cycles of stars: Sun-like and massive stars
Key Concepts: Sun-like star: nebula → protostar → main sequence star → red giant → white dwarf. Massive star: nebula → protostar → main sequence → red supergiant → supernova → neutron star or black hole. Stars on the main sequence are balanced between gravity (inward) and radiation pressure from fusion (outward).

Section A — Formation of Stars

1. Describe how a protostar forms from a nebula. [3]
2. Explain what causes a protostar to begin nuclear fusion and become a main sequence star. [3]
3. Explain why a main sequence star is stable. Refer to the two forces in balance. [2]

Section B — Sun-Like Stars

4. List all stages in the life of a Sun-like star in the correct order. [4]
5. Explain what causes a main sequence star to expand into a red giant when it runs out of hydrogen. [3]
6. Describe the final stage of a Sun-like star and explain why it no longer produces light by fusion. [2]

Section C — Massive Stars

7. List all stages in the life of a star much more massive than the Sun, in the correct order. [5]
8. Describe what happens during a supernova. [3]
9. State the two possible end stages after a supernova and explain what determines which one forms. [2]
10. Explain the significance of supernovae for the formation of new stars and planets. [2]

Total marks: 29

Mark Scheme

1. A nebula is a cloud of gas and dust [1]; gravity pulls the material together [1]; as it contracts, the gas heats up to form a protostar [1] [3]
2. As the protostar contracts, the core temperature and pressure rise [1]; when the temperature is high enough (~10 million °C), hydrogen nuclei have sufficient kinetic energy to overcome electrostatic repulsion [1]; hydrogen fusion begins, releasing large amounts of energy [1] [3]
3. Inward gravitational force [1] is balanced by outward radiation pressure from nuclear fusion [1] [2]
4. Nebula → protostar → main sequence star → red giant → white dwarf [4 — 1 mark per correct stage in order; accept 3/4 if one stage is missing or slightly misplaced]
5. When hydrogen in the core runs out, fusion slows and radiation pressure decreases [1]; the core contracts under gravity and heats up [1]; the outer layers expand and cool, making the star a red giant [1] [3]
6. The star becomes a white dwarf — a hot, dense, slowly cooling remnant [1]; there is no longer any fusion taking place, so it only glows from residual heat [1] [2]
7. Nebula → protostar → (massive) main sequence star → red supergiant → supernova → neutron star / black hole [5 — 1 mark per stage]
8. When a massive star's core collapses rapidly [1], the outer layers are thrown off in a massive explosion [1]; releasing enormous energy and heavy elements into space [1] [3]
9. A neutron star or a black hole [1]; the more massive the remnant core after the supernova, the more likely it forms a black hole rather than a neutron star [1] [2]
10. Supernovae scatter heavy elements (formed in the star) into space [1]; these elements become part of new nebulae from which future stars and planetary systems (including Earth) can form [1] [2]