A student states: 'All stars go through exactly the same stages in their life cycle.' Explain why this statement is not fully correct, referring to the stages that follow the main sequence for stars of different sizes.

OCR GCSE Physics A: Gateway Science (J249) — P6.3 Astronomy · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Stars spend most of their lifetime in the main sequence stage, during which gravitational forces and outward pressure are balanced. However, what happens after this stage depends on the mass of the star.

Model answer (4 marks)

All stars are not identical in their evolution. During the main sequence the balance between gravity and pressure keeps a star stable. When the core hydrogen is exhausted a low‑mass star (≈1 M⊙) expands into a red giant, then sheds its outer layers and becomes a white dwarf. A high‑mass star (≫1 M⊙) instead continues to fuse heavier elements, becomes a red supergiant and eventually explodes as a supernova. The remnant of this explosion can be a neutron star or a black hole, whereas a Sun‑like star ends as a white dwarf.

Examiner tips

  • State the main‑sequence stability first. Mention the red‑giant phase for low‑mass stars. Contrast with the supernova and neutron star/black hole outcome for massive stars. Use the exact terms ‘red giant’, ‘red supergiant’, ‘supernova’, ‘neutron star’, ‘black hole’, ‘white dwarf’.
  • Keep the answer brief – 4 marks – so avoid extra detail such as planetary nebulae. Use the word ‘massive’ to distinguish the two cases.

Common mistakes

  • Confusing the red giant phase of low‑mass stars with the red supergiant phase of massive stars.
  • Using the word ‘explodes’ without specifying a supernova or the type of remnant.
  • Failing to mention that a Sun‑like star ends as a white dwarf, not a neutron star or black hole.

Mark scheme (4 marks)

  1. In the main sequence, gravitational forces and outward pressure are balanced (accept: stars are stable during the main sequence)
  2. When hydrogen is used up, a star similar in size to the Sun expands to form a red giant
  3. A massive star (much larger than the Sun) explodes as a supernova after the main sequence (accept: after the red supergiant stage)
  4. After a supernova, a massive star can form a neutron star or black hole, whereas a Sun-like star ends as a white dwarf (accept either contrast stated clearly)

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