Describe the life cycle of a star similar in size to the Sun, starting from a nebula and ending at its final stage.

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

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

Scientists study stars at different stages of their life cycles to understand how our own Sun formed and how it will eventually change. By observing many different stars, astronomers can piece together the sequence of stages a star like the Sun passes through during its lifetime.

Model answer (4 marks)

A nebula of gas and dust collapses under its own gravity to form a protostar.
The protostar heats up until hydrogen nuclei fuse, creating a main‑sequence star where gravitational pressure balances the outward radiation pressure.
When core hydrogen is exhausted, the core contracts and the outer layers expand, turning the star into a red giant.
The red giant eventually sheds its outer layers, leaving a hot core that cools into a white dwarf, the final stage.

Examiner tips

Use the sequence: nebula → protostar → main sequence → red giant → white dwarf. Mention fusion of hydrogen and balance of forces. Show the core‑envelope changes (expansion, shedding).
Keep each stage brief but include key processes (gravity, fusion, expansion, shedding).

Common mistakes

Confusing the order of stages (e.g., putting red giant before main sequence). Omitting the role of hydrogen fusion or the balance of forces. Using vague terms like "star changes" without specifying red giant or white dwarf.

Mark scheme (4 marks)

A nebula (cloud of dust and gas) is pulled together by gravitational attraction to form a protostar
The protostar becomes hot enough for hydrogen nuclei to undergo fusion, forming a main sequence star where gravitational forces and outward pressure are balanced
When hydrogen runs out, the star expands to form a red giant (as larger nuclei are produced by fusion)
The red giant eventually collapses and sheds its outer layers to leave behind a white dwarf as the final stage

Key terms in this question

nebula