The life cycle of stars traces how a cloud of gas and dust becomes a star, then a white dwarf or a black hole. This is Extended (Supplement) only: Core candidates can skip it, but Extended candidates face it regularly as a 4-6 mark sequencing question. Examiners test it because the two branches, low mass and high mass, are easy to confuse under pressure.
How is a star born and what keeps it stable?
Stars form from interstellar clouds of gas and dust called nebulae. Gravity pulls the cloud inwards. As it collapses, the core gets denser and hotter, forming a protostar. When the core becomes hot enough, nuclear fusion starts: hydrogen nuclei fuse into helium, releasing energy. This is the same fusion that powers our Sun, which consists mostly of hydrogen and helium and radiates most of its energy as infrared, visible and ultraviolet light.
The star is now a main sequence star. It is stable because two influences balance:
| Direction | Cause |
|---|---|
| Inward | Gravitational attraction (weight of outer layers) |
| Outward | Pressure from the hot core, sustained by fusion |
The Sun has been on the main sequence for about 4.6 billion years and is roughly halfway through that stage. Most stars in our galaxy, the Milky Way, are main sequence stars. One light-year, the distance light travels in a year, is ; the Milky Way is about 100,000 light-years across.
What happens when the hydrogen runs out?
The path splits by mass. Learn both sequences as flowcharts.
A star like the Sun (low mass): main sequence → red giant → planetary nebula (outer layers ejected) → white dwarf, which slowly cools.
A star much more massive than the Sun: main sequence → red supergiant → supernova (a huge explosion) → neutron star, or a black hole if the core is massive enough.
A supernova matters beyond the explosion. It scatters hydrogen, helium and heavier elements into space as a nebula. New stars and planets form from this material: the elements in your body were made in stars.
Worked Exam Question
A star has a mass much greater than the Sun and has used up the hydrogen in its core. (a) Describe the remaining stages in the life cycle of this star. [3] (b) Explain why a main sequence star does not collapse under its own gravity. [2]
Solution, set out the way we teach it:
(a) The star expands into a red supergiant. It then explodes as a supernova. The core that remains becomes a neutron star, or a black hole if the core mass is large enough.
(b) Fusion in the core keeps it extremely hot, producing an outward pressure. This outward pressure balances the inward gravitational attraction, so the star stays stable.
Mark scheme:
- B1: red supergiant stage named
- B1: supernova explosion
- B1: neutron star OR black hole as final stage (both for safety)
- B1: outward force/pressure from fusion or hot core
- B1: balances inward gravitational force/attraction
Common Mistakes
- Mixing the branches. A Sun-like star never goes supernova. Red giant pairs with white dwarf; red supergiant pairs with supernova.
- Skipping the protostar. “Nebula → star” loses a mark. The sequence is nebula → protostar → main sequence.
- Saying stars “burn”. Burning is a chemical reaction. Stars release energy by nuclear fusion of hydrogen into helium. Write “fusion” every time.
- “Pressure equals gravity” with no source. State where the outward pressure comes from: the hot core, heated by fusion.
- Black hole as the only ending. Massive stars can leave a neutron star instead. Give both unless the question fixes the core mass.
Exam Technique Tip
Answer life-cycle questions as a numbered chain, one stage per line, with an arrow or “then” between stages. Examiners award B1 marks per correct stage in the correct order, so a clean list collects marks that a paragraph hides. Before the exam, practise writing both branches from memory in under 60 seconds.
How This Is Examined
This subtopic appears only on Extended papers: Paper 2 (multiple choice) and Paper 4 (theory). Paper 2 favours single-step questions: what balances gravity, what follows a supernova. Paper 4 sets 3-6 mark “describe the life cycle” questions, sometimes splitting low-mass and high-mass stars into parts (a) and (b). There is no Core version and no practical angle, so Papers 1, 3, 5 and 6 ignore it. Since many Malaysian schools teach Space Physics last, in the term before the exam, check this chapter has actually been covered. It is one of the most self-studyable in 0625.
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