Physics • Year 12 • Module 8 • Lesson 6
The Stellar Life Cycle
Apply your understanding of stellar evolution pathways, fusion stages, and nucleosynthesis to real data, stellar scenarios, and a sequencing task.
1. Sequence the fusion stages — massive star
The table below lists nine events in the life of a massive star (> 8 M☉), in shuffled order. In the “Order” column, write the correct sequence number (1 = first, 9 = last). 9 marks (1 per correctly placed event)
| Event | Order (1–9) |
|---|---|
| Silicon burning produces an iron-nickel core; the core exceeds the Chandrasekhar limit and collapses. | |
| Gravitational collapse of a molecular cloud forms a protostar; core temperature rises toward 107 K. | |
| Carbon and oxygen burning in the core at temperatures ~6×108 K; outer layers of hydrogen and helium shell burning continue. | |
| Hydrogen fusion (p-p chain or CNO cycle) ignites in the core; the star enters the main sequence in hydrostatic equilibrium. | |
| The outer layers are ejected in a supernova explosion; heavy elements are dispersed into the interstellar medium. | |
| Core hydrogen is exhausted; the core contracts and hydrogen shell burning begins; the outer layers expand into a red supergiant. | |
| Helium burning (triple-alpha process) ignites in the core at ~108 K, producing carbon-12 and oxygen-16. | |
| The collapsed core stabilises as a neutron star (if remnant < 3 M☉) or continues collapsing into a black hole. | |
| Neon, oxygen, and silicon burning proceed in sequential shells; the star develops a layered “onion” core structure. |
2. Interpret stellar data — classifying stellar remnants
The table below lists six stars with their initial masses and observed outcomes. 9 marks
| Star | Initial mass (M☉) | Main sequence lifetime (approx.) | Expected final remnant | Highest fusion stage reached |
|---|---|---|---|---|
| Star A | 0.3 | > 1 trillion yr | ||
| Star B | 1.0 (our Sun) | ~10 billion yr | ||
| Star C | 3 | ~350 million yr | ||
| Star D | 12 | ~20 million yr | ||
| Star E | 25 | ~8 million yr | ||
| Star F | 60 | ~3 million yr |
2.1 Complete the “Expected final remnant” and “Highest fusion stage reached” columns for all six stars. 6 marks (1 per row)
2.2 Describe the general relationship between initial stellar mass and main sequence lifetime, using data from at least two stars to support your answer. 2 marks
2.3 Which star (or stars) in the table could potentially contribute gold atoms to the interstellar medium? Explain why. 1 mark
3. Interpret graph — binding energy per nucleon
The graph below shows the binding energy per nucleon (MeV/nucleon) as a function of atomic mass number (A) for selected nuclei. 7 marks
Figure 3.1. Binding energy per nucleon versus mass number. Based on nuclear data from the National Nuclear Data Center (NNDC). Note: curve has been smoothed for clarity.
3.1 Describe the shape of the binding energy curve from hydrogen (A = 1) to uranium (A = 238), identifying the nucleus at the peak. 2 marks
3.2 Using the graph, explain why stellar fusion can release energy only up to iron. What happens energetically when nuclei heavier than iron are fused? 3 marks
3.3 A student states: “Gold-197 must be produced by fusion in stellar cores because it has a reasonably high binding energy.” Use the graph and your knowledge of nucleosynthesis to evaluate this claim. 2 marks
4. Predict and justify — a 15 M☉ star scenario
A star is observed on the main sequence with an estimated initial mass of 15 M☉. Astronomers predict it has approximately 10 million years remaining on the main sequence. 6 marks
4.1 Predict the complete future evolutionary sequence for this star, from its current main sequence phase to its final remnant. Name each stage and the key physical process that drives the transition between them. 4 marks
4.2 The star’s supernova will disperse heavy elements into a nearby molecular cloud. Predict which elements heavier than iron this explosion contributes and explain the nucleosynthesis process responsible. 2 marks
5. Compare low-mass and massive star evolution
Complete the two-column table below. For each feature, write a concise description contrasting the two types of star. 10 marks (1 per cell)
| Feature | Low-mass star (0.5–8 M☉) | Massive star (> 8 M☉) |
|---|---|---|
| Main sequence lifetime | ||
| Post-main sequence stage | ||
| Highest fusion stage | ||
| Final remnant | ||
| Contribution to heavy element inventory |
Q1 — Sequence the fusion stages (correct order)
1 = Gravitational collapse of a molecular cloud (protostar). 2 = Hydrogen fusion ignites; main sequence begins. 3 = Core hydrogen exhausted; core contracts; red supergiant forms. 4 = Helium burning (triple-alpha) in core at ~108 K. 5 = Carbon and oxygen burning at ~6×108 K in sequential shells. 6 = Neon, oxygen, silicon burning; layered onion-shell structure develops. 7 = Silicon burning produces iron core; core exceeds Chandrasekhar limit and collapses. 8 = Outer layers ejected in supernova; heavy elements dispersed. 9 = Remnant stabilises as neutron star or black hole.
Q2.1 — Complete table
Star A (0.3 M☉): Final remnant — eventually a white dwarf (but lifetime exceeds age of universe so still on main sequence); highest fusion — hydrogen only (p-p chain). Star B (1 M☉): White dwarf; hydrogen (core) + helium (triple-alpha, briefly). Star C (3 M☉): White dwarf; hydrogen and helium burning. Star D (12 M☉): Neutron star (after supernova); full sequence to iron. Star E (25 M☉): Neutron star or black hole (after supernova); full sequence to iron. Star F (60 M☉): Black hole (after supernova); full sequence to iron.
Q2.2 — Mass vs lifetime relationship
There is an inverse relationship between initial stellar mass and main sequence lifetime [1]. More massive stars are far more luminous and burn through their hydrogen fuel at a much higher rate: Star B (1 M☉) lives ~10 billion years, while Star F (60 M☉) lives only ~3 million years — roughly 3000 times shorter despite having 60 times more fuel [1].
Q2.3 — Which stars contribute gold
Stars D, E, and F (masses 12, 25, and 60 M☉) can contribute gold atoms because they are massive enough (> 8 M☉) to end in Type II supernovae, which provide the extreme neutron flux required for the r-process to build nuclei heavier than iron, including gold-197.
Q3.1 — Binding energy curve description
The binding energy per nucleon rises steeply for low-A nuclei (hydrogen to helium), continues increasing more gradually through carbon and oxygen, reaches a broad maximum peak at iron-56 (and nickel-62) at approximately 8.8 MeV/nucleon, then decreases gradually for heavier elements through gold and uranium [1 for trend; 1 for identifying iron-56 as the peak].
Q3.2 — Why fusion releases energy only up to iron
Fusion releases energy when lighter nuclei combine to form a product with a higher binding energy per nucleon (more tightly bound). On the rising portion of the curve (A < 56), fusion moves nuclei toward the peak, releasing the difference in binding energy as radiation [1]. Iron-56 sits at the peak; fusing iron would produce a product with lower binding energy per nucleon, meaning energy must be input to make the reaction occur — it is endothermic [1]. This endothermic process absorbs the energy that was supporting the star against gravity, triggering core collapse [1].
Q3.3 — Evaluating the claim about gold
The student’s claim is incorrect [1]. Gold-197 does have a binding energy of ~7.9 MeV/nucleon, but this is lower than iron-56 (8.8 MeV/nucleon). Fusing nuclei to form gold would require energy input, not release it — so gold cannot be produced by fusion in a stellar core. Gold is produced by the r-process (rapid neutron capture) in supernovae or neutron star mergers, not by thermonuclear fusion [1].
Q4.1 — Evolutionary sequence for a 15 M☉ star
Main sequence (hydrogen fusion in core, hydrostatic equilibrium) → core hydrogen exhaustion triggers core contraction and hydrogen shell burning, outer layers expand forming a red supergiant → sequential core burning: He → C → O → Ne → Si → iron core formation → iron core exceeds Chandrasekhar limit, electron degeneracy fails, core collapses → Type II supernova explosion → remnant becomes a neutron star (remnant mass likely < 3 M☉) or possibly a black hole. [1 per correct transition with process, max 4]
Q4.2 — Heavy elements from supernova
The supernova will contribute elements heavier than iron, including elements such as gold, platinum, lead, and uranium [1], produced by the r-process (rapid neutron capture). During the supernova, the extreme neutron flux bombards nuclei faster than they can beta-decay, building successively heavier neutron-rich nuclides that later decay to stable heavy isotopes [1].
Q5 — Compare and contrast table
Main sequence lifetime: Low-mass: billions of years (Sun ~10 Gyr); Massive: millions of years (< 50 Myr). Post-main sequence stage: Low-mass: red giant → planetary nebula; Massive: red supergiant → supernova. Highest fusion stage: Low-mass: helium burning (triple-alpha; carbon-oxygen core for solar-mass stars); Massive: full sequence to silicon burning, producing iron-nickel core. Final remnant: Low-mass: white dwarf (supported by electron degeneracy pressure); Massive: neutron star (< 3 M☉ remnant) or black hole (> 3 M☉ remnant). Contribution to heavy elements: Low-mass: carbon, oxygen, nitrogen from helium/carbon burning and CNO cycle; Massive: elements up to iron from core burning, plus elements heavier than iron via r-process in supernova.