Trace fuel depletion into giant stages
Relate central hydrogen depletion to red-giant or red-supergiant evolution according to mass.
Extended only (Supplement); Physics 0625, 2026–2028. Use supplied distances, periods and stated orbital models. Drawings are not to scale. Written practice is self-checked.
Before you begin
- Identify the stable stellar stage.
- Distinguish initial mass from apparent brightness.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which property selects the broad evolutionary branch?
What you will learn
- Relate central hydrogen depletion to red-giant or red-supergiant evolution according to mass.
- Use central fuel depletion without claiming all hydrogen everywhere suddenly vanishes.
Change the central fuel condition
During the stable stage, hydrogen in the central reaction region is converted into helium. Eventually the central hydrogen supply is depleted enough that the stable-stage conditions change. This does not mean that every hydrogen nucleus throughout the star disappears simultaneously.
Most stars in the syllabus’s less-massive branch expand into red giants. More massive stars follow the red-supergiant branch. The branch depends on stellar mass and evolution, not merely whether the distant star looks bright in an uncalibrated photograph.
Read a branching sequence
Both branches follow formation and a stable stage, but their later outcomes differ. Do not draw red giant → red supergiant as a compulsory sequence for every star. A branching diagram gives alternative routes for differently massive stars.
A larger outer radius does not require a hotter-looking outer surface. The label red refers to the evolved surface colour/temperature description, while the internal core conditions are a separate issue. Exact mass thresholds and detailed shell reactions are not required by this section.
Pause and explain
What does central hydrogen depletion mean?
Worked example
Choose the next branch for a Sun-like star and a much more massive star after central fuel changes.
Show the worked solution
- Both have undergone a stable stage.
- Use the less-massive branch for the Sun-like example.
- Use the more-massive branch for the massive example.
Answer Sun-like: red giant; more massive: red supergiant.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Give the giant-stage names for the two mass branches.
Give me a hint
Treat these as alternatives, not successive stages for every star.
Compare my reasoning
- Less massive follows a red-giant route.
- More massive follows a red-supergiant route.
- Keep both after the stable stage.
Less massive: red giant; more massive: red supergiant.
Look for these in your work
- Name both stages.
- Retain alternative branches.
Correct “red giant then red supergiant is every star’s fixed route”.
Give me a hint
Find the missing decision point in the proposed chain.
Compare my reasoning
- Initial mass affects the later route.
- Red giant belongs to the less-massive branch.
- Red supergiant belongs to the more-massive branch.
Replace the compulsory chain with mass-dependent alternative branches.
Look for these in your work
- Identify mass as the condition.
- Reject the universal sequential route.
A star’s outer hydrogen remains while its centre has mostly converted hydrogen to helium. Is giant-stage evolution ruled out?
Give me a hint
The required fuel condition concerns the centre.
Compare my reasoning
- Identify central depletion.
- Do not require all outer hydrogen to be absent.
- Apply the relevant mass branch.
No; central depletion can change the star’s structure even with outer hydrogen remaining.
Look for these in your work
- Use the central-region boundary.
- Avoid a whole-star disappearance claim.
Common mistakes
- Red giant and red supergiant are alternative mass routes.
- Central depletion is not instant whole-star hydrogen loss.
- Apparent brightness alone cannot select the route.
What can you explain now?
What does central hydrogen depletion mean?
Compare with the explanation
The central stable-stage fuel has been largely converted
The relevant central reaction region changes; outer hydrogen need not vanish at once.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Use central fuel depletion without claiming all hydrogen everywhere suddenly vanishes.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.