Explain the balance in a stable star
Explain how inward gravity is balanced by outward effects of high central temperature.
Section 6.2.2 · Stars
Exam scope and assessment notes
Extended only (Supplement) · Physics 0972, 2026–2028. Use supplied distances, periods and stated orbital models. Drawings are not to scale. Written practice is self-checked. Human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Use resultant-force reasoning.
- Recall the hot contracting protostar.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a zero resultant force establish?
What you will learn
- Explain how inward gravity is balanced by outward effects of high central temperature.
- Distinguish stable mechanical balance from absence of energy transfer.
Compare the opposing effects
A protostar enters a stable stellar stage when inward gravitational attraction is balanced by an outward force associated with high temperature in its centre. Hot material provides pressure support. “Outward force” here describes the pressure effect opposing contraction, rather than a solid external support or a rocket engine.
The star continues to release and radiate energy during approximate balance. Balanced opposing forces do not mean zero individual forces, zero temperature or zero radiation. For a supplied representative shell, balance means no net accelerating force on that shell in the simplified model.
Retain the evolutionary meaning
Stable means a long-lived approximate configuration, not that every part is permanently motionless or that its composition cannot change. Nuclear fuel is used during this stage, so the conditions supporting the balance eventually change.
If inward gravitational effect exceeds outward support in a supplied comparison, the tendency is contraction. If outward support exceeds gravity, the tendency is expansion. These are model tendencies; they do not specify the full time-dependent stellar response or a numerical lifespan.
Pause and explain
What supports a stable star against gravity?
Worked example
A model shell experiences inward force 40 units and outward force 40 units. State the resultant and what it does not imply.
Show the worked solution
- Choose outward positive.
- Calculate 40 − 40 = 0.
- Retain both non-zero forces and continuing radiation.
Answer Zero resultant in the shell model; neither zero forces nor zero energy emission.
Try a topic test
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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.
State the two opposing effects in a stable star.
Give me a hint
Name an inward interaction and its outward countereffect.
Compare my reasoning
- Identify gravitational attraction inward.
- Identify pressure support from high central temperature outward.
- State their approximate balance.
Inward gravity balances outward high-temperature pressure effects.
Look for these in your work
- Name both directions.
- Explain approximate balance.
A shell model has outward support 65 units and inward gravity 80 units. Find the resultant and tendency.
Give me a hint
Use one direction convention before subtracting the forces.
Compare my reasoning
- Take outward positive.
- Resultant = 65 − 80 = −15 units.
- The resultant is inward.
15 units inward; a contraction tendency.
Look for these in your work
- Subtract consistently.
- Give direction and tendency.
A student infers “no radiation because the star is stable”. Correct the inference.
Give me a hint
Mechanical balance and an energy budget answer different questions.
Compare my reasoning
- Opposing effects balance in the stable structure.
- The star still produces and radiates energy.
- Stable is not synonymous with no changes or no transfer.
The inference is false; a stable star still releases and radiates energy.
Look for these in your work
- Separate force balance from energy flow.
- Retain finite fuel.
Common mistakes
- Balance does not remove gravity.
- Stable does not mean no radiation.
- A shell model is not a full dynamical simulation.
What can you explain now?
What supports a stable star against gravity?
Compare with the explanation
Outward effects of high central temperature
Hot material produces pressure support that balances inward gravitational attraction.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Distinguish stable mechanical balance from absence of energy transfer.
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.