OCR A Level Physics A: Stars, stellar evolution and the HR diagram (H556) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover section 5.5.1 (Stars), outcomes (a) to (g), of the OCR A Level Physics A (H556) specification, Version 3.0 (March 2026). Learn the content from the study guide and revision notes. The course hub and checklist show the rest of the course.

Data: c = 3.00 × 10⁸ m s⁻¹; G = 6.67 × 10⁻¹¹ N m² kg⁻²; 1 solar mass = 1.99 × 10³⁰ kg.

Questions

1. State what is meant by each term.

(a) planetary satellite [1] (b) comet [1] (c) galaxy [1]

2. Explain how an interstellar cloud of gas and dust turns into a stable star. [4]

3. A main sequence star radiates 6.0 × 10²⁶ W, all supplied by fusion. Calculate the mass converted to energy each second. [3]

4. Describe what happens to a star like the Sun from the time hydrogen in its core is used up until it becomes a white dwarf. [5]

5. A white dwarf has a mass of 1.1 solar masses and a radius of 6.0 × 10⁶ m.

(a) Explain how electron degeneracy pressure supports a white dwarf. [2] (b) State the Chandrasekhar limit. [1] (c) Calculate the mean density of this white dwarf. [3]

6. Explain how a massive star ends its life in a supernova. [6]

7. A neutron star has mass 2.2 × 10³⁰ kg and radius 11 km.

(a) Calculate its mean density. [3] (b) Calculate the gravitational field strength at its surface. [2] (c) Calculate the escape velocity from its surface and state whether it is a black hole. [3]

8. (a) Describe two characteristics of a neutron star. [2] (b) State what is meant by the event horizon of a black hole. [2] (c) Show that the event horizon radius is 2GM/c², then find it for a black hole of 12 solar masses. [3]

9. Four stars have these luminosities (in solar units) and surface temperatures.

Star	Luminosity	Temperature / K
W	2 × 10⁻³	9 000
X	8 × 10²	4 500
Y	5 × 10⁴	25 000
Z	1 × 10⁵	3 000

(a) Name the region of the HR diagram for each star. [4] (b) Find how many times more luminous Z is than X, and suggest why Z is more luminous although it is cooler. [2]

10. Compare a white dwarf and a neutron star in terms of what supports them, their size, their density and how each forms. [7]

11. (a) State where a star like the Sun lies on the HR diagram during its main sequence life, as a red giant and as a white dwarf. [3] (b) Explain why the star is more luminous as a red giant than on the main sequence even though its surface is cooler. [2] (c) State one difference between a white dwarf and a main sequence star of the same surface temperature. [1]

Answers

1. (a) A smaller body that orbits a planet (a moon is a natural example) [1] (b) A small icy body in an elongated orbit around a star, developing a tail of gas and dust near it [1] (c) A very large group of stars, gas and dust held together by gravity [1] Examiner insight: One precise idea per mark gains credit; a comet described only as “a rock” does not distinguish it.

2. Gravity pulls the cloud into clumps, which collapse [1]; gravitational potential energy becomes thermal energy, heating the protostar [1]; the core becomes hot enough for hydrogen nuclei to overcome repulsion and fuse into helium [1]; outward gas pressure and radiation pressure balance gravity, so the star is stable [1] Examiner insight: Answers on stability gain credit by naming both pressures and saying they balance gravity, not just that fusion occurs.

3. E = mc² per second, so m = P t / c² [1]; m = 6.0 × 10²⁶ × 1 / (3.00 × 10⁸)² [1]; 6.7 × 10⁹ kg each second [1] Examiner insight: Show the unit kg per second or the time of 1 s; a bare number leaves the working unclear.

4. Core hydrogen runs out, so the core contracts and heats [1]; hydrogen fuses in a shell around the core and the outer layers expand and cool, giving a red giant [1]; the outer layers are ejected as a planetary nebula [1]; the core remains as a white dwarf [1]; no fusion occurs in it and it cools slowly [1] Examiner insight: A named sequence with no physical reason for each change gains little; link each stage to the pressure balance.

5. (a) Electrons cannot occupy the same quantum state (Pauli exclusion principle) [1]; squeezing them forces them to higher energies, giving an outward pressure independent of temperature [1] (b) The maximum mass of a stable white dwarf, about 1.4 solar masses [1] (c) M = 1.1 × 1.99 × 10³⁰ = 2.19 × 10³⁰ kg [1]; V = (4/3)π(6.0 × 10⁶)³ = 9.05 × 10²⁰ m³ [1]; ρ = M/V = 2.4 × 10⁹ kg m⁻³ [1] Examiner insight: A limit given without its unit (solar masses or kg) is incomplete; cubing the radius before multiplying avoids a common slip.

6. The star becomes a red supergiant [1]; heavier nuclei fuse in layers up to iron [1]; fusing iron does not release energy, so the core loses its outward pressure [1]; the core mass is too large for electron degeneracy pressure to support it, so it collapses rapidly [1]; the outer layers are thrown off in a supernova [1]; the remnant is a neutron star or a black hole, depending on its mass [1] Examiner insight: The step explaining why collapse occurs is the one most often missing; a list of stage names alone does not explain.

7. (a) V = (4/3)π(1.1 × 10⁴)³ = 5.58 × 10¹² m³ [1]; ρ = 2.2 × 10³⁰ / 5.58 × 10¹² [1]; 3.9 × 10¹⁷ kg m⁻³ [1] (b) g = GM/r² = 6.67 × 10⁻¹¹ × 2.2 × 10³⁰ / (1.1 × 10⁴)² [1]; 1.2 × 10¹² N kg⁻¹ [1] (c) v = √(2GM/r) = √(2 × 6.67 × 10⁻¹¹ × 2.2 × 10³⁰ / 1.1 × 10⁴) = 1.6 × 10⁸ m s⁻¹ [1]; this is 0.54 of c [1]; it is below c, so light can escape and the object is not a black hole [1] Examiner insight: Convert 11 km to metres before substituting, and finish by comparing your answer with c so the conclusion is justified.

8. (a) Any two from: radius about 10 to 15 km; almost entirely neutrons; density about 10¹⁷ kg m⁻³ like a nucleus. One mark each [1] [1] (b) The boundary around a black hole [1]; within it the escape velocity exceeds c, so nothing, including light, can escape [1] (c) Set v = c in v² = 2GM/r, so r = 2GM/c² [1]; M = 12 × 1.99 × 10³⁰ = 2.39 × 10³¹ kg and r = 2 × 6.67 × 10⁻¹¹ × 2.39 × 10³¹ / (3.00 × 10⁸)² [1]; 3.5 × 10⁴ m [1] Examiner insight: In “show that” questions, start from the stated condition and arrive at the given result; do not begin with the formula already written.

9. (a) W: white dwarf (dim, fairly hot) [1]; X: red giant [1]; Y: main sequence (hot and luminous) [1]; Z: super red giant [1] (b) 1 × 10⁵ / 8 × 10² = 125 times [1]; Z has a much larger surface area (it is a bigger star) [1] Examiner insight: Place a star using both coordinates; naming only the luminosity does not separate a giant from a hot main sequence star.

10. A white dwarf is supported by electron degeneracy pressure [1]; a neutron star by pressure between neutrons [1]; white dwarf radius is about that of Earth, 10⁶ m [1]; neutron star radius about 10 to 15 km [1]; densities of the order of 10⁹ kg m⁻³ and 10¹⁷ kg m⁻³ respectively [1]; a white dwarf is the core left after a low-mass star ejects a planetary nebula [1]; a neutron star is the remnant of a massive star’s core after a supernova [1] Examiner insight: Compare like with like (support with support, size with size); two separate descriptions without comparison rarely earn full credit.

11. (a) Main sequence: a diagonal band from upper left to lower right [1]; red giant: upper right [1]; white dwarf: lower left [1] (b) The surface is cooler, so each square metre emits less power [1]; the surface area is vastly greater, so the total power is greater [1] (c) The white dwarf has much lower luminosity because it is much smaller [1] Examiner insight: Full explanations of luminosity use both temperature and surface area, not only one.

Where marks are usually lost

• Not converting km to m before calculating density, field strength or escape velocity.
• Giving the Chandrasekhar limit with no unit or a unit of kg alone with the wrong value.
• Saying a white dwarf is held up by fusion.
• Describing the planetary nebula as made of planets.
• Omitting the reason the core collapses in a massive star.
• Reading the temperature axis the wrong way round.
• Using only one of gas pressure and radiation pressure.
• Failing to compare an escape velocity with c.

Next steps

• Revision notes
• Study guide
• OCR A Level Physics hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

OCR Level 3 Advanced GCE in Physics A (H556), Version 3.0 (March 2026), first assessment 2017, published by OCR. Section 5.5.1, outcomes (a) to (g).