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

These notes condense section 5.5.1 (Stars), outcomes (a) to (g), of the OCR A Level Physics A (H556) specification, Version 3.0 (March 2026). Full explanations are in the study guide. Test yourself with the practice questions, tick off topics on the checklist, check the course hub, and use the free diagnostics to find weak areas.

Definitions (5.5.1(a))

Term	Definition to learn
Planet	Orbits a star; massive enough to be near-spherical; has cleared its orbital neighbourhood
Planetary satellite	Smaller body orbiting a planet (a moon is a natural one)
Comet	Small icy body in an elongated orbit; tail of gas and dust forms near the star and points away from it
Solar system	A star plus the bodies orbiting it, bound by gravity
Galaxy	Huge group of stars, gas and dust bound by gravity
Universe	All space, matter, radiation and energy

Size order: satellite, planet, solar system, galaxy, universe.

Star formation (5.5.1(b))

Method in steps:

1. Cloud of interstellar dust and gas (mainly hydrogen) clumps under gravity.
2. Gravitational collapse: gravitational potential energy becomes thermal energy, so the clump heats and forms a protostar.
3. Core reaches about ten million kelvin: hydrogen fuses into helium.
4. Gravity inwards balances gas pressure plus radiation pressure outwards: a stable main sequence star.

Pressure	Source
Gas pressure	Collisions of hot particles
Radiation pressure	Photons released by fusion

Why high temperature? Nuclei repel each other electrostatically, so they need high kinetic energy to get close enough to fuse.

Low-mass star (5.5.1(c))

Main sequence, then red giant, then planetary nebula, then white dwarf.

Stage	Key point
Core hydrogen runs out	Core contracts and heats; hydrogen fuses in a shell around it
Red giant	Outer layers expand and cool; luminous because large
Planetary nebula	Outer layers ejected as a shell of gas
White dwarf	Exposed core; no fusion; cools slowly

White dwarfs (5.5.1(d))

• Mass up to 1.4 solar masses, radius about that of Earth, very dense, hot but dim.
• Electron degeneracy pressure: electrons cannot share quantum states (Pauli exclusion principle), so compressing them forces them into higher energy states. This gives an outward pressure that does not depend on temperature.
• Chandrasekhar limit: maximum mass of a stable white dwarf, about 1.4 solar masses (2.8 × 10³⁰ kg).

Massive star (5.5.1(e))

Main sequence, then red supergiant, then supernova, then neutron star or black hole.

Method in steps:

1. After core hydrogen runs out, the star expands to a red supergiant and fuses heavier nuclei in layers.
2. Fusion stops at iron because fusing iron does not release energy.
3. The core is above the limit electron degeneracy pressure can support, so it collapses very quickly.
4. Outer layers are thrown off in a supernova.
5. Remnant core below a few solar masses becomes a neutron star; above that it becomes a black hole.

Neutron stars and black holes (5.5.1(f))

Feature	Neutron star	Black hole
Size	Radius about 10 to 15 km	Event horizon radius depends on mass; singularity inside
Composition	Almost entirely neutrons	Described by mass; matter inside cannot be observed
Density	About 10¹⁷ kg m⁻³, like a nucleus	Not defined as a single value
Escape velocity	Below c	Equal to c at the event horizon; greater inside
Seen how	Radiation from the star or its surroundings	Effects on nearby matter; radiation from infalling matter

HR diagram (5.5.1(g))

• Vertical axis: luminosity (often in solar units, log scale). Horizontal axis: surface temperature, decreasing left to right.
• Main sequence: diagonal band, top left to bottom right.
• Red giants: upper right. Super red giants: top right. White dwarfs: bottom left.

Region	Temperature	Luminosity	Size
Main sequence	Wide range	Wide range	Wide range
Red giant	Low	High	Large
Super red giant	Low	Very high	Very large
White dwarf	High	Low	Small

Evolution tracks: Sun-like star, main sequence to red giant region to white dwarf region. Massive star, main sequence to supergiant region, then supernova.

Worked reminders

Density. ρ = M / ((4/3)πr³). Example: a star of mass 2.4 × 10³⁰ kg and radius 9.0 × 10⁶ m. Volume = 3.05 × 10²¹ m³. Density = 7.9 × 10⁸ kg m⁻³, typical of a white dwarf.

Comparing with the limit. 1.4 solar masses = 1.4 × 1.99 × 10³⁰ = 2.8 × 10³⁰ kg. A remnant core of 3.1 × 10³⁰ kg is greater, so it cannot be a white dwarf.

Escape condition. A body is a black hole when the escape velocity from its surface would exceed c. Using v = √(2GM/r), the horizon radius is r = 2GM/c². For a mass of 6.0 × 10³⁰ kg: r = 2 × 6.67 × 10⁻¹¹ × 6.0 × 10³⁰ / (3.00 × 10⁸)² = 8.9 × 10³ m.

Model explanations to learn

Why does a main sequence star not collapse? Gravity acts inwards. Fusion in the core keeps the gas hot, so gas pressure and radiation pressure act outwards. The two balance, so the star’s size stays steady.

Why does a star become a red giant? Core hydrogen runs out, so the core contracts and heats. Hydrogen fuses in a shell around it, which pushes the outer layers outwards. They expand and cool, so the surface looks red.

Why can a white dwarf not exceed 1.4 solar masses? Electron degeneracy pressure is the only thing resisting gravity. Above the Chandrasekhar limit it is too small to balance gravity, so collapse continues.

Why does a supernova follow iron formation? Iron fusion absorbs energy rather than releasing it. The core loses its outward support and collapses. The rebound throws off the outer layers.

Timeline checklist

Star	Sequence
Low-mass	Gas cloud, then protostar, then main sequence, then red giant, then planetary nebula leaving a white dwarf
Massive	Gas cloud, then protostar, then main sequence, then red supergiant, then supernova leaving a neutron star or black hole

Must-know distinctions

• Planetary nebula versus planet: the nebula is a shell of ejected gas.
• Red giant versus red supergiant: giant from a low-mass star, supergiant from a massive star.
• White dwarf versus neutron star: electron degeneracy pressure versus neutron-based support; radius about Earth’s versus about 12 km.
• Gas pressure versus radiation pressure: particle collisions versus photons.
• Nuclear fusion in the core (main sequence) versus in a shell (giant stage).

Quick self-test

1. Name the two outward pressures in a main sequence star.
2. Why must a protostar reach a very high core temperature before fusion starts?
3. What forms from the ejected outer layers of a low-mass star?
4. State the cause of electron degeneracy pressure.
5. Convert the Chandrasekhar limit to kilograms (1.99 × 10³⁰ kg per solar mass).
6. Why does fusion in a massive star stop at iron?
7. State the escape velocity at the event horizon of a black hole.
8. On an HR diagram, which corner has hot, dim stars?
9. A star has luminosity 500 times the Sun’s and a surface cooler than the Sun’s. Where is it, and why is it so luminous?
10. Calculate the mean density of a neutron star of mass 2.8 × 10³⁰ kg and radius 1.3 × 10⁴ m.

Answers

1. Gas pressure and radiation pressure.
2. Nuclei repel electrostatically; they need high kinetic energy to get close enough to fuse.
3. A planetary nebula; the remaining core becomes a white dwarf.
4. Electrons cannot occupy the same quantum state (Pauli exclusion principle), so compression forces them into higher energy states.
5. 1.4 × 1.99 × 10³⁰ = 2.8 × 10³⁰ kg.
6. Fusing iron does not release energy, so no outward pressure is produced to resist gravity.
7. Equal to the speed of light, 3.00 × 10⁸ m s⁻¹.
8. Bottom left (white dwarfs).
9. Upper right, in the red giant region. Its surface is cool, so it is luminous because its surface area is very large.
10. V = (4/3)π(1.3 × 10⁴)³ = 9.20 × 10¹² m³; ρ = 2.8 × 10³⁰ / 9.20 × 10¹² = 3.0 × 10¹⁷ kg m⁻³.

Where marks are usually lost

• Saying “the star runs out of fuel” with no mention of what happens to the balance of forces.
• Describing a planetary nebula as a nebula that makes planets.
• Naming electron degeneracy pressure for a neutron star.
• Giving the limit as 1.4 kg or 1.4 with no unit; say 1.4 solar masses.
• Reading the temperature axis the wrong way round.
• Saying a white dwarf is “burning” or “fusing”.
• Describing a supernova as the end of the Sun.
• Forgetting to convert km to m in density or escape calculations.
• Writing that a black hole “sucks in” everything, instead of saying that escape is impossible from inside the event horizon.
• Giving a density answer to too many significant figures when the data has two.

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).