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

This guide covers section 5.5.1 (Stars), outcomes (a) to (g), of the OCR A Level Physics A (H556) specification, Version 3.0 (March 2026). It is full A level content. Spectra, Wien’s law, Stefan’s law and stellar radius (5.5.2) and distances, redshift and the Big Bang (5.5.3) are separate units and are not repeated here.

Use it with the revision notes and the practice questions. The course hub and the printable checklist show where this unit sits, and the free diagnostics help you find weak spots first. The density and field calculations lean on the unit skills in the foundations guide.

What this unit covers

Outcome	What you must be able to do
5.5.1(a)	Define planets, planetary satellites, comets, solar systems, galaxies and the universe
5.5.1(b)	Explain star formation: gravitational collapse, hydrogen fusion into helium, radiation pressure and gas pressure
5.5.1(c)	Describe a low-mass star becoming a red giant, then a white dwarf, with a planetary nebula
5.5.1(d)	Describe white dwarfs; explain electron degeneracy pressure and the Chandrasekhar limit
5.5.1(e)	Describe a massive star becoming a red supergiant, a supernova, then a neutron star or black hole
5.5.1(f)	Describe the characteristics of a neutron star and a black hole
5.5.1(g)	Use the HR diagram: luminosity against temperature, main sequence, red giants, supergiants, white dwarfs

The specification links several outcomes to how ideas about stars developed and are tested by observation. Expect to be asked to explain, not just name.

5.5.1(a) The objects

• Planet: a body that orbits a star, is massive enough for gravity to pull it into a near-spherical shape, and has cleared the neighbourhood around its orbit of other bodies.
• Planetary satellite: a smaller body in orbit around a planet. A moon is a natural satellite.
• Comet: a small body of ice and dust in an elongated orbit around a star. When it is near the star, heating releases gas and dust that form a tail pointing away from the star.
• Solar system: a star together with the planets, satellites, comets and other bodies that orbit it, held by gravity. Ours is the Sun and its orbiting bodies.
• Galaxy: a very large collection of stars, gas and dust held together by gravity. The Sun belongs to the Milky Way.
• Universe: all of space and everything in it: matter, radiation and energy.

Order of size, smallest to largest: planetary satellite, planet, solar system, galaxy, universe. A common slip is to call a galaxy a “solar system” because both contain stars.

5.5.1(b) Forming a star

A star begins in an interstellar cloud of dust and gas, mostly hydrogen.

1. Gravity pulls the cloud together. Denser clumps attract more matter, so they collapse faster. This is gravitational collapse.
2. As the clump shrinks, gravitational potential energy transfers to the thermal energy of the particles. The clump heats up and becomes a protostar.
3. When the core temperature is high enough (of order ten million kelvin), hydrogen nuclei approach closely enough to overcome their electrostatic repulsion. Hydrogen fuses into helium, releasing energy.
4. The star settles on the main sequence.

On the main sequence the star is stable because two effects balance. Gravity acts inwards. Outward pressure comes from the hot gas (gas pressure, from particle collisions) and from the photons released by fusion (radiation pressure). If the core cooled, outward pressure would fall and gravity would shrink the star until it heated again. This self-correcting balance is called equilibrium.

Worked example: energy from fusion

Data supplied: when four hydrogen nuclei fuse into one helium nucleus, the mass of the products is 0.0287 u less than the mass of the four nuclei. 1 u = 1.66 × 10⁻²⁷ kg, c = 3.00 × 10⁸ m s⁻¹, mass of a proton = 1.67 × 10⁻²⁷ kg. A star has a luminosity of 2.0 × 10²⁷ W. Find the number of fusion events each second and the mass of hydrogen consumed each second.

Mass lost per event: 0.0287 × 1.66 × 10⁻²⁷ = 4.76 × 10⁻²⁹ kg.

Energy per event: E = mc² = 4.76 × 10⁻²⁹ × (3.00 × 10⁸)² = 4.29 × 10⁻¹² J.

Events per second: 2.0 × 10²⁷ / 4.29 × 10⁻¹² = 4.7 × 10³⁸ s⁻¹.

Hydrogen used per second: 4 × 1.67 × 10⁻²⁷ × 4.66 × 10³⁸ = 3.1 × 10¹² kg s⁻¹.

5.5.1(c) A low-mass star such as the Sun

1. Main sequence: hydrogen fuses to helium in the core for most of the star’s life.
2. Hydrogen runs out in the core. Fusion there slows, so outward pressure falls and the core contracts and heats up. Hydrogen fusion continues in a shell around the core.
3. Red giant: the outer layers expand greatly and cool. The surface is cooler and redder, but the star is far more luminous because the surface area is so large. The core gets hot enough for helium to fuse into heavier nuclei such as carbon.
4. Planetary nebula: the outer layers are blown off and drift away as a glowing shell of gas. The name is historical and has nothing to do with planets.
5. White dwarf: the exposed core remains. No fusion takes place in it. It slowly cools.

5.5.1(d) White dwarfs

A white dwarf has a mass of the order of the Sun’s but a radius similar to Earth’s, so it is extremely dense. It is hot and white or blue-white at first, but dim because it is small.

What stops it collapsing? Not fusion. Electron degeneracy pressure supports it. Electrons obey the Pauli exclusion principle: no two can occupy the same quantum state. Squeezing the electrons together forces them into higher energy states, and this produces an outward pressure that does not depend on temperature.

The Chandrasekhar limit is the maximum mass of a stable white dwarf, about 1.4 solar masses (2.8 × 10³⁰ kg). Above it, electron degeneracy pressure cannot balance gravity.

Worked example: density of a white dwarf

A white dwarf has mass 0.60 solar masses and radius 8.7 × 10⁶ m. One solar mass is 1.99 × 10³⁰ kg. Find its mean density, then say whether a core of 1.9 solar masses could form a stable white dwarf.

M = 0.60 × 1.99 × 10³⁰ = 1.19 × 10³⁰ kg.

V = (4/3)πr³ = (4/3)π(8.7 × 10⁶)³ = 2.76 × 10²¹ m³.

ρ = M/V = 1.19 × 10³⁰ / 2.76 × 10²¹ = 4.3 × 10⁸ kg m⁻³.

A 1.9 solar mass core (3.8 × 10³⁰ kg) is above the limit of 2.8 × 10³⁰ kg, so electron degeneracy pressure could not support it. It could not settle as a white dwarf.

5.5.1(e) A massive star

A star of much greater mass than the Sun (roughly eight solar masses or more) begins the same way but is hotter and denser in the core, so it burns its fuel faster and lives a shorter life.

1. Red supergiant: after hydrogen runs out in the core, the star expands to an enormous size with a cool, red surface. Core temperatures become high enough to fuse heavier and heavier nuclei, forming layers.
2. Iron core: fusion up to iron releases energy. Fusing iron does not, so the core stops producing energy.
3. Core collapse: the core is too massive for electron degeneracy pressure to hold it. It collapses in a fraction of a second.
4. Supernova: the outer layers rebound and are thrown into space in a huge explosion that is extremely luminous. It scatters heavy elements that later form new stars and planets.
5. Remnant: a neutron star if the remaining core is below a limit of a few solar masses, otherwise a black hole.

5.5.1(f) Neutron stars and black holes

Neutron star

• Radius only about 10 to 15 km, with a mass above the Chandrasekhar limit.
• Made almost entirely of neutrons. During collapse electrons and protons combine into neutrons.
• Density of the order of 10¹⁷ kg m⁻³, comparable to the density of an atomic nucleus.
• Supported against gravity by pressure between the neutrons (neutron degeneracy pressure).

Black hole

• A region where gravity is so strong that nothing, not even electromagnetic radiation, can escape from inside the event horizon. The escape velocity at the horizon equals the speed of light.
• The mass is thought to be concentrated at a point called the singularity.
• It emits no light itself. It is detected through its gravitational effect on nearby stars or through radiation from matter heated as it falls in.

Worked example: a neutron star

A neutron star has mass 1.5 solar masses and radius 12 km. G = 6.67 × 10⁻¹¹ N m² kg⁻², c = 3.00 × 10⁸ m s⁻¹.

M = 1.5 × 1.99 × 10³⁰ = 2.99 × 10³⁰ kg. V = (4/3)π(1.2 × 10⁴)³ = 7.24 × 10¹² m³.

Density = 4.1 × 10¹⁷ kg m⁻³.

Surface field strength: g = GM/r² = 6.67 × 10⁻¹¹ × 2.99 × 10³⁰ / (1.2 × 10⁴)² = 1.4 × 10¹² N kg⁻¹.

Escape velocity: v = √(2GM/r) = 1.8 × 10⁸ m s⁻¹, which is 0.61 of c. It is below c, so light escapes and this object is not a black hole. A black hole of the same mass would have an event horizon radius of 2GM/c² = 4.4 km, smaller than this star. Compressing the same mass inside that radius would make it a black hole.

5.5.1(g) The HR diagram

The Hertzsprung–Russell diagram plots the luminosity of stars against their surface temperature. Luminosity is on the vertical axis, often as a multiple of the Sun’s, on a logarithmic scale. Temperature is on the horizontal axis and it decreases from left to right, so the hottest stars are on the left.

Region	Position	Description
Main sequence	Diagonal band, top left to bottom right	Stars fusing hydrogen in the core; the Sun is here
Red giants	Upper right	Cool but highly luminous because they are very large
Super red giants	Top right	Even more luminous and larger
White dwarfs	Bottom left	Hot but dim because they are very small

A star moves across the diagram as it evolves. A low-mass star leaves the main sequence for the red giant region, then ends at the white dwarf region. A massive star moves to the supergiant region.

Worked example: reading the diagram

Star P: 2 × 10⁴ times the Sun’s luminosity, surface temperature 3600 K. Star Q: 1 × 10⁻³ times the Sun’s luminosity, 12 000 K. Star R: 0.8 times the Sun’s luminosity, 5500 K.

• P is very luminous and cool, so it lies in the top right: a super red giant.
• Q is hot but dim, so it lies in the bottom left: a white dwarf.
• R has luminosity and temperature close to the Sun’s: a main sequence star.
• P is 2 × 10⁴ / 1 × 10⁻³ = 2 × 10⁷ times as luminous as Q. P is cooler, so its much greater luminosity comes from a far larger surface area.

Common errors

• Writing that the Sun will become a supernova. It lacks the mass.
• Saying a white dwarf is held up by fusion. It is held up by electron degeneracy pressure.
• Reading the temperature axis from low to high. It runs from high on the left to low on the right.
• Saying “gravity stops” at equilibrium. Gravity still acts. It is balanced by outward pressure.
• Saying nothing escapes a black hole’s gravity at any distance. Escape is impossible only from inside the event horizon.

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).