OCR GCSE Physics A: Red-shift, the Big Bang and star formation (J249)

This guide follows the OCR GCSE (9-1) Gateway Science Physics A specification (J249), Version 5.0 (July 2026), for first assessment in 2018. It covers statements P8.3a, P8.3b and P8.3c in topic P8.3, Beyond Earth. All three are printed in bold in the specification, so they are tested only in the Higher Tier papers and are labelled “Higher Tier only” below. Other statements in P8.3 are taught on separate pages and are not repeated here.

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What this unit covers

Statement	What you must be able to do	Tier
P8.3a	Explain the red-shift of light from receding galaxies (qualitative only), including changes in frequency and wavelength. Explain how the change of each galaxy’s speed with distance is evidence of an expanding universe	Higher Tier only
P8.3b	Explain how red-shift and other evidence, including the cosmic microwave background radiation (CMBR), link to the Big Bang model	Higher Tier only
P8.3c	Recall that the Sun formed from dust and gas drawn together by gravity. Explain how this caused fusion reactions, leading to equilibrium between gravitational collapse and expansion due to the energy released during fusion. The specification also lists the life cycle of a star	Higher Tier only

Red-shift is qualitative only. You are not asked to calculate a speed from a wavelength. You are asked to describe what changes, which way, and what that tells you.

Light from galaxies and the idea of red-shift (P8.3a, Higher Tier only)

Light is an electromagnetic wave. Visible light runs from violet (shortest wavelength, highest frequency) to red (longest wavelength, lowest frequency). The wave equation connects the two: wave speed = frequency × wavelength. Light travels through space at a fixed speed, so if the wavelength gets longer, the frequency gets lower. See electromagnetic waves and the spectrum if this needs refreshing.

Atoms of each element absorb and emit light only at particular wavelengths. In a laboratory you can measure these wavelengths accurately. Light from a star or galaxy contains dark lines at the wavelengths of the elements in its atmosphere. These lines act as a fingerprint.

When astronomers look at light from distant galaxies, the pattern of lines is the same, but every line has moved to a longer wavelength. The whole pattern has shifted towards the red end of the spectrum. This is red-shift.

Why the shift happens

Waves from a moving source are squashed together in front of it and stretched out behind it. You meet this with sound: a siren seems to drop in pitch as an ambulance passes you and moves away. The sound waves reaching you are stretched, so their frequency is lower.

The same effect occurs with light. If a galaxy is moving away from us, each wave crest is emitted from a position further away than the last, so the waves reach us stretched out. The observed wavelength is longer and the observed frequency is lower than the values emitted. The faster the galaxy recedes, the bigger the change.

The specification suggests a Doppler ball to model red-shift and a balloon to illustrate why galaxies are moving away from us.

Key idea

Galaxy is…	Observed wavelength	Observed frequency
Receding	Longer than emitted	Lower than emitted
Stationary	Same as emitted	Same as emitted

Almost all galaxies that astronomers observe, apart from a few close to our own, show red-shift. A galaxy moving towards us would show the opposite change, a shift to shorter wavelengths.

Speed, distance and an expanding universe (P8.3a, Higher Tier only)

The size of the shift tells you how fast a galaxy is moving away. The distance to the same galaxy can be found by other methods. When both are plotted for many galaxies, a clear pattern appears: the more distant a galaxy, the greater its red-shift, so the faster it is receding.

Worked example 1: reading the pattern

Light from a spectral line that has a laboratory wavelength of 656 nm is measured in five galaxies. The recession speeds are worked out from the shifts.

Galaxy	Distance (million light years)	Recession speed (km/s)	Observed wavelength (nm)
A	80	1680	660
B	160	3450	664
C	320	6810	671
D	640	13 870	686
E	900	19 540	699

Describe the pattern and say what it suggests.

1. Every observed wavelength is longer than 656 nm, so every galaxy shows red-shift and every galaxy is moving away from us.
2. The shift grows with distance. Galaxy A is shifted by 4 nm and galaxy E by 43 nm.
3. The speed grows with distance. Doubling the distance from A to B, from B to C and from C to D roughly doubles the speed each time.
4. If space is expanding, any two galaxies move apart, and the further apart they are, the more space lies between them to stretch, so the faster they separate. This is the pattern in the table.

The data are supplied for this example. The pattern, not any calculation, is what the specification asks for.

Explaining it with a balloon

Draw dots on a balloon and inflate it. Every dot moves away from every other dot. A dot twice as far from yours as another moves away about twice as fast, because there is twice as much stretching rubber between them. No dot has to be moving across the surface: the surface itself is stretching. The same pattern is seen from any galaxy, so Earth is not in a special position.

The balloon is a model, so it has limits. The galaxies themselves do not grow, only the spaces between them do.

What the evidence tells us

• Galaxies are moving away from us, shown by red-shift (P8.3a).
• More distant galaxies recede faster, which is what an expanding universe predicts (P8.3a).
• Therefore the universe is expanding.

The Big Bang model (P8.3b, Higher Tier only)

If the universe is expanding today, then in the past everything was closer together. Imagine running the expansion backwards. All matter and space would be squeezed into a very small, extremely hot and dense region. The Big Bang model says the universe began from such a state and has been expanding ever since. Current estimates put the beginning at about 13.8 billion years ago.

Avoid two common errors:

• The Big Bang was not an explosion of matter into empty space. It was the expansion of space itself.
• There is no centre of the universe from which the galaxies fled. Observers in every galaxy see the same pattern.

Evidence 1: red-shift

Red-shift and the speed-distance pattern show that the universe is expanding. This is what the Big Bang model predicts. It does not by itself prove a hot, dense start, so other evidence matters.

Evidence 2: cosmic microwave background radiation (CMBR)

CMBR is microwave radiation that reaches Earth from every direction in space. It is almost the same in all directions. The Big Bang model explains it like this:

1. The early universe was extremely hot, so it was filled with high-energy, short-wavelength radiation.
2. As space expanded, the wavelengths of this radiation were stretched. This is red-shift on a huge scale.
3. Today the wavelengths are in the microwave part of the spectrum, corresponding to a very low temperature of about 2.7 K.

CMBR is “left over” radiation from the early universe. An alternative to the Big Bang would need to explain it just as well.

Why two lines of evidence matter

A scientific model gains support when different kinds of evidence agree. Red-shift shows expansion. CMBR shows a hot early universe that has cooled and stretched. Both fit the Big Bang model. New evidence could still change the model, which is how scientific theories work.

Worked example 2: linking evidence to the model

Explain how the CMBR is evidence for the Big Bang.

1. The model says the early universe was extremely hot and dense, filled with radiation of very short wavelength.
2. The universe has expanded since, which stretches the wavelengths of that radiation.
3. The radiation should now reach us from all directions with a much longer wavelength, in the microwave region. CMBR is observed at exactly these wavelengths, from all directions.

Formation of the Sun and equilibrium in stars (P8.3c, Higher Tier only)

From cloud to star

1. Space contains clouds of dust and gas, mostly hydrogen. Gravity attracts the particles towards each other.
2. A region of the cloud that is denser than its surroundings pulls in more material and contracts.
3. As the cloud falls inward, gravitational potential energy is transferred to thermal energy. The particles move faster and the temperature rises.
4. The centre becomes very hot and very dense. A hot ball of gas forms, called a protostar.

The Sun formed this way from a cloud of dust and gas, about 4.6 billion years ago.

Fusion starts

At a high enough temperature and pressure in the core, hydrogen nuclei move fast enough to overcome the repulsion between their positive charges. They fuse to form helium nuclei, and energy is released. This is nuclear fusion. For the nuclear physics, see fission and fusion.

Equilibrium

Two effects act on the star:

Effect	Direction	Cause
Gravitational collapse	Inward	Gravity pulls all the star’s matter towards the centre
Expansion	Outward	Energy released by fusion heats the gas, which tries to expand

For most of its life a star is stable because these two effects are balanced. The Sun is in this state now.

If fusion speeds up, the core heats and expands slightly, which lowers the density and slows fusion again. If fusion slows, gravity compresses the core, which raises the temperature and speeds fusion up again. The star stays near equilibrium.

Worked example 3: explaining stability

Explain why the Sun does not collapse or blow apart.

1. Gravity pulls the Sun’s matter inward, tending to make it collapse.
2. Fusion in the core releases energy, which heats the gas and makes it expand outward.
3. The two effects are balanced, so the Sun’s size stays about constant.

Life cycle (listed under “To include”)

When hydrogen in the core runs low, fusion no longer balances gravity. What happens next depends on the star’s mass.

• A star like the Sun swells into a red giant. It then sheds its outer layers and the core shrinks to a white dwarf.
• A much more massive star swells into a red supergiant, then explodes as a supernova. The core may become a neutron star or a black hole.
• A supernova scatters heavier elements. New stars and planets can later form from dust and gas that includes these elements.

Common errors

• Writing “the galaxy appears red”. The point is a longer wavelength and lower frequency.
• Saying that red-shift means the light slows down. Light speed does not change.
• Describing the Big Bang as an explosion in space or placing Earth at the centre.
• Treating CMBR as visible light. It is microwaves.
• Saying fusion happens “because gravity is strong” without hot, dense conditions.
• Writing only “balanced forces” for equilibrium. Name the inward and outward effects.

Official syllabus

OCR, GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0 (July 2026), for first assessment in 2018, topic P8.3 Beyond Earth, statements P8.3a to P8.3c.