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

These notes condense statements P8.3a to P8.3c of the OCR GCSE (9-1) Gateway Science Physics A specification (J249), Version 5.0 (July 2026), for first assessment in 2018. All three statements are printed in bold in the specification, so they are tested only in the Higher Tier papers. For full explanations, read the study guide, then try the practice questions.

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Key terms

Term	Meaning
Red-shift	Light from a receding source is observed at a longer wavelength and lower frequency than it was emitted
Spectral line	A wavelength absorbed or emitted by a particular element, used as a fingerprint
Expanding universe	Space itself is stretching, so galaxies move apart from each other
Big Bang model	The universe began from a very small, hot, dense state about 13.8 billion years ago and has expanded since
CMBR	Cosmic microwave background radiation: microwaves arriving from all directions, left over from the early universe
Protostar	A hot ball of gas formed as a cloud of dust and gas collapses under gravity, before fusion begins
Equilibrium (star)	Inward gravitational collapse balanced by outward expansion due to energy released by fusion

Red-shift (P8.3a, Higher Tier only)

Qualitative only: no calculation of speed from wavelength is asked for.

Source	Observed wavelength	Observed frequency
Moving away	Longer	Lower
Not moving	Same	Same

• Light speed does not change, so a longer wavelength means a lower frequency.
• Each spectral line moves towards the red end of the spectrum. The pattern of lines stays the same, so the elements can still be identified.
• Sound comparison: a siren drops in pitch as the vehicle moves away.
• Doppler ball and balloon are the specification’s suggested models.

Method: describe red-shift in steps

1. Name the change: the observed wavelength is longer.
2. Name the matching change: the observed frequency is lower.
3. Link it to motion: the source is moving away from the observer.
4. Link size to speed: a bigger shift means a faster recession.

Speed and distance: evidence for expansion (P8.3a, Higher Tier only)

Observation	What it shows
Nearly every galaxy is red-shifted	Galaxies are moving away from us
More distant galaxies have bigger red-shifts	More distant galaxies move away faster
Same pattern from any galaxy	Space is expanding, with no special centre

Balloon model

• Dots on a balloon move apart as it inflates.
• A far dot has more rubber between it and you, so it moves away faster.
• The dots do not grow, and neither do galaxies. Only the spaces between them stretch.

Small worked reminder

A line of 656 nm in the laboratory is observed at 661 nm from galaxy P and 679 nm from galaxy R. Both are red-shifted. R has the larger shift, so R recedes faster. If the expansion pattern holds, R is the more distant galaxy.

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

• Expansion run backwards: everything was once closer together, in a very small, hot, dense state.
• The Big Bang was the expansion of space, not an explosion of matter into empty space.
• There is no centre of the universe to point to.

Two links between evidence and model

Evidence	How it fits the Big Bang model
Red-shift and speed-distance pattern	The universe is expanding, as the model predicts
CMBR	The early universe was hot and full of short-wavelength radiation. Expansion stretched it into microwaves, now at about 2.7 K, seen from all directions

Method: explain how CMBR supports the Big Bang

1. The early universe was very hot and dense and full of radiation.
2. Expansion stretched the wavelengths of this radiation.
3. It should now be seen from all directions as microwaves, and it is.

Two or more independent lines of evidence agreeing makes a model more convincing. Further evidence could still change it.

Star formation and equilibrium (P8.3c, Higher Tier only)

Order of events

1. A cloud of dust and gas, mostly hydrogen, is pulled together by gravity.
2. The cloud contracts. Gravitational potential energy is transferred to thermal energy, so the temperature rises.
3. The core becomes hot and dense enough for hydrogen nuclei to fuse to helium. Energy is released.
4. The energy released heats the gas and makes it expand outward.
5. Inward gravity balances outward expansion. The star is stable.

The Sun formed this way about 4.6 billion years ago and is in this stable stage now.

Balance table

Effect	Direction	Cause
Gravitational collapse	Inward	Gravity pulls matter towards the centre
Expansion	Outward	Energy released by fusion heats the gas

Self-correcting equilibrium

• Fusion speeds up: the core expands slightly and fusion slows down.
• Fusion slows: gravity squeezes the core and fusion speeds up.

Life cycle (listed under “To include”)

Star	Sequence
Similar to the Sun	cloud, protostar, stable star, red giant, white dwarf
Much more massive	cloud, protostar, stable star, red supergiant, supernova, neutron star or black hole

A supernova spreads heavier elements, and later stars and planets can form from this material.

Worked reminders

Reading a table of galaxies

Galaxy	Distance (million light years)	Observed wavelength of a 656 nm line (nm)
W	100	661
X	200	666
Y	400	674

1. All three wavelengths are longer than 656 nm, so all three galaxies are receding.
2. The shift is bigger for the greater distance, so Y recedes fastest and W slowest.
3. Conclusion: speed increases with distance, which fits an expanding universe.

A strong answer quotes at least one pair of values from the table, for example “W at 100 is shifted to 661 nm but Y at 400 is shifted to 674 nm”.

Explaining why the Sun is stable

1. Gravity pulls the Sun’s matter inward.
2. Fusion in the core releases energy, heating the gas so that it expands outward.
3. The two effects balance, so the Sun’s size stays about constant.

Explaining how a star starts

1. Gravity pulls dust and gas together, and the cloud contracts.
2. The temperature of the core rises as particles speed up.
3. When it is hot and dense enough, hydrogen nuclei fuse and release energy.
4. Fusion begins, and the star settles into equilibrium.

Exam language

Use these command words correctly.

Command word	What to write
Describe	Say what is seen or what changes, with no reasons needed
Explain	Give the change and the reason, linked with “because” or “so”
Suggest	Use your physics on an unfamiliar case, for example a data set you have not seen
Evaluate	Weigh evidence for and against, then reach a conclusion

For a question that asks how evidence supports a model, state what the model predicts, then state what is observed, then say that they match.

Must-know distinctions

Idea	Not this
Longer wavelength	“The light turned red” or “light slowed down”
Space expanding	Galaxies moving into empty space from a centre
CMBR is microwaves	CMBR is visible light
Fusion joins nuclei	Fission splits them
Equilibrium needs two named effects	“Forces are balanced” with nothing named

Quick self-test

1. How does the wavelength of light reaching us from a receding galaxy compare with the wavelength emitted?
2. State what happens to its frequency.
3. A galaxy shows a larger red-shift than another. What does that say about its speed?
4. What does the pattern of speed with distance suggest about the universe?
5. Name the radiation that is observed from all directions as evidence for the Big Bang.
6. In which part of the electromagnetic spectrum is CMBR found?
7. State why running the expansion backwards leads to a hot, dense start.
8. What is pulled together by gravity to form a star?
9. What reaction in the core releases the energy that stops a star collapsing?
10. Name the two effects in the equilibrium of a stable star.

Answers

1. It increases (is longer than emitted).
2. It decreases (is lower than emitted).
3. It is moving away faster.
4. The universe is expanding.
5. Cosmic microwave background radiation (CMBR).
6. Microwaves.
7. Everything was closer together in the past, so matter and energy were squeezed into a very small region.
8. Dust and gas, mostly hydrogen.
9. Nuclear fusion of hydrogen into helium.
10. Inward gravitational collapse and outward expansion due to energy released by fusion.

Where marks are usually lost

• Writing that light “becomes red” without naming wavelength and frequency.
• Stating the wavelength change but leaving out frequency, or swapping the two.
• Saying that red-shift shows galaxies are far away, rather than moving away.
• Not linking a bigger shift to a faster recession.
• Describing the Big Bang as an explosion at a point in space.
• Giving CMBR as evidence without saying that the early radiation was stretched by expansion.
• Writing “gravity and fusion balance” without saying that fusion releases energy that causes outward expansion.
• Confusing fusion in stars with fission in power stations.

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.