AQA GCSE Physics Physics Paper 2 (Higher), June 2022: Question 6

💡 Key Knowledge: The Common Stages

Both types of stars begin in exactly the same way:

1. Nebula: A giant cloud of dust and gas (mainly hydrogen) collapses inwards due to the pull of gravity.
2. Protostar: As mass gathers, temperature and pressure rise until nuclear fusion begins.
3. Main Sequence: Inward gravitational force is precisely balanced by outward radiation pressure created by nuclear fusion (hydrogen fusing into helium). This is a long, stable period.

✅ Model Answer: Comparison of Divergent Paths

🧠 Exam Technique: Securing Level 3 (5–6 Marks)

Level 2 Descriptor (Mark Scheme): Scientifically relevant features are identified; similarities and differences are clearly contrasted with accurate sequence.

• Structure your answer in two halves: "Similarities in formation" and "Differences in later stages".
• Always state what fusion occurs: hydrogen nuclei fuse into helium nuclei during the main sequence.
• Use comparison terms clearly (e.g. "Whereas a Sun-like star becomes a white dwarf, massive stars explode as a supernova").

❌ Common Student Errors

• Mixing up pathways: Saying a massive star turns into a white dwarf or black dwarf.
• Omitting the balance of forces: Forgetting to state that gravity acts inwards and fusion radiation pressure acts outwards during the main sequence.
• Confusing Red Giant and Red Supergiant: They are distinct stages based strictly on initial stellar mass.

✅ Mark Scheme Answer

• There is an (observed) increase in the wavelength of light from distant galaxies. [1 mark]
• Because the galaxies are moving away from us (the observer). [1 mark]

💡 Key Knowledge: The Doppler Effect in Space

When a source of light moves away from an observer, the emitted waves are stretched out:

• Wavelength (λ) increases → shifts towards the red end of the visible spectrum.
• Frequency (f) decreases because wave speed remains constant ( v = f × λ ).

❌ Common Errors & Examiner Notes

• Rejected phrasing: The mark scheme explicitly states: "ignore light waves are stretched". You must specify that the wavelength increases.
• Do not say "the light turns red"—the spectral absorption lines are simply displaced towards longer (redder) wavelengths.

✅ Mark Scheme Answer

1. Observation from diagram: The furthest galaxies have the longest velocity arrows, meaning the furthest galaxies are moving away the fastest. [1 mark]
2. Link to Big Bang: This shows that the universe is expanding, which suggests that at some point in the past, all galaxies / all matter started from a single point. [1 mark]

🧠 Exam Technique: Two-Step Evidence Logic

To score both marks on "evidence" questions:

• Step 1: State the trend shown in the visual aid (arrow length = velocity; distance = distance from Milky Way).
• Step 2: Wind the clock backwards. If everything is currently moving outwards and faster the further away it is, it must have originated from a common origin point in the past.

✅ Accepted Answers

• There are new observations or new evidence that do not fit into the current theory/model. [1 mark]
• Alternative allowed: Giving a specific accepted example, such as observations that led to hypotheses of dark matter or dark energy. [1 mark]

💡 Scientific Method Recap

When newly discovered experimental or observational data conflicts with existing predictions, the scientific model must either be refined or completely replaced (peer review & falsification).