A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Hertzsprung-Russell Diagram and Stellar Evolution — mark scheme explained
The short answer
The Hertzsprung-Russell (HR) diagram is a fundamental tool in astrophysics that plots stars based on their absolute magnitude (brightness) against their surface temperature or spectral class. This diagram helps us understand the life cycle of stars, including those similar to our Sun.
The question
A star has an absolute magnitude of +3.0 and a surface temperature of 6,000 K. Identify the region on the HR diagram where this star is located. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
1. Locate the absolute magnitude (+3.0) on the y-axis of the HR diagram.
- S2
2. Locate the surface temperature (6,000 K) on the x-axis of the HR diagram.
- S3
3. Find the intersection of these two values on the HR diagram.
- S4
4. The star is located in the main sequence region.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
1. Locate the absolute magnitude (+3.0) on the y-axis of the HR diagram.
- S2
2. Locate the surface temperature (6,000 K) on the x-axis of the HR diagram.
- S3
3. Find the intersection of these two values on the HR diagram.
- S4
4. The star is located in the main sequence region.
Final answer: Main sequence
Common mistakes
- Confusing absolute magnitude with apparent magnitude — Absolute magnitude is a measure of intrinsic brightness, while apparent magnitude depends on distance. Always use absolute magnitude for the y-axis on the HR diagram.
- Misidentifying the main sequence region — The main sequence is a diagonal band from the top left (hot and bright) to the bottom right (cool and dim). The Sun is a G-type star located in the middle of this region.
- Incorrectly placing red giants — Red giants are located in the upper right corner of the HR diagram. They have a high absolute magnitude (bright) and a low surface temperature (cool).
- Misunderstanding the path of stellar evolution — The path for a Sun-like star is: formation → main sequence → red giant → helium flash and helium burning → planetary nebula → white dwarf. Ensure you understand each stage and its corresponding position on the HR diagram.
- Confusing spectral classes with temperature — Spectral class O is the hottest (50,000 K), while M is the coolest (2,500 K). Use this order to correctly place stars on the HR diagram.
- Incorrectly identifying white dwarfs — White dwarfs are located in the lower-left of the HR diagram. They have a high surface temperature (hot) but low luminosity (faint) — a high (more positive) absolute magnitude — because their Earth-sized radius gives a very small radiating surface area.
Where the marks go
- Full worked solution (all marking points)4 marks