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
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For identification questions, ensure you clearly state the region on the HR diagram. For explanation questions, provide a step-by-step description of the process, including relevant stages and their positions on the diagram. Use specific terms like 'main sequence', 'red giant', and 'white dwarf' to gain marks.
What the command words demand
- Identify
- Locate and name the region on the HR diagram.
- Explain
- Provide a detailed description of the process or concept.
- Describe
- Give a clear account of the characteristics or features.
- Compare
- Highlight similarities and differences between two or more items.
- Outline
- Provide a brief summary of key points.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate about 2-3 minutes per mark for detailed explanations and 1 minute per mark for identification questions.
- 1. Locate the absolute magnitude (+3.0) on the y-axis of the HR diagram.1 mark
- 2. Locate the surface temperature (6,000 K) on the x-axis of the HR diagram.1 mark
- 3. Find the intersection of these two values on the HR diagram.1 mark
- 4. The star is located in the main sequence region.1 mark
Final answer: Main sequence
Work through every step correctly and you earn all 4 marks.
Another worked example
Explain the path a star similar to our Sun takes on the HR diagram from formation to white dwarf.
- 1. The star forms and moves towards the main sequence as it contracts and heats up.1 mark
- 2. It spends most of its life in the main sequence, fusing hydrogen into helium.1 mark
- 3. As the core hydrogen is depleted, the core contracts and heats up, causing the outer layers to expand and cool, moving the star to the red giant phase.1 mark
- 4. Helium fusion begins in the core, leading to further changes in structure.1 mark
- 5. The outer layers are ejected, forming a planetary nebula, leaving behind a white dwarf.1 mark
- 6. The white dwarf cools and fades over billions of years.1 mark
Final answer: Formation → Main Sequence → Red Giant → Helium Flash and Helium Burning → Planetary Nebula → White Dwarf
Work through every step correctly and you earn all 6 marks.
Common mistakes
Confusing absolute magnitude with apparent magnitude
Why it happens: Students often mix up these two concepts, leading to incorrect placement of stars on the HR diagram.
Fix: 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
Why it happens: Students may not recognize that most stars lie in this region, leading to incorrect placement of stars like the Sun.
Fix: 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
Why it happens: Students may place red giants too far to the left or right on the HR diagram, not understanding their high luminosity and low temperature.
Fix: 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
Why it happens: Students may not grasp the sequence of events in a star's life cycle, leading to incorrect descriptions of its path on the HR diagram.
Fix: 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
Why it happens: Students may not fully grasp the relationship between spectral classes (OBAFGKM) and surface temperatures, leading to incorrect placement of stars on the x-axis.
Fix: 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
Why it happens: Students may not recognise that white dwarfs are small and very hot yet appear faint, because their tiny (roughly Earth-sized) radius gives a small radiating area.
Fix: 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
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| HR Diagram Placement | Use absolute magnitude and temperature to locate a star's region on the HR diagram. | 4 |
| HR Diagram Evolution | Describe a Sun-like star's evolutionary path across the HR diagram, naming each stage and position. | 6 |
| HR Diagram Identification | Use absolute magnitude and surface temperature to locate a star's region on the HR diagram. | 4 |
| HR Diagram Identification | Locate a star's region on the HR diagram using its magnitude and temperature. | 4 |
| Total across these question types | 18 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.