Exam-style question
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Which of the following best describes the path a star follows on the Hertzsprung–Russell diagram after it has exhausted hydrogen in its core and begins helium fusion?.
- A.It moves downwards and to the left, becoming a white dwarf.
- B.It moves upwards and to the right, becoming a red giant.
- C.It moves downwards and to the right, becoming a brown dwarf.
- D.It moves upwards and to the left, becoming a blue supergiant.
Model answer
What a good answer should say
- It moves upwards and to the right, becoming a red giant.
Explanation
Why this works
When core hydrogen is depleted, the core contracts and heats while the outer layers expand and cool. On the HR diagram this is an upward (higher luminosity) and rightward (lower temperature) shift, placing the star on the red‑giant branch.
This is the standard post‑main‑sequence evolution for low‑ to intermediate‑mass stars.
Common mistake
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