Exam-style question
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Which spectral class corresponds to the hottest stars on the main sequence?.
- A.O
- B.B
- C.A
- D.F
Model answer
What a good answer should say
- O
Explanation
Why this works
Define both: Spectral class O stars are the hottest, with surface temperatures above 30 000 K, while classes B, A and F are progressively cooler. Identify key difference: O stars have the highest temperatures and most ionised absorption lines.
Explain when each applies: O stars are used to describe the most massive, luminous, and hottest main‑sequence stars; B, A and F classes describe cooler stars. Conclude: The question asks for the hottest class, so O is correct.
Common mistake
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