Consider the following statements : Statement-I : Giant stars live much longer than dwarf stars. Statement-II : Compared to dwarf stars, giant stars have a greater rate of nuclear reactions. Which one of the following is correct in respect of the above statements ?
- (a) Both Statement-I and Statement-II are correct and Statement-II explains Statement-I
- (b) Both Statement-I and Statement-II are correct, but Statement-II does not explain Statement-I
- (c) Statement-I is correct, but Statement-II is incorrect
- (d) Statement-I is incorrect, but Statement-II is correct ✓ UPSC's answer
Why the answer is (d)
• A star's lifetime depends on how fast it burns its fuel: massive giant stars have far higher core temperatures and pressures, so nuclear fusion proceeds at a much greater rate — Statement II is correct.
• Because they consume their hydrogen so quickly, giant stars live only millions of years, whereas small dwarf stars (like red dwarfs) burn slowly and can last hundreds of billions of years — so Statement I, claiming giants live longer, is incorrect.
• The rule of thumb is that lifetime falls steeply with mass (roughly as mass to the power −2.5).
• Statement II actually explains why Statement I is false.
• Hence option (d).
Why the other options are wrong
- (a) Both Statement-I and Statement-II are correct and Statement-II explains Statement-I
- Statement I is wrong: giant stars have shorter lives.
- (b) Both Statement-I and Statement-II are correct, but Statement-II does not explain…
- Statement I is wrong, so both cannot be correct.
- (c) Statement-I is correct, but Statement-II is incorrect
- It is Statement I that is incorrect and Statement II that is correct.
Asked in the GS Paper I of the UPSC Civil Services Preliminary Examination 2024, held on 16 June 2024. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.