Consider the following pairs : Objects in space : Description 1. Cepheids : Giant clouds of dust and gas in space 2. Nebulae : Stars which brighten and dim periodically 3. Pulsars : Neutron stars that are formed when massive stars run out of fuel and collapse How many of the above pairs are correctly matched?
- (a) Only one ✓ UPSC's answer
- (b) Only two
- (c) All three
- (d) None
Why the answer is (a)
• Pair 3 is correct: pulsars are rapidly rotating neutron stars, the collapsed cores left when massive stars exhaust their fuel and explode as supernovae; they emit beams of radiation like a lighthouse.
• Pair 1 is wrong: Cepheids are variable stars that brighten and dim in regular periods; their period–luminosity relation makes them 'standard candles' for measuring cosmic distances.
• Pair 2 is wrong: nebulae are giant clouds of gas and dust, the birthplaces of stars — the descriptions in pairs 1 and 2 have been swapped.
• Hence only one pair, option (a).
Why the other options are wrong
- (b) Only two
- Pairs 1 and 2 have swapped descriptions; only pair 3 is correct.
- (c) All three
- Cepheids and nebulae are misdescribed.
- (d) None
- Pair 3 on pulsars is correctly matched.
Asked in the GS Paper I of the UPSC Civil Services Preliminary Examination 2023, held on 28 May 2023. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.