UPSC Prelims 2023 Q56: Science & Technology – Space Technology & Astronomy
- (a) Only one
- (b) Only two
- (c) All three
- (d) None
Answer: (a) Only one
Explanation
Objects in Space — Classification
| Pair | Description Given | Verdict | Correct Description |
|---|---|---|---|
| 1. Cepheids | 'Giant clouds of dust and gas' | WRONG | Cepheids are variable stars that periodically brighten and dim — used as 'standard candles' to measure cosmic distances. Giant clouds of dust and gas = Nebulae. |
| 2. Nebulae | 'Stars which brighten and dim periodically' | WRONG | Nebulae are giant clouds of interstellar dust and gas — they are the raw material for star formation. Stars that brighten and dim = Cepheid variables. |
| 3. Pulsars | 'Neutron stars formed when massive stars run out of fuel and collapse' | TRUE | Pulsars are rotating neutron stars that emit regular beams of electromagnetic radiation. They form when massive stars (>8 solar masses) exhaust their fuel and undergo supernova collapse. |
Only Pair 3 is correctly matched → Only one.
Answer: (a) Only one.
Why each option is right or wrong
- (a) CORRECT. Only Pulsars (pair 3) are correctly described as neutron stars formed from collapsed massive stars. Cepheids and Nebulae descriptions are swapped.
- (b) Incorrect. 'Only two' would require one more pair to be correct — but pairs 1 and 2 have definitively swapped descriptions.
- (c) Incorrect. 'All three' — Cepheids are NOT clouds of dust and gas, and Nebulae are NOT variable stars.
- (d) Incorrect. 'None' — Pulsars are accurately described as neutron stars from supernova collapse of massive stars.
Key point
Cepheids = variable stars (not clouds). Nebulae = clouds of dust/gas (not variable stars). Pulsars = neutron stars from massive star collapse ✓. Pairs 1 and 2 have swapped descriptions — classic UPSC astronomy trap.
Remember
Cepheids are variable stars that brighten and dim periodically. Nebulae are giant clouds of dust and gas. Pulsars are neutron stars formed when massive stars collapse — only Pulsars are correctly described in the given pairs.
Common mistakes
- Accepting the Cepheid description without verification — Cepheids are one of the most important distance measurement tools in astronomy; they are pulsating stars, not clouds.
- Thinking Nebulae are variable stars — nebulae are diffuse clouds (Orion Nebula, Eagle Nebula); variable stars are a completely different stellar phenomenon.
- Not knowing Pulsars = neutron stars — when a massive star goes supernova, the core collapses into a neutron star which, if it rotates and emits beams, becomes a pulsar.
Exam tip
Cepheids = variable stars (standard candles for distance). Nebulae = gas/dust clouds (star nurseries). Pulsars = rotating neutron stars. Pairs 1 and 2 in the question are swapped. Only pulsar description is correct.
Sources
- NCERT Physics Class 11/12 — Universe and Stellar Evolution
- ISRO — Space Science Educational Resources