UPSC Prelims 2021 Q76: Science & Technology – Space Technology & Astronomy
- (a) Distances among stellar bodies do not change.
- (b) Gravity of stellar bodies does not change.
- (c) Light always travels in straight line.
- (d) Speed of light is always same.
Answer: (d) Speed of light is always same.
Explanation
Light-Year: Why Used for Astronomical Distances
| Option | Reason for Light-Year | Correct? |
|---|---|---|
| a) Distances among stellar bodies do not change | Stars move (proper motion) | NO |
| b) Gravity of stellar bodies does not change | Gravity varies | NO |
| c) Light always travels in straight line | Not always (gravitational lensing bends light) | NO |
| d) Speed of light is always same | Constant c = 299,792,458 m/s in vacuum | YES |
Light-year = distance light travels in one year (~9.46 trillion km). It works as a unit because the speed of light in vacuum is a universal constant (Einstein's postulate of Special Relativity, 1905). This constancy lets us convert time (years) into distance reliably.
Answer: (d) Speed of light is always same.
A light-year is about 9.46 x 10^12 km. Proxima Centauri (nearest star) is 4.24 light-years away.
Why each option is right or wrong
- (a) Incorrect. Stars have proper motion - Barnard's Star moves ~10 arcseconds per year across the sky.
- (b) Incorrect. Gravity of stellar bodies varies (mass loss, mergers, black hole formation).
- (c) Incorrect. Light bends near massive bodies (gravitational lensing). Einstein's 1919 eclipse observation proved this.
- (d) CORRECT. Speed of light in vacuum (c = 299,792,458 m/s) is a universal constant per Special Relativity (1905). This constancy allows us to convert 'time light takes' into 'distance' reliably. One light-year = ~9.46 trillion km.
Key point
Light-year works because speed of light is a UNIVERSAL CONSTANT (c = 3 x 10^8 m/s in vacuum). 1 ly = ~9.46 trillion km. Convention: 1 parsec = 3.26 ly. Alpha Centauri = 4.37 ly; Andromeda Galaxy = 2.5 million ly away.
Remember
Astronomical distances are measured in light-years because the SPEED OF LIGHT IS ALWAYS THE SAME (universal constant c = 3 x 10^8 m/s in vacuum, per Einstein's Special Relativity). This constancy lets us convert 'time light takes' into a reliable distance unit. Correct answer: (d).
Common mistakes
- Confusing 'speed of light constant' (universal, in vacuum) with 'light travels straight' (not true near massive objects).
- Not knowing gravitational lensing and cosmic expansion complicate light travel over cosmological distances.
- Missing units: 1 parsec = 3.26 light-years = distance at which 1 AU subtends 1 arcsecond.
Exam tip
Astronomical distance units: AU (Earth-Sun = 150M km), Light-year (~9.46 trillion km), Parsec (3.26 ly), Kiloparsec (1000 pc, galaxy scale), Megaparsec (1M pc, intergalactic scale). All rely on speed of light being constant.
Sources
- NCERT Class XI Physics - Chapter on Units and Measurements
- NASA Space Science - Astronomical Distance Measurement Units