UPSC Prelims 2021 Q76: Why Astronomers Use Light-Years — Explained
Which one of the following is a reason why astronomical distances are measured in light-years?
- Distances among stellar bodies do not change.
- Gravity of stellar bodies does not change.
- Light always travels in straight line.
- Speed of light is always same.
Answer: (d) Speed of light is always same.
Statement-wise explanation
- a: incorrect — Distances between stellar bodies change all the time. Stars move through the galaxy, and galaxies outside our Local Group are moving apart as the universe expands, as Hubble showed in 1929. Even if distances were fixed, that would not explain why light is used to measure them.
- b: incorrect — Gravity has nothing to do with how a unit of distance is defined. A light-year is the speed of light multiplied by one year; gravity does not enter that calculation. Stars also lose mass over their lives through stellar winds and nuclear fusion, so their gravity is not fixed either.
- c: incorrect — Light does not always travel in a straight line. Massive objects bend it; this is called gravitational lensing, and it was first confirmed during the 1919 solar eclipse. Light also bends by refraction when it passes from one medium into another. The light-year depends on how fast light travels, not on the shape of its path.
- d: correct — A unit of length has to stay the same, and the speed of light in vacuum is a universal constant: exactly 299,792,458 m/s, fixed by the SI definition of the metre since 1983. The International Astronomical Union defines a light-year as the distance light travels in vacuum in one Julian year (365.25 days), about 9.46 trillion km. The unit is only reliable because that speed never changes.
How to eliminate
Ask what any unit of measurement needs: it must stay the same. A light-year is distance = speed × time. The year is fixed, so the unit can only be reliable if the speed of light is fixed. Only the fourth statement is about the speed of light, which the unit is built on. The other three are about the objects being measured or the path light takes, not the measuring unit. Basic science also rules them out. Stars and galaxies move, so distances change. Stars lose mass, so their gravity changes. Gravitational lensing and refraction show that light bends. Beware of the absolute word 'always': it makes the third statement false, but it is accurate in the fourth, because the speed of light in vacuum really is constant. Fact to remember: the speed of light in vacuum is constant, about 3 lakh km/s, and the light-year is a unit of distance, not time.
Know more
Why astronomers need special units
Space is so large that kilometres give unmanageable numbers. Astronomers use units built on something that never changes: the speed of light in vacuum, exactly 299,792,458 m/s (about 3 lakh km/s). This value has been fixed by definition since 1983, when the metre itself was redefined in terms of light.
Light-year = speed of light × one Julian year (365.25 days) ≈ 9.46 trillion km. It is a unit of distance, not time. This is a favourite UPSC trap.
| Unit | Value | Typical use |
|---|---|---|
| Astronomical Unit (AU) | 149,597,870.7 km (mean Earth–Sun distance, fixed by IAU in 2012) | Distances within the Solar System |
| Light-year (ly) | ≈ 9.46 × 10¹² km ≈ 63,241 AU | Distances to stars and galaxies; popular science |
| Parsec (pc) | ≈ 3.26 ly | Professional astronomy; based on stellar parallax |
Useful benchmarks
- Sunlight takes about 8 minutes 20 seconds to reach Earth (1 AU).
- Proxima Centauri, the nearest star after the Sun: about 4.2 ly.
- Milky Way: about 1 lakh ly across.
- Andromeda galaxy: about 2.5 million ly away.
Looking back in time. Because light takes time to travel, we see a star 100 ly away as it was 100 years ago. Distant galaxies therefore show the early universe.
Fine print. Light slows down in media such as water or glass. It also bends near massive bodies (gravitational lensing) and when it passes from one medium into another (refraction). The constant that defines the light-year is the speed of light in vacuum.
NCERT Concepts to Revise
- Light year: The question is about what a light-year measures and why it can serve as a standard unit.
- Electromagnetic radiation: Light is electromagnetic radiation, and its fixed speed of about 300,000 km per second is what the light-year is built on.
- Galaxy: Distances between and within galaxies are the main reason astronomers need a unit as large as the light-year.