The correct option is The distance light travels in one year..
Explanation
In astronomy, the distances between celestial objects are too vast to be conveniently expressed in standard units like kilometers. Consequently, astronomers use specific units such as the Light Year, Astronomical Unit (AU), and Parsec to measure these distances.
Detailed Analysis:
- Definition: A Light Year is defined as the total distance that light travels in a vacuum during one Julian year (365.25 days).
- Value: Since light travels at a speed of approximately 300,000 kilometers per second ($3 \times 10^8$ m/s), one light year is equivalent to approximately 9.46 trillion kilometers ($9.46 \times 10^{12}$ km).
- Application: It is the standard unit for measuring interstellar and intergalactic distances. For instance, Proxima Centauri, the nearest star to the Solar System, is located about 4.24 light years away.
- Clarification on Incorrect Options:
- The time taken by light to travel from Sun to Earth.: This describes the specific time light takes to reach Earth from the Sun (approximately 8 minutes and 20 seconds), not the definition of the unit.
- The speed of light per second.: This refers to the speed of light ($c$), not the distance covered.
- The average age of a star.: This refers to the age of a celestial body, which is a measure of time, not length.
Key Takeaway:
Despite containing the word "Year," a Light Year is strictly a unit of distance (length), not a unit of time.