In astronomy, a “light year” is a unit used to measure which physical quantity?

Introduction / Context:
Astronomers deal with extremely large separations between stars and galaxies. Ordinary units like kilometres rapidly become inconvenient, so special large-scale units are used. One of the most common is the light year. Despite the word "year" suggesting time, a light year is actually a measure of length. This question tests whether you understand that a light year is a distance unit, not a unit of time, speed or any other quantity.

Given Data / Assumptions:
- The term "light year" refers to the distance light travels in one year in vacuum.
- Options include acceleration, distance, light intensity and weight.
- We assume the usual value of the speed of light in vacuum, approximately 3 * 10^8 m/s, although the exact number is not needed for the concept.

Concept / Approach:
By definition, a light year is the distance travelled by light in vacuum in one Julian year. Since speed multiplied by time gives distance, and the speed of light is multiplied by one year, the result is a large length, not a time interval. It is therefore a unit of distance commonly used to express separations between stars and galaxies. Acceleration measures rate of change of velocity, intensity measures power per unit area, and weight is a force; none of these match the definition of a light year.

Step-by-Step Solution:
Step 1: Recall that speed * time = distance. Step 2: A light year is defined as the distance light covers in one year. Step 3: Substitute: distance = speed of light * one year of time. Step 4: The result must be a length or distance, not time or acceleration. Step 5: Therefore, among the options, distance is the only correct physical quantity measured in light years.

Verification / Alternative check:
You often see statements such as "the nearest star Proxima Centauri is about 4.24 light years away from the Sun". Here light year clearly describes how far the star is, not how long light travels in time units. Another example is that the Milky Way galaxy is roughly one hundred thousand light years across, which again expresses a linear size, confirming that the light year is a unit of length.

Why Other Options Are Wrong:
Acceleration: Usually measured in m/s^2. A light year does not involve time squared or change of velocity.
Light intensity: Expressed in units like candela or watts per square metre. Light year does not measure brightness or power per area.
Weight: Weight is a force and in SI is measured in newtons, not in light years. A light year has nothing to do with gravitational force.

Common Pitfalls:
The name "light year" misleads many learners into thinking it is a unit of time, because of the word "year". Remember that it is defined using the distance travelled in that year, not the duration itself. The key idea is that light year answers the question "how far", not "how long". Keeping this distinction clear will help you avoid confusion in astronomy questions.

Final Answer:
A light year is a unit of distance in astronomy.