What does an astronomical unit (AU) measure?

An astronomical unit (AU) is specifically designed to measure distances within our Solar System, making it the appropriate answer to this question. The defined distance of one astronomical unit is equivalent to the average distance from the Earth to the Sun, which is approximately 93 million miles or about 150 million kilometers. This unit provides a convenient way to express and compare distances between various celestial bodies, such as planets and asteroids, making it invaluable for astronomical calculations and navigation.

In the context of the other options, mass is typically measured in kilograms or solar masses rather than astronomical units, which are strictly for distance. Brightness of stars is measured in units like magnitudes or luminosity rather than AU. Lastly, the volume of planets is quantified in cubic kilometers or similar metrics, not in astronomical units. This underscores the specific role of AU in providing clarity and standardization when discussing distances in the vastness of space.