Answer:
Astronomers use light-years to describe distances between celestial objects that are outside of our solar system. This is because light-years are a much larger unit of measurement than astronomical units.
An astronomical unit (AU) is the average distance between the Earth and the Sun. It is equal to about 93 million miles. A light-year is the distance that light travels in one year. It is equal to about 5.88 trillion miles.
So, a light-year is about 63,000 times as long as an astronomical unit. This means that light-years are a more convenient unit of measurement for talking about distances between objects that are very far away.
For example, the nearest star to Earth is Proxima Centauri, which is about 4.2 light-years away. This means that it would take light 4.2 years to travel from Proxima Centauri to Earth.
The Milky Way galaxy is about 100,000 light-years across. This means that it would take light 100,000 years to travel from one side of the Milky Way to the other.
And the Andromeda galaxy, which is the nearest major galaxy to the Milky Way, is about 2.5 million light-years away. This means that it would take light 2.5 million years to travel from the Milky Way to Andromeda.
As you can see, these are all very large distances. That's why astronomers use light-years to describe them.