Astronomical Units or AU
If you start measuring distances between stars, the AU becomes less useful. This is because our galaxy is so much bigger than our Solar System. The nearest star to the Sun is Proxima Centauri. It is roughly 268,000 AU from the Sun. To give you an idea of the size of our galaxy, the centre of Milky Way is around 1,700,000,000 AU from the Sun. When we measure distances across galaxies, we need bigger units like light years.