A light year is a unit of measurement used by astronomers and represents the distance light travels in one year. This distance is approximately 6,000,000,000 miles, the speed of light being just over 186,000 miles a second.
Astronomers adopted this unit because they found ordinary measurements impractical for coping with the vast distances of space. Using this means, scientists calculate that the Milky Way must be about 100,000 light years in diameter.
The light from some stars takes millions of years to reach us. Andromeda is about 1,800,000 light years distant from earth, although it can be seen with the naked eye. Our nearest star, Proxima Centauri, is about four light years away, while the most brilliant star, Sirius (the Dog Star), is eight and a half light years from us.
So, when you look up at a star in the sky, you're like looking back in time.