What is a light-year? The fastest thing that we know of is light which travels at a speed of 186,000 miles or 300,000 kilometers per second in empty space. To get an idea of how fast this is, light can travel about seven times around Earth in one second! Astronomer use the speed of light to measure how far away things are in space. They use a unit called the light-year. A light-year (ly) is the distance that light can travel in one year. In one year light travels about 5,880,000,000,000 miles or 9,460,000,000,000 kilometers. So, this distance is 1 light-year. For example, the nearest star to us is about 4.3 light-years away. Our galaxy, the Milky Way, is about 150,000 light-years across, and the nearest large galaxy, Andromeda, is 2.3 million light-years away.