Astronomers say that looking at the stars is like looking back in time: the star-light we see has often spent millions of years traveling to Earth. The farther away an object is, the farther the light has to travel to reach Earth, and the older the image of the object. So when we say a galaxy is a million light-years away, we literally mean that the image we see is actually a million years old. This ability to look back in time is a useful tool for astronomers studying the history of the universe.