Light Years E=mc2

Astronomers have more practical units of measurement, such as the light-year, to describe the vast distances between objects in the universe. Because light does not travel instantaneously—in a vacuum, light travels at a finite speed of about 300,000 km/s (186,000 mi/s)—it takes time for light to travel through the universe. One light-year is equal to the distance that light can travel in one year's time, which is nearly 10 trillion km. Thus, Proxima Centauri is about four light-years away. That means that it takes four years for the light emitted from Proxima Centauri to reach Earth. Andromeda is about two million light-years away—meaning it takes two million years for its light to reach Earth.