What Is a Light-year?
Space is so vast that scientists don’t use miles to measure how far stars are away from us. The number of miles would be too large to work with easily. Instead, scientists use what they call “light-years” to measure great distances in space.
A light-year is the number of miles that light can travel in a year’s time. Light travels very fast – about 186,000 miles every second! To get an idea of how fast this is, light can travel about seven times around Earth in one second!
If we multiply this speed by the number of seconds in a year, we find that it adds up to roughly six million, million miles. The nearest star beyond our sun, Alpha Centauri, is nearly 4.3 light-years away.
Astronomers use the speed of light to measure how far away things are in space. 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. 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.