What is a light-year?
A light-year is the distance light travels in one year, about 9.46 trillion kilometers (5.88 trillion miles). Despite its name, it measures distance, not time.
Light travels at about 300,000 kilometers per second. Multiplying that by the number of seconds in a year gives a light-year. Astronomers use it because distances between stars are so large that kilometers become unwieldy.
The nearest star after the Sun, Proxima Centauri, is about 4.2 light-years away. The center of the Milky Way is about 26,000 light-years away, and the Andromeda Galaxy is about 2.5 million light-years away.
Looking back in time
Because light takes time to travel, we see distant objects as they were in the past. When you look at a star 100 light-years away, you see light that left it a century ago. Astronomers also use the parsec, equal to about 3.26 light-years.
Did You Know?
The most distant galaxies seen by the James Webb Space Telescope sent out their light more than 13 billion years ago, when the universe was very young.