What is a light-year?

Despite the word “year,” a light-year measures distance, not time. It is defined as how far light travels through a vacuum in one Earth year: roughly 9.46 trillion kilometers (about 5.88 trillion miles). Astronomers use it because cosmic distances are so vast that kilometers become unwieldy. Looking into space is also looking back in time — when you see a star 100 light-years away, you see light that left it a century ago. The nearest star system, Alpha Centauri, is about 4.25 light-years away, and the Milky Way galaxy spans roughly 100,000 light-years.