Past the planets, distances get so huge that even astronomical units are too small. Astronomers use the light-year: the distance light travels in one year. Light moves about 300,000 km every second, so one light-year is about 9.5 trillion km.
- The Sun is a star, an ordinary one: medium in size and brightness.
- The nearest other star, Proxima Centauri, is about 4.2 light-years away.
- The Sun is one of hundreds of billions of stars in the Milky Way, a spiral galaxy about 100,000 light-years across. We are about 26,000 light-years from its center.
- The Milky Way is one of billions of galaxies. The nearest big one, the Andromeda Galaxy, is about 2.5 million light-years away.
Key idea
A light-year is a distance, about 9.5 trillion km. Stars are light-years apart; galaxies are huge groups of billions of stars, millions of light-years apart.
Why it works
A light-year sounds like a time, but it is built like "an hour's drive": a distance measured by how far something goes in a set time. It also means we see the past. Light from a star 100 light-years away left it 100 years ago, so we see the star as it was then.
Worked example: In kilometers
Proxima Centauri is about 4.2 light-years away. About how far is that in kilometers?
- 1 light-year is about 9.5 trillion km.
- $4.2 \times 9.5 \approx 40$, so about 40 trillion km.
Answer: About 40 trillion km
Worked example: Looking back in time
The Andromeda Galaxy is about 2.5 million light-years away. How old is the light we see from it tonight?
- Light covers one light-year in one year.
- So it took about 2.5 million years to get here.
Answer: About 2.5 million years old
Common mistake
Thinking a light-year measures time, or that the stars lie just past the planets. A light-year is a distance. And the nearest star is about 9,000 times farther than Neptune. Check: if the answer to "how far?" is in light-years, it is a distance; the time is in plain years.