What is a light-year?
A light-year is the distance that light can travel in one year – that is, in the time it takes the planet Earth to make one full trip around the Sun. Light travels 186,000 miles in a second, so in a minute it can go about 11 million miles. In an hour, light travels about 670 million miles. In a day, light travels almost 26 billion miles. So in a year, light travels about 9.5 trillion miles (9,500,000,000,000). That’s a lot of zeroes, so we just shorten it and say “one light-year”.
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What if we could travel at the speed of light?
Alpha Centauri is the nearest star to the Earth (other than our Sun), and it’s about 4.4 light-years away. If we had a spaceship that could travel at the speed of light (which is probably impossible), it would still take almost four and a half years to get there. The whole universe is much, much bigger than that. It’s probably about 13.7 billion light-years across.
You’re looking at the past
Because even light can only travel one light-year in a year, when you look at Alpha Centauri in the sky, you’re actually seeing what Alpha Centauri looked like four and a half years ago, not what it looks like now. If Alpha Centauri blew up right now, you wouldn’t see it for four and half more years. It takes that long for the light to get from Alpha Centauri to your eyes.
Finding out about the Big Bang
That means that looking through a telescope at things that are very far away, on the other side of the universe, is the same thing as looking at things that happened billions of years ago. That’s how we know what the universe was like when it was first forming, after the Big Bang.
Astronomers build bigger and bigger telescopes, on top of mountains or sometimes out in space so they don’t have to look through the Earth’s atmosphere, in order to see farther and farther away. It’s not that they are so interested in faraway stars and planets, but when we look at a star that is billions of light-years away, we’re seeing what that star looked like billions of years ago. If we can look far enough away, we can see the beginning of time, which was probably almost 14 billion years ago. Right now, the best telescopes can see about 10-15 billion light years away, so we are beginning to be able to see the Big Bang itself.