What Is a Light-Year? — For Kids

What Is a Light-Year? — Elementary

A tricky name

The word year sounds like time.

But a light-year does not measure time.

A light-year measures distance.

Distance means how far away something is.

That is the big idea on this page.

The short answer

A light-year is how far light travels in one Earth year.

That trip is about 6 trillion miles.

In kilometers, it is about 9 trillion km.

That is a 6 with 12 zeros behind it.

We use light-years for most space objects.

They tell us how far away those objects are.

Light moves very fast

Light travels 186,000 miles per second.

That is 300,000 km per second.

That sounds very fast.

But space objects are very far away.

So their light still takes a long time to reach us.

Looking back in time

When we look at faraway objects, we look back in time.

The farther away an object is, the farther back we see it.

The Sun is the closest star to us.

It is about 93 million miles away.

Its light takes about 8.3 minutes to reach us.

So we always see the Sun as it was 8.3 minutes ago.

The next closest star is about 4.3 light-years away.

We see that star as it was 4.3 years ago.

Far, far away

Polaris is 320 light-years away.

The center of the Milky Way is 26,000 light-years away.

Andromeda is 2.5 million light-years away.

Each number tells a distance.

Each number also tells how long ago we are seeing.

Big groups of stars

Stars live in large groups.

Those groups are called galaxies.

One galaxy can have millions of stars.

One galaxy can have billions of stars.

GN-z11 is a very far galaxy.

It is 13.4 billion light-years away.

We see it as it was 13.4 billion years ago.

That was only 400 million years after the big bang.

In 2016, the Hubble Space Telescope saw GN-z11.

It was the farthest galaxy Hubble had seen.

What Is a Light-Year? — Middle

A name that sounds like time

It is easy to hear the word “year” and think a light-year must be a length of time, but it is not.

A light-year measures distance — how far away something is — and scientists use it because space distances are far too large for ordinary miles or kilometers alone to be handy.

What a light-year measures

For most space objects, we use light-years to describe their distance.

A light-year is the distance light travels in one Earth year.

One light-year is about 6 trillion miles (9 trillion km). That is a 6 with 12 zeros behind it!

Looking back in time

When we use powerful telescopes to look at distant objects in space, we are actually looking back in time.

Light travels at a speed of 186,000 miles (or 300,000 km) per second.

This seems really fast, but objects in space are so far away that it takes a lot of time for their light to reach us.

The farther an object is, the farther in the past we see it.

Our Sun is the closest star to us. It is about 93 million miles away, so the Sun’s light takes about 8.3 minutes to reach us.

This means that we always see the Sun as it was about 8.3 minutes ago.

The next closest star to us is about 4.3 light-years away, so when we see this star today, we are actually seeing it as it was 4.3 years ago.

All of the other stars we can see with our eyes are farther, some even thousands of light-years away.

Distances from the chart

The source chart gives three more examples of how this scale grows.

Polaris is 320 light-years away, and the center of the Milky Way is 26,000 light-years away.

Andromeda, the nearest large galaxy to us, is 2.5 million light-years away, so we see Andromeda as it was 2.5 million years in the past.

Galaxies and the farthest one Hubble saw

Stars are found in large groups called galaxies. A galaxy can have millions or billions of stars.

The universe is filled with billions of galaxies, all farther away than Andromeda, and some of these galaxies are much farther away.

In 2016, NASA’s Hubble Space Telescope looked at the farthest galaxy ever seen, called GN-z11.

It is 13.4 billion light-years away, so today we can see it as it was 13.4 billion years ago.

That is only 400 million years after the big bang, and it is one of the first galaxies ever formed in the universe.

Learning about the very first galaxies that formed after the big bang, like this one, helps us understand what the early universe was like.