A light year is a way of measuring distance in space (rather than time as the name implies). A light year is the distance that light can travel in a year.
Measuring distance in light years is essential in the vast area of space. The nearest galaxy Andromeda is 21 quintillion km away (21,000,000,000,000,000,000)which is equivalent to 2.3 light years. The milky way is 150,000 light years across.
Light years can also help to tell the age of items in space too. A star that is one million light years away is one million years old, as it has taken the stars light one million years to reach us. The light was therefore created on million years ago, meaning that we do not see an accurate image of what is there but an image of what used to be there all that time ago.
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