Looking Into Space Is Looking Back In Time

You may have heard that it takes 8 minutes for light from the sun to reach the Earth. That's because, as fast as light is, it still takes time to get to where it's going. So if it's 8 minutes from the nearest star to us, then how long does it take for light from more distant stars to reach our eyes? And if the light really takes so long to get to you, then is what you're seeing what's really there, or only what used to be there?

Related: At The Event Horizon Of A Black Hole, Time And Space Fundamentally Change

A Window To The Past

It's true: every time you look at a star in the sky, you are looking at it as it was, not as it is. Actually, that's true of everything that you see, but light moves so quickly that it usually doesn't make any difference. But if what you're looking at is one light-year away, then what you see is what was there one year ago. Now, the farthest star that can be seen with the naked eye is in the constellation Cassiopeia, and the light that it produces takes no less than 4,000 years to reach us. That means that we're looking at the star as it was when woolly mammoths still roamed the planet. Once you start to factor in high-power telescopes, that time gap grows exponentially—today, the farthest object spotted is a galaxy approximately 13.4 billion light-years away, meaning that our image of it is older than the earth itself.

Related: What Happens When Galaxies Die?

What's There, What's Not

One of the first questions we had when we started delving into this question was this: are any of the stars we see at night still there? After all, it seems like most of them could have gone supernova hundreds of years ago and we wouldn't know it yet.

Related: There's Going To Be A New Star In The Sky

The bad news is that there are almost certainly some stars that have sparked out but that we still see twinkling. The good news is that most stars are still there. Stars live a long time—some as long as trillions of years—so a couple of thousands of years is just a drop in the bucket. Beyond that, there are stars being born in the distant universe all the time that we can't see yet, but they'll take the place of whatever "dead" stars we currently wish upon in due time.

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Written by Curiosity Staff April 4, 2017

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