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The Question

(Submitted July 10, 1997)

How long does it take for the light from stars to be visible here on Earth?

The Answer

That's an interesting question. Light travels at 300,000 kilometers per second or 186,000 miles per second. The time it takes for light from stars to reach us is the distance to the star divided by this speed. The nearest star to us is the Sun and it takes about 8.3 minutes for its light to reach us here on Earth.

Other stars are so much farther away that it is convenient to express the distance to them in units of the distance traveled by light in one year. This unit is called a light year. The next closest star to us is Proxima Centauri. This star is 4.3 light years away which means that light from it takes 4.3 years to reach us. Our galaxy is about 100,000 light years across. This means that it can take tens of thousands of years for light from some stars in our galaxy to reach us. For stars that we can see in nearby galaxies it can take millions of years. The farthest objects we can see are quasars. They are so distant that the light we see from them today left billions of years ago.

So when we look up at the stars we are looking back in time. This is useful for astronomers because when we look at very distant objects we can see what the Universe was like a long time ago.

Damian Audley
for the Ask an Astrophysicist team

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