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Physics - Light Years

The distances to the stars and other objects beyond the planets of our Solar System are measured in light years. As a measurement of actual length, a light year is about 5.9 trillion miles long.

People confuse a light year with a length of time, because the term contains the word ‘year’. But a light year is really a distance measurement – the length that light travels, zipping through space at 186,000 miles per second, over the course of a year.

When you view an object in space you see it as it appeared when the light left the object. Consider these examples:

When Astronomers spot an explosion on the sun, we don’t see it in real time; the light from the explosion takes about eight minutes to get to Earth.

The nearest star beyond out sun, Promixa Centauri, is about 4 light years away or 61 trillion miles.

Look up at the Andromeda Galaxy, the most distant object that you can readily see with the unaided eye, on a clear, dark night in the autumn. The light your eye receives left that galaxy about 2.6 million years ago. If the galaxy disappeared by some mysterious means, we wouldn’t even know for over two million years.

Here’s the bottom line:

When you look into space you are looking back in time.

Astronomers don’t have a way to know exactly what an object out in space looks like right now.

When you look at bright stars in a faraway galaxy, you must entertain the possibility that those particular stars don’t even exist anymore.

Some massive stars only live for 10 or 20 million years. If you see them in a galaxy that exists 50 million light years away, you are looking at dead stars because you are looking at an object that is 50 million years in the past.

If Astronomers send a flash of light toward one of the most distant galaxies, found with Hubble and other major telescopes, the light would take at least 12 billion years to arrive, because the farthest galaxies are at least 12 billion light years away. Furthermore, the universe is expanding, so those galaxies will be farther away by the time the light gets there.

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