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Counting Galaxies

March 13, 2018

Leo, the lion, springs across the sky on March nights. He’s in the east at nightfall, marked by his prominent “heart,” the bright star Regulus, which is a third of the way up the sky.

If you scan the sky below and to the left of Leo with a telescope, you’ll see clusters of galaxies. They contain thousands of galaxies in all. Each galaxy is an “island universe” similar to the Milky Way — a vast assemblage of millions or billions of stars.

A couple of years ago, a group of astronomers estimated that, if we had big enough telescopes, we could see about two trillion galaxies. Most of those are small “puffballs” from the very early universe, when the first galaxies were being born.

We see those galaxies as they looked when the universe was only about a billion years old. In other words, we see what they looked like about 13 billion years ago. By today, none of them look as they did then. Most of them probably have merged with other galaxies to form giant galaxies similar to our own.

On the other hand, there probably are many more galaxies that are out of sight — they’re so far away that their light, traveling at a limited speed, hasn’t had time to reach Earth. And for many of those galaxies, it never will. Because the universe is expanding, light can’t travel fast enough to cover the ever-widening gulf. So there are vast regions of the universe that we can’t see — and never will.

We’ll have more about the early universe tomorrow.


Script by Damond Benningfield

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