Wilhelm Olbers

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Why is the night sky dark?

That's a question that a lot of six-year-olds ask. And for centuries, it was one that astronomers asked, too.

Today, the question is known as "Olbers' Paradox" -- named for German astronomer Heinrich Wilhelm Olbers, who was born 250 years ago tomorrow.

Olbers was a physician with a love of astronomy. He built an observatory at his house, and spent many nights hunting comets. He also hunted for a planet between Mars and Jupiter. He didn't find one, but he did find two of the first four asteroids, which inhabit that region of the solar system. Olbers thought they were the remnants of an exploded planet.

Later, he turned his attention to the dark sky.

Olbers reasoned that if the universe were infinite, as many scientists thought, then we should see stars in every direction, so the night sky would be filled with light. Since we see only a few stars, he decided that dust between the stars must be blocking their light.

In fact, dust clouds do absorb the light of the stars behind them. But other factors are more important. For one, the observable universe isn't infinite, so there aren't stars in every direction you look. For another, the universe is expanding, which stretches the light of distant stars and galaxies to wavelengths that we can't see. And for another, the universe is so large that the light from many of its stars hasn't had time to reach us.

All of those factors combine to keep the night sky dark.

Script by Damond Benningfield, Copyright 2008

For more skywatching tips, astronomy news, and much more, read StarDate magazine.