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The Question

(Submitted August 15, 1998)

I'm a middle school geography teacher with no formal expertise in, but a lifelong fascination with, astronomy and space in general. I seem to remember from a long ago college astronomy course a discussion of Oblers' Paradox that explains why we don't have perpetual daylight despite the billions of bright stars that presumably send their light to all parts of the earth. In trying to explain this concept to my eight-year old daughter, I get tongue-tied by all the technical jargon involved. Can you help me put my explanation in layman's terms?

The Answer

In an infinite universe, which has existed forever, we shouldn't have night. Imagine a universe divided into shells, with stars of a single brightness distributed evenly --- if you look at a shell twice as far, each star is only a quarter as bright, but there are four times as many stars, so each shell is equally bright. If you have an infinite number of shells, you end up with infinite brightness!

The big bang cosmology solves this, mainly by the implied age of the universe. We only see light emitted within the last 12 billion years (or whatever the age of the universe might be). This is a long time, but certainly not infinite, and not enough to make the night sky bright.

Koji Mukai & Maggie Masetti
for Ask an Astrophysicist

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