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Dear Dr. Staff: If the universe is, for all intents and purposes, infinitely big, and stars are scattered randomly throughout it, how come the night sky isn't completely bright? It seems to me that if I drew a line from an observer out to any point in the sky, I should eventually encounter a star if I can make the line infinitely long. Every spot in the sky should be lit! --Astral Inquisitor

Dear Astral: You've found an intent and a purpose for which the universe is not infintely big. If you think of the universe as a sphere, then there's a very good chance that the earth is not at the exact center. (Besides, if it were at the center today, then by tomorrow the earth would have moved around the sun so it would no longer be at the exact center anymore.) Since the earth is closer to one edge of the universe than the other, you can see all the way to the end of the universe on the close side. This is the part of the sky that you see as dark at night--all the dark spots where you don't see any stars are places where you are actually looking past the end of the universe. But the earth rotates every 24 hours, and during the day, your side of the earth is looking toward the far end of the universe--and there is a star every place you look, so the sky is bright. (The light comes to earth white, just like starlight at night, but the atmosphere makes it blue as I describe in the March 13, 1989 issue.) If you don't understand, think of the sun--it's just an especially close star. Stars at night only light up tiny spots in the sky; since the sun is closer, it lights up a lot more, but it still doesn't fill the whole sky. The rest of the sky is lit because of all the stars farther away between here and the edge of the universe.

If you're still having difficulty understanding, consider one of the interesting side-effects of all this. When the sun sets or rises, the stars that are lighting up the sky are those that are farthest away from earth. If you've taken a physics course, you'll know that the farther away a star is, the more red shift it has--so, instead of being white, the stars there look red. That's why the sky is somewhat red at sunrise, and very red at sunset. (It's the other way around south of the Equator.) --Astral Expert

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