Sun Goes Down. Up Comes A Mystery : Krulwich Wonders... A child's question and an old paradox: If the sky is filled with stars, why is it dark at night? Physicists say the answer lies in those deep dark spaces between stars — evidence of our expanding universe.

# Sun Goes Down. Up Comes A Mystery

Here's a question you probably didn't know was a question: Why is the sky dark at night?

My daughter asked me this about 10 years ago. We were looking up at the night sky, and she said, "There's lots of stars up there." And I said, "Yes."

Then she said, "Are there stars everywhere?" And I said there were. Then she said, "Well, if there are stars everywhere, all of them shining, why don't they fill the sky and make the sky shiny?" In her mind, the sky, instead of being dark, should look like this, brightness everywhere.

Thirteen-year-old girls aren't the only ones asking this question. In 1823, a German astronomer, Heinrich Olbers, wondered, "Why is there dark between the stars?" And ever since, this is called Olbers' Paradox (which sounds better than My Daughter's Too Hard To Answer Question.)

The key to the paradox is, if you look closely at any patch of sky, you will see shining stars. If you look through a telescope, you will see more. The nearer stars will be brighter, of course, but behind them will be more stars, and behind those, even more. Even if those distant stars are dimmer, there are so many of them, their combined glow should add up to a considerable brightness.

So why is it so dark at night? Where does the darkness come from?

Here's an answer. It comes from Henry Reich and his team at Minute Physics. They do very short videos about very big questions.

They pack a lot into this "minute." Actually, they cheated. It's almost four minutes long. I found it fascinating.

The big revelation — for me — is that the darkness between stars suggests that there are chunks of deep space where stars are missing, either because they aren't there (wrong), or because they are so far away, light from those stars hasn't reached us yet (right).

In other words, the darkness tells us something about the size and character of our universe. We're expanding. In a few billion years, those dark patches between stars will grow. There will be fewer stars to see. A few billion years after that, you'll be standing on a hill looking up on a clear night, and the sky will be close to pitch black, no Milky Way, no constellations, just blackness, because the stars we see today are speeding away from us, leaving a faint, infrared light. It will be very lonely. (Unless you're a telescope-wielding butterfly. Butterflies can see infrared. Distant retreating stars will be visible to them. To us, those stars will be invisible.)

Ah me. I'm glad I'm living now. 10 billion years from now, I'd rather be a butterfly.

If you found the Minute Physics video intriguing and you want to spend five more minutes thinking about this, there's another, really good, though more traditional, video that covers the same ground. It's from DeepAstronomy.com and you can find it here.

Also, we came upon a startling Milky Way image, shot from the surface of Mars. It comes, (we think) from independent filmmaker, Rich Smyth, in Britain. Deep space photographers should all be so daring.

## Krulwich Wonders

Robert Krulwich On Science