Stars Are Suns, Suns Are Bright—So, Why Is the Night Sky Dark?

Why is the night sky dark? This seemingly simple question has some significant implications. 

We take for granted that the night sky is dark. At the day’s end, the closest and brightest star in our sky, the sun, sinks below the horizon. It leaves behind the vast blackness of space, which is dotted with the light from distant stars, sometimes the Moon, and maybe, if you’re lucky or in the southern hemisphere, a look at the rest of our Milky Way galaxy and our extragalactic neighbors, the Small and Large Magellanic Clouds. So why should we expect anything different? 

Should the night sky be dark?

In the 1820s, the astronomer Heinrich Olbers pointed out that, based on our understanding of the universe at the time, the night sky should not be dark at all. We considered the universe to be steady and unchanging. And this is a very reasonable assumption. We can be sure that at the start of every day, the sun will rise and that the stars in the sky will move in predictable patterns. We also considered the universe to be uniform and infinite in extent. The idea that there could be a Truman-show-esque edge beyond which the universe ceased to exist seemed ridiculous. 

Olbers’ Paradox tells us that the entire night sky should not only not be dark, but it should be about as bright as the surface of the sun in all directions.

But if that were all true, then looking out into the night sky into an infinite universe should mean that in any direction, we will eventually see a star. Imagine looking into a dense forest. Although there's space between the trees, you see what appears to be a wall of trees. That's because a tree always falls within your line of sight. In our infinite universe, you may have to look back increasingly far to see a star in a given direction, which could mean that star is too faint to see.

But, Olbers’ Paradox tells us that if the universe is infinite and unchanging, it must also be infinitely old. Even light from the most distant stars has had time to reach our eyes as they look out into the night sky. The entire night sky should not only not be dark, but it should be about as bright as the surface of the sun in all directions. 

This seeming contradiction became known as Olbers’ Paradox, even though Olbers was far from the first person to bring up the problem. We've all been there—a few different people may have a similar idea, but whoever says it the loudest and with the most confidence gets the credit. The trouble with the night sky being dark had been mentioned before by other astronomers, including Kepler...

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