It might seem obvious that the sky is black at night because there’s no Sun to illuminate it.
But what about all those billions of stars that are out there in the Universe, shining brightly? Shouldn't they be creating a huge amount of light of their own, significantly brightening the night sky?
If you've ever considered this, you're not alone.
More cosmic puzzles
Heinrich Olbers
In 1823, German astronomer Heinrich Olbers concluded that the night sky should be as bright as the surface of the Sun.
If the Universe were infinite and filled with stars, he reasoned, wherever you look you’d be looking at a star.
Although light from more distant stars is weaker, there are more of them, which should result in a bright sky.
Resolving Olber's paradox
How is ‘Olber’s paradox’ resolved?
An oft-quoted explanation says the expansion of the Universe weakens the light on its journey towards us.
This is true, but it’s by no means enough to make the sky black.
The real answer is that galaxies and the stars within them have a finite age, having first formed a few hundred million years after the Big Bang.
They’ve only emitted a limited amount of light, so it’s truer to say the night sky is black because the Universe has not existed forever.
