Why is the night sky black?

Why does the night sky appear black? Why isn’t it white-hot? This is an important question.

When we look up on a moonless night, except for the small number of stars we can see with our unaided eyes (about 2500 at any one time), why is it so pitch black?  The answer may not be as obvious as you might think.

whyisthenigh

The blackness of space. Beth Scupham/flickr, CC BY-SA

The 19th century astronomer Olbers posed a paradox. If you imagine as you look out in space, even though galaxies and hence stars are great distances from each other, if space extends far enough, eventually every line of sight in every direction should finish on a star. If the Universe was infinite in size and filled with stars this would have to be the case. Thus why isn’t the night sky burning bright? Why isn’t it white-hot like the sun? Continue reading