If you have sharp eyes or good binoculars, you can sometimes see double in the night sky – two stars that are quite close together. In some cases, the stars really are close – they can be bound to each other by gravity, forming a binary.

In other cases, though, it’s just a coincidence. The stars are unrelated, but they just happen to line up in the same direction. These stars are known as optical doubles. And some of them are beautiful sights – especially when viewed through a telescope. The two members can show different colors, providing a nice contrast.

One of the best-known examples is Albireo. It marks the head of Cygnus, the swan, and it’s high overhead at nightfall. A small telescope reveals one blue star and one gold star. The best measurements to date indicate that the stars are about 30 light-years apart.

An example that’s visible to the eye alone is Algedi, in Capricornus. It’s due south at nightfall. The eye can just make it out as two separate points of light. The stars are hundreds of light-years apart.

Sometimes, it’s hard to tell if a double star is a binary or not. The best example is Mizar and Alcor, in the handle of the Big Dipper. It’s fairly easy to see the stars as two points of light. They actually move through space together, so they probably were born together. But astronomers aren’t sure if they’re a true binary – or just two related stars moving through space on their own.

Script by Damond Benningfield