There are stars in the night sky that look brighter than the others, and when you look at them through a telescope you realize you are looking at twins. The two stars rotate around each other, sometimes taking nearly a hundred years to do it. They create so much gravitational pull there's no room around for anything else. You might see a blue star, for example, and realize only later that it has a white dwarf as a companion - that first one shines so bright, by the time you notice the second one, it's too late.
