As the name suggests, it is a pair of stars close together in the sky, in fact so close together that they can only be separated using some kind of optical aid. Some wide, bright doubles can be seen in binoculars, but most need a telescope of some kind. It used to be thought that they were chance alignments of stars in the sky, but it is now known that such alignments are rare and most doubles are actually close to each other in space and bound by gravity, either orbiting each other (when they tend to be referred to as binary stars), or having common proper motion - that is, they are moving through space together. In fact more than half the stars we see in the sky are actually multiple star systems. In that sense, our own sun is in the minority because (as far as we know) it has no companion.

Alpha Centauri is one of the most famous doubles, and only accessible to us blessed southern hemisphere observers. Alp Cen is actually a triple system, with the third member, Proxima Centauri (quite far away from the bright pair and not visible in the same field), being the closest star to our sun. The bright pair are orbiting each other relatively quickly, completing a full orbit in only 79.2 years. So motion along the orbit is measurable in much less than an observer’s lifetime, which is not always true of visual doubles.