Our star the Sun travels through space with its system of planets, but it has no close stellar companions. The nearest star, Proxima Centauri, lies over 4 light-years away. But the majority of stars - two out of three - travel through space with one or more companions. They form multiple star systems, in which the various component stars orbit around one another. The most common multiple stars are double stars, or binaries, but as many as six stars may be closely associated. On a broader scale, looser groupings of stars travel through space together, spread over several light-years. They are called open clusters. Examples are the Pleiades, or Seven Sisters, and the Double cluster in Perseus. Typically, open clusters have at maximum a few hundred members. By contrast, circulating around the centre of the galaxies are very dense globular clusters. They contain hundreds of thousands of stars packed together in a globe-shaped mass. Some can be seen with the naked eye. Examples are Omega Centauri in the southern heavens and M13 in Hercules in northern skies. The stars in the patterns of bright stars we call the constellations are not associated in any way in space. They just happen to lie in the same direction.
