Binary star system, artwork. Two stars (top right) forming a binary star system within an open star cluster. Other members of the star cluster are seen in the background. Open star clusters contain stars that formed from the same cloud of gas and dust. Over millions of years, the stars gradually move apart as their different trajectories take them in different directions. Some stars form at the same time and close to each other. These stars are likely to remain orbiting each other, and two such stars will form a binary star system. A large proportion of stars are thought to be members of binary star systems. The different colours of the stars can indicate their age, mass and composition.