It’s thought to be down to the way they form. Up to 50 per cent of all stars are in binary systems. However, some astronomers estimate that an even higher percentage of stars are born in pairs. Our theories of star formation suggest that most stars form in clusters rather than singularly. In the denser part of clusters, stars could have paired up, forming binary stars. Another way in which stars can end up in pairs is by the breaking up of discs around newly formed stars. Discs of gas and dust are a natural result of the star formation process due to the initial rotation of the interstellar cloud that collapses to make a star. A disc increases in mass as more material from the cloud falls onto it and can become unstable, or too heavy, to be maintained and fragments, breaking up into stars. If this happens quickly after the first star is formed, then the two stars may have similar masses.
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