A team of astronomers may have answered one of the long-held mysteries in the universe - why some stars have companions while others are singles.

In general, stars are born together in groups, and virtually all newborn stars have a companion. The clustered groups would then disperse into the galaxy. However, different star formation environments were the determinants of whether stars stay with their companions or not, according to astronomers from Bonn University and the Max-Planck-Institute for Radio astronomy, whose work is published in the journal

Monthly Notices of the Royal Astronomical Society.

Depending on how the binary stars interact with other stars gravitationally as they travel the birth sites before dispersing into the galaxy, the stars end up with different systems.

In many cases the pairs are torn apart into two single stars, in the same way that a pair of dancers might be separated after colliding with another couple on a crowded dance floor, said Michael Marks, a PhD student and member of the International Max-Planck Research School for Astronomy and Astrophysics.

Not every collision leads to separation of stars.

The outcome of binary-binary collisions are most often one binary and two single stars, says Pavel Kroupa, a professor at Bonn University's Institute for Astronomy. Sometimes one gets four single stars and no binary. The calculations of how often an outcome occurs are very involved and can be compared to the scattering or collision events studied in the Large Hadron Collider, where subatomic particles are crashed together and physicists observe the various particles that then fly out from the crash.

In the case of a triple system colliding with a binary system, most often one will get three stars flying out and one binary, and sometimes a triple or even a quadruple system will result, said Kroupa.

Stellar nurseries vary in their density and form, and more interaction takes place within higher density groups where the most binaries form, splitting them to single stars. Thus the composition of single and binary stars is different in every group, according to the initial density of stars.

Computer models can help calculate the composition of stars and binaries in areas with various densities, giving clues to how different types of birth sites  generate stars to the wider Galaxy.

This model can be used much more extensively than just our Galaxy, said the astronomers. We can now, for the first time, calculate the content in binary systems of any galaxy, ranging from the tiniest dwarf spheroidals to the most massive galaxies up to a thousand times more massive than our Milky Way, according to Kroupa.

This is the first time we have been able to compute the stellar content of a whole galaxy, something that was simply not possible until now. With our new method we can now calculate the stellar contents of many different galaxies and work out how many single and binary stars they have, said Marks.