Why some stars prefer to be single, while others are either paired up or in trios, could have been answered by a team of astronomers at the Max-Planck-Institute for Radio astronomy and the University of Bonn with the help of sophisticated computer models.

The general idea, which features in the journal Monthly Notices of the Royal Astronomical Society, is that different star formation environments decide whether or not stars keep hold of their companions. Since newly formed stars are not born singularly, but in groups within clouds or nebulae, just how some find themselves alone has mystified astronomers, especially when it comes to the type of stellar systems that exist in our Galaxy.

Before groups of hot, newly born stars disperse into the Milky Way, they travel around their birth sites, interacting with their companions in a somewhat aggressive gravitational nature. “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,” says PhD student and member of the International Max Planck Research School for Astronomy and Astrophysics, Michael Marks.

However, it appears that not all colliding groups or pairs are unlucky enough to lose their partners, even if the outcome of these stellar smashes can not always be predicted. “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.”

The outcome of triple systems colliding with that of a binary is even more complicated still, and is believed to momentarily form a compact system of five stars that become unstable, eventually decaying or falling apart over ten to a hundred thousand years to form several possible outcomes. “If a triple system collides with a binary system, then most often one will get three stars flying out and one binary, and sometimes a triple [with two single stars] or even a quadruple system [with one single free floating star] will result,” adds Kroupa.

However, according to Kroupa, the vast majority of stars form as binary systems. “Even our Sun may have formed like that, and it may have lost its distant companion in a collision in the young star cluster where it was born,” he tells Astronomy Now. “The companion would have been a distant one, because otherwise the Solar System would not have been able to form.” It seems that if this is true, then as stars are being torn apart, the number of binaries diminishes before the stars spread out into the wider galaxy.

Since stellar nurseries do not all look the same and are not crowded to the same extent, the stellar density varies. The most binaries form within higher density groups where more interaction is likely to take place and with this, these double stars are split into single stars. This explains why every group varies in composition of single and binary stars when the stellar-birthing area disperses, depending on the initial density of stars.

The computer models that the Bonn University astronomers have employed to discover this finding enables the calculation of the resulting composition of stars and binaries in regions of varying densities. This allows the determination of how different types of birth sites will contribute binary systems and hence single stars to the wider Galaxy. “Working out the composition of the Milky Way from these numbers is simple: We just add up the single and binary stars in all the dispersed groups to build a population for the wider galaxy,” says Kroupa.

However, Kroupa and Marks believe that this model does not stop at our Galaxy, but can be used much more extensively. “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,” concludes Kroupa.