Sound Science Bite: November 25.Recently (Science 346,732(2014)) it has been discovered that perhaps half of all stars do not belong in galaxies, but exist in intergalactic space. How did this happen? After all, star formation is routinely observed in galaxies, including our own Milky Way. How did they get way out "there"? It has long been known that galaxies occasionally collide. Wouldn't that lead to the destruction of those galaxies? Or would it explain the "rogue" intergalactic stars?

Stars in galaxies are very apart. For example, the nearest star to our sun is about four light years away, or 9.5 × 10¹⁵ kilometers of separation. The diameter of the sun is close to 1,400,000 km. Hence the ratio of starless space to the space occupied by the sun is about 600 thousand trillion trillion to one. Two gnats might have a greater chance of colliding in the Grand Canyon than the sun colliding with another star if the Milky Way were in collision with another galaxy. How do stars then get free of a galaxy during a collision? Gravity.

Consider a body such as a star bound to another body, say a galaxy. To physicists this means the combined energy of gravitational interaction of these bodies is negative. For them to separate permanently, the combined energy must at least be zero. For this to happen gravitational energy must be added to the system. In galaxy collision this energy does not come from direct collisions of stars with stars, but from the gravitational interactions of a star with the stars of the colliding galaxy, resulting in so-called "tidal" interactions as streams of stars escape their host galaxies. This complex process can be understood in the "simple" case of what physicists call a "three-body interaction". (Not so simple, really, as no general solution for this problem exists.)

In a three-body interaction you have two objects, such as two stars, in a mutual orbit, that is, orbiting each other. (If the stars have equal masses they will orbit a point halfway between them.) Another star comes along. If the interaction between this star and the orbiting two occurs in a certain way, the orbiting stars' energy may increase while that of intervening star may decrease. (Actually, the energy of the intervening star interacting with the other two decreases.) If the orbiting stars' mutual energy becomes positive, they will separate. In the meantime, if the mutual energy of the intervening star and one of the previously orbiting stars decreases to less than zero, the intervening star and that star will form a new orbiting pair, while the third formerly orbiting star goes off its merry way.

This is in fact how it is thought large clusters of stars, called globular clusters due to their spherical shape, evolve and lose stars. Three-body interactions should be relatively common in these dense clusters, resulting in the clusters losing energy as stars are expelled by these interactions. Perhaps many of the intergalactic stars come not only from colliding galaxies, but also from such clusters.