Many of the stars in our galaxy don't orbit alone; systems of two, three, or even more stars are common. These have provided us important windows into the Universe in general. Type-Ia supernovae, which we use to measure cosmic distances, are triggered when two orbiting stars collide. Pulsars get sped up by the gas of a companion star until they rotate every few milliseconds. And changes in a star's motion can reveal an otherwise hidden companion, such as a stellar-mass black hole.

Despite their importance, however, we haven't really learned much about the formation of multi-star systems. While it was once proposed that they formed when one star gravitationally captured a second, surveys show that groups of orbiting bodies are more common when stars are young. This suggests that these systems form together before gravitational interactions kick some of the members out of the group. The challenge with imaging their formation is that stars form very quickly in astronomical terms, taking on the order of a hundred thousand years.

Now, researchers have been able to image one of these systems in the process of formation. By looking at a star forming field in the Perseus region, they've been able to spot a single protostar that's accompanied by three additional condensing regions, all of which should reach the protostar stage within the next 40,000 years.

The region that's the focus of this work is called Barnard 5, and we'd previously observed a protostar that is currently 0.1 solar masses in the area (called B5-IRS1). The researchers behind the new work decided to get a look at the dynamics in the region by imaging a spectral line produced by ammonia, which would be shifted slightly by the turbulent motion of the gas in this area. Their imaging revealed that the Barnard 5 region contains several filaments of denser gas and that condensations were forming in these. Each of these condensations were about a third the mass of the Sun, and the distances between them and the protostar were on the order of 3,000 to 10,000 astronomical units (for scale, Neptune is about 30 astronomical units from the Sun).

Tracking the motion of the gas, they found that, relative to each other, all four bodies were barely moving (relative motions were on the order of 0.2km/s). Based on the distances and masses involved, this was enough to determine that all of the objects were gravitationally bound in a single system.

The motion of the gas also revealed that there was very little turbulence in the area that the gas is able to reach free-fall velocities into any gravity wells nearby. The authors calculate that there's enough gas around that each condensation is likely to draw in enough to clear the brown dwarf limit (which is 80 Jupiter masses) and form a star, taking approximately 40,000 years to do so.

It's impossible to tell at this point whether the system will continue to evolve as a four-star collective. The remaining gas will help draw the stars closer together initially and provide some drag that will slow down their relative motion. But, once the gas is cleared out by the light, the gravitational interactions among the stars themselves will dominate, and these can either enter stable resonances or slingshot one of the members out of the system.

Further observations of the new system should help refine our knowledge of how multi-star systems form. Given that we now know these systems can also produce planets, it's possible that these observations can also provide some insight into that process.

Nature, 2015. DOI: 10.1038/nature14166 (About DOIs).