Multiple-star systems can be born from gravitationally unstable conditions, according to observations of a triple protostar system described in Nature this week. The findings help us to understand the evolution of multiple-star systems.

Almost half of all Sun-like stars have at least one companion star, a common outcome of the star-formation process. There are two main theories to explain this process. One explanation, for which observational evidence has recently emerged, is that turbulence within the molecular cloud core that is forming a star can cause clumps of material to fragment and form companion stars. John Tobin and colleagues provide evidence to support the second theory - that gravitational instability causes fragmentation of the protostellar disk (gas and dust that surrounds the star).

The authors study L1448 IRS3B, a young triple protostar system (thought to be less than 150,000 years of age) that is in an early stage of star formation. The system has been studied before, but new images from the ALMA telescope are of much higher quality than previous studies, allowing the authors to observe a spiral arm feature in the disk around these stars. This observation indicates that the protostellar disk has recently undergone gravitational instability, giving birth to at least one of the companion stars, the authors conclude.