Detection of a septuple stellar system in formation via disk fragmentation

Stellar multiple systems play a pivotal role in cluster dynamics and stellar evolution, producing intense astronomical phenomena like X-ray binaries, gamma-ray bursts, type Ia supernova and stellar mergers, which are prime sources of gravitational waves. However, their origin remains poorly understood. Here we report the discovery of a septuple protostellar system embedded in a Keplerian disk within the high-mass star-forming region NGC 6334IN, with close separations of 181–461 au. A stability analysis reveals that the disk surrounding the septuple system is dynamically unstable, indicating that the septuple system formed through disk fragmentation. Previous studies have typically found only two or three members forming through disk fragmentation in both low- and high-mass star-forming regions. Our findings provide compelling observational evidence that the fragmentation of a gravitationally unstable disk is a viable mechanism for the formation of extreme high-order multiplicity, confirming what was previously only a theoretical concept. The results shed new light on the formation of extreme high-order multiplicity in cluster environments.