ALMA spots a nine-member stellar family in the act of formation

June 24, 2026 report ALMA spots a nine-member stellar family in the act of formation Shreejaya Karantha Author Gaby Clark Scientific Editor Robert Egan Associate Editor Massive stars much bigger than our sun always come in pairs or groups, not alone. But astronomers don't fully understand how these groupings form. In a new study, astronomers using ALMA have serendipitously discovered a young system containing nine baby stars forming together, and they have detailed a rare glimpse of the formation of such a stellar family in its earliest assembly stage in a paper submitted to the arXiv preprint server on June 2. Breaking to build There are a few competing theories of how groups of stars form: disk, core and filament fragmentation. That is, a spinning disk of gas around a young star breaks apart into multiple pieces, or a big core of gas fragments into multiple star-forming clumps before the stars even form, or a long filamentary cloud of gas breaks into clumps along its length, each clump forming a star. Massive stars, growing together as a group, are hard to study because they are far away, buried in thick dust and grow up fast while still in dense clouds. So astronomers have not caught many of them "in the act" of forming. In this new study, astronomers did not set out to search for multiple-star systems. Instead, they were using ALMA telescope data from the CoCCoA survey, a project designed to study the chemistry of complex organic molecules around 25 hot cores in massive star-forming regions. One of these targets was NGC 6334-43, a known hot core within a larger star-forming complex about 4,340 light-years away. An accidental find While analyzing the high-resolution dust and gas emission in this field, the researchers noticed something unexpected: The data revealed nine compact, closely spaced sources (out of 12 detected in the field) strung along a single long filament of gas, suggesting they may be bound together as one large multiple-star system. Upon further analysis, the team found that the nine sources are not randomly scattered. They form a single, gravitationally bound system, confirmed by a stability check comparing gravitational and kinetic energy, with a mean separation between pairs of about 7,930 astronomical units (AU, 1 AU = 150 million kilometers). These sources appeared to share a common gas structure, lying along a single large-scale filamentary structure roughly 24,700 AU long. Within this larger system, however, two smaller subgroups tell a different story. "The nine sources considered here display a range of evolutionary signatures," the team writes in the paper. The ALMA2 triple—comprising the close hot-core pair ALMA2a/b plus the younger ALMA2c—shows no evidence of a shared disk, consistent instead with core fragmentation. Similarly, the ALMA6 binary, with components 1,530 AU apart and an unusually long spiral-arm-like structure, also appears to have formed via core fragmentation, with ALMA6a more evolved than its likely companion ALMA6b, which has not yet collapsed into a star (pre-stellar stage). The remaining sources in the system show a range of masses and evolutionary stages, with some driving active outflows—signatures of newborn stars. For the hot-core pair ALMA2a/b, however, there's some ambiguity over which star is actually driving the outflow: "At the resolution of these archival data, it is unclear whether this outflow is driven by ALMA2a, ALMA2b, or the combined system," the researchers note. Puzzling ages At first glance, the wide range of maturity among these nine objects seems to argue against them all forming together when a single filament broke apart, since that scenario permits only a half-million-year age gap between sibling stars. But the researchers note that the age spread expected from filament fragmentation is comparable to the entire time span it takes to form a single massive star, meaning the differences in maturity seen here fall within what the theory could still produce. As they put it, "a filament-fragmentation origin for the nine-member system remains feasible." Together, these results suggest filament fragmentation built the system as a whole, while core fragmentation shaped its closer subsystems. This accidental discovery highlights how surveys built for one purpose can still reveal rare, very young examples of multiple-star formation, which are otherwise difficult to catch so early. Written for you by our author Shreejaya Karantha, edited by Gaby Clark, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you. Publication details D. J. Taylor et al, A nine-member protostellar system forming via filament fragmentation in the high mass protocluster NGC 6334-43, arXiv (2026). DOI: 10.48550/arxiv.2606.03261 Journal information: arXiv © 2026 Science X Network