ALMA watches a massive binary star system form in real time

The majority of massive stars are not solitary. Indeed, at least nine out of ten are born with companions and thus form binary or more complicated systems. Such partnerships matter because they influence galaxy development through supernova explosions and the production of heavy elements. Yet astronomers usually study such systems only long after they’ve formed, so the moment of assembly has remained elusive.

Astronomers have taken a remarkable step forward in watching a massive binary star system assemble in real time. Using ALMA, they tracked the motions of two young, massive stars for nearly eight years, building one of the most detailed 3D views ever captured of such a system. What they found was striking: the stars follow a highly elongated orbit and are each surrounded by gas disks that are strongly tilted, both relative to each other and to the orbital plane.

This challenges the traditional view that massive binary stars form together when a single rotating disk breaks up. The evidence indicates these stars formed separately and then ended up near each other through a close gravitational interaction. This scenario therefore offers a new way for close massive binaries to form.

The team, led by Yichen Zhang of Shanghai Jiao Tong University, focused on the system IRAS 07299−1651, containing two massive protostars still growing by accreting gas and dust. Back in 2019, ALMA observations hinted at this unusual behavior, but the misaligned disks were the clue that something didn’t fit the traditional model. Now, with years of data, the case for a gravitational “meeting” rather than co‑formation has grown much stronger.

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To push beyond earlier hints, the researchers committed to nearly eight years of patient observation, measuring the tiniest shifts in the stars’ positions with ALMA’s extraordinary precision.

As Yichen Zhang explained, “For the first time, we were able to watch two massive stars move around one another while they were still being born.”

They went even further. The team combined ALMA’s long-term monitoring with radio data from the VLA, infrared imaging from the James Webb Space Telescope (JWST), and observations from ESO’s Very Large Telescope (VLT), which followed jets of material streaming away from the stars.

They reconstructed the system’s full three-dimensional structure by combining these different datasets, thus showing how the stars orbit each other, how their disks are tilted, and how their jets are oriented in space.

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By combining data from multiple observatories, astronomers got their clearest look yet at how massive binary stars form. The observations revealed orbital movement of stars and interaction between their disks and jets, which leads to early growth.

“Each telescope revealed a different piece of the puzzle,” said Rubén Fedriani, a co-author of the study. “The combination of radio and infrared observations provides the most exquisite detail on the formation of this massive protobinary system.”

The team found a system far more complicated than they expected. Rather than moving in a smooth, circular orbit, the two stars follow a highly eccentric path, the best orbital solutions being nearly parabolic. Moreover, the disks around them are not aligned; each is strongly tilted with respect to the other and to the orbit.

As first author Yao Wang put it, “It felt like solving a three‑dimensional puzzle. Each new observation added another piece, and eventually the orbit, disks, and jets all came together into a single, coherent picture.”

The idea that the stars formed in a single disk is not compatible with this disordered structure, since such a scenario usually results in aligned spins and disks. The evidence, rather, suggests an alternative origin: the stars most probably started to form separately in different pockets of gas and were then brought together later on by a near-gravitational encounter.

In fact, as the representative orbital solutions show, they were about 60 years before the observations at their closest to one another, a moment which is effectively instantaneous on cosmic timescales. Remarkably, their compact disks have survived the encounter and still show distinct rotational structures.

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“This study demonstrates that the early lives of stars can be quite chaotic, with a chance encounter leading to this gravitational dance and stellar coupling,” said Jonathan C. Tan, a co-author of the study.

It’s still not certain whether the two stars will stay gravitationally bound for good; their current motion sits close to the dividing line between a bound orbit and one that could eventually fly apart, and interactions with the surrounding gas may still nudge their fate one way or the other. Future observations will help pin that down.

More broadly, the study opens a new way of investigating how massive binary stars come together. By applying the same long-term monitoring approach to other young massive binaries, astronomers hope to learn how often close encounters like this one, rather than shared birth in a single disk, build the massive binary systems seen across our galaxy.

Journal Reference:

1. Wang, Y., Zhang, Y., Fedriani, R. et al. An eccentric massive protobinary assembled via a core-merger parabolic encounter. Nat Astron (2026). DOI: 10.1038/s41550-026-02953-z