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Massive Binary Stars May Form Through Cosmic Encounters

Astronomers have caught a rare moment in the birth of massive binary stars—and the process appears messier than the standard scenario suggests. Using the Atacama Large Millimeter/submillimeter Array, or ALMA, researchers studied the young system IRAS 07299−1651, about 5,300 light-years from Earth. The pair’s elongated orbit and sharply misaligned feeding disks suggest that the protostars may have formed separately before a close encounter brought them into gravitational partnership.

The research, published in Nature Astronomy on Sept. 8, leaves one crucial question unresolved: whether this encounter will produce a lasting binary system or whether the stars will eventually drift apart.

IRAS 07299−1651: A Young Massive Binary in the Making

IRAS 07299−1651 contains two massive protostars that are still gathering material. The team first observed the system in 2019, then tracked subtle changes in the stars’ positions over eight years. That long observational baseline allowed the researchers to follow their motion rather than rely on a single snapshot.

“For the first time, we were able to watch two massive stars move around one another while they were still being born,” said team leader Yichen Zhang of Shanghai Jiao Tong University in China.

Massive stars eventually explode as supernovae and reshape their surroundings, making their origins important to models of both stellar evolution and galaxies. Roughly half of sun-sized stars in the Milky Way have companion stars, while the proportion rises to around 90% among massive stars.

Why Massive Binary-Star Formation Matters

The usual explanation is that one collapsing cloud forms a disk that fragments into two stars. In that scenario, the resulting stars and their surrounding disks should generally share a similar orientation. IRAS 07299−1651 does not fit that tidy picture.

The system therefore supports an alternative possibility: two protostars born in separate pockets of gas and dust may have encountered one another early in life and become gravitationally linked. If confirmed across other systems, this pathway would show that chaotic encounters—not just fragmentation—help build binary stars.

How ALMA, JWST, VLA and VLT Built a 3D Picture

ALMA supplied crucial radio and submillimeter observations of the cold material feeding the protostars. The Very Large Array added complementary radio data, while infrared observations from the James Webb Space Telescope and Very Large Telescope helped reveal the embedded young stars and their environments.

“Each telescope revealed a different piece of the puzzle,” said Rubén Fedriani of the Instituto de Astrofísica de Andalucía in Granada, Spain. “The combination of radio and infrared observations provides the most exquisite detail on the formation of this massive protobinary system.”

Why Multi-Wavelength Observations Were Essential

No single instrument could reconstruct the system’s complete architecture. Together, the observations allowed the team to map the stars’ orbit, determine how their disks are aligned and measure the angles at which they launch jets into space.

“It felt like solving a three-dimensional puzzle,” said Yao Wang of Shanghai Jiao Tong University. “Each new observation added another piece, and eventually the orbit, disks, and jets all came together into a single, coherent picture.”

Eccentric Orbits and Misaligned Disks Point to Chaos

The researchers initially expected the protostars to occupy relatively neat, near-circular orbits. Instead, they found an orbit described as highly eccentric—so elongated that it is almost parabolic. The disks feeding the stars are also tilted at sharp angles to one another and to the orbital path.

That combination is difficult to explain if both stars formed from the same disk. A disk-born pair should inherit a more coherent orientation unless some later event disturbed it. The observed geometry instead suggests that the system’s present arrangement may record a recent and violent assembly process.

Separate Births and Gravitational Capture

The leading theory is that each protostar formed in an individual cloud of gas and dust. Their respective natal environments later approached one another, and gravity coupled the two young stars into the orbit observed today. This would be a case of dynamical capture during the earliest stage of stellar life.

Tracing the orbit backward led the researchers to estimate that the encounter occurred around 60 years ago. That is an instant compared with the billions of years over which stars live. Remarkably, the disk-shaped material around the protostars appears to have survived with well-defined structures intact.

The 60-Year Timeline Is an Inference, Not Direct Observation

The team did not witness the encounter itself. The date is a reconstruction based on the present orbit and modeling of how the system evolved. Even so, it provides a surprisingly recent window into the dynamical processes that can shape young stars.

“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 of the University of Virginia.

The Future of the IRAS 07299−1651 Protobinary Remains Uncertain

The observations do not yet establish whether the stars are destined to remain together. Interactions with the gas surrounding the system could reshape the orbit, while other outcomes remain possible. In other words, the final fate of this young pairing has not been sealed.

Further monitoring may show how the orbit, disks and jets continue to evolve. Those changes could reveal whether surrounding material is stabilizing the pair or allowing them eventually to separate.

Long-Term Monitoring Could Transform Young-Massive-Binary Research

Young stellar systems can change too slowly to be understood from isolated observations. A sustained program combining ALMA, the VLA, JWST and the VLT can instead track orbital motion and structural changes over time.

The researchers hope to apply this approach to other young massive binary systems. If similar eccentric orbits and misaligned disks are found elsewhere, astronomers could determine whether chance encounters are uncommon exceptions or an important route to forming binary stars.

Massive Binary Stars: What Is Confirmed and What Remains Unknown

• Confirmed: Two massive protostars in IRAS 07299−1651 move around each other while still accreting material.
• Observed: Their orbit is highly eccentric, and their disks and jets have markedly different orientations.
• Inferred: The stars likely formed separately and underwent a close encounter around 60 years ago.
• Unknown: The system may either settle into a permanent binary or evolve toward separation.
• Unresolved: Researchers do not yet know how common this formation pathway is.

IRAS 07299−1651 does not prove that all massive binaries begin through capture; disk fragmentation may remain the dominant route. What it does is expose an overlooked possibility during the brief, turbulent first chapter of a star’s life. By watching a young system change over time, astronomers are gaining a new way to test how stellar partnerships begin.