TL;DR: Two massive newborn stars now orbiting each other first grew apart and then were pulled together by gravity, eight years of telescope tracking shows.

Two massive young stars that now move as a pair did not start life together. Long-term tracking shows the objects formed on their own and then drew together in a close gravitational meeting, giving astronomers a direct look at a massive binary still taking shape, according to the University of Virginia report titled Astronomers Catch Two Massive Stars Forming a Binary Pair.

The work centered on a young, massive protobinary known as IRAS 07299-1651. The international team watched the relative motion of its two protostars for nearly eight years, combining high-resolution radio measurements with infrared views of gas, dust and outflows, that University of Virginia report said.

The result was published in Nature Astronomy as “An eccentric massive protobinary assembled via a core-merger parabolic encounter.” A listing of Nature Astronomy research articles identifies that study as using multi-epoch ALMA and VLA observations to reveal three-dimensional orbital motion, highly eccentric and close to parabolic orbits, and strong disk-orbit misalignment.

Watched for eight years across four observatories

The observing campaign ran from 2016 to 2024, a time baseline described as necessary to determine the parameters of relative motion between the two protostars, according to the Chilean account published by Radio Nuevo Mundo under the title Astrónomo U. de Chile participa en primera reconstrucción 3D de la órbita de estrellas masivas en formación.

That account names the Atacama Large Millimeter/submillimeter Array, the Karl G. Jansky Very Large Array, the James Webb Space Telescope and the Very Large Telescope as the instruments used. The University of Virginia account likewise names the Atacama Large Millimeter/submillimeter Array, or ALMA, along with other telescopes including the James Webb Space Telescope, or JWST.

In that combined approach, ALMA supplied the high-resolution observations needed to track the stars’ positions and movements, while JWST and other instruments supplied information about surrounding material and outflows, the University of Virginia account said. Observations at different wavelengths helped the researchers piece together the system’s history.

Where the pair sits and how far apart it is

IRAS 07299-1651 is described as a young and massive protobinary located 1,680 parsecs away, in the Chilean university account. Another Chilean account, published by FayerWayer, expresses the same distance as 5,500 light-years from Earth, or 1,680 parsecs.

The two protostars are separated by a distance near 200 astronomical units, or AU, that Radio Nuevo Mundo account said. That separation equals 200 times the distance between the Sun and Earth, the same account explained.

Each protostar retains its own surrounding disk of gas and dust. The Chilean account states that each object has around it its own disk of gas and dust, while the University of Virginia account describes disks surrounding the two stars.

Who led the measurement

The project was led by professor Yichen Zhang of Shanghai Jiao Tong University in China, who was a postdoctoral researcher at Cerro Calán between 2014 and 2016, according to the Radio Nuevo Mundo account. The FayerWayer account also identifies Yichen Zhang of Shanghai Jiao Tong University as leader.

The team included Jonathan C. Tan, professor of astronomy at the University of Virginia, who studies how stars form and evolve, including the formation of massive binary star systems, the University of Virginia account said. His research on the young binary system used observations collected over nearly eight years.

It also included Guido Garay, winner of the 2017 National Prize for Exact Sciences and academic in the Department of Astronomy of the FCFM, that Radio Nuevo Mundo account said. The same account identifies Garay as a full professor of the FCFM and scientific director of the South American Center for Astronomy of the Chinese Academy of Sciences, known as CASSACA.

First three-dimensional orbit for such a massive newborn pair

The researchers reconstructed for the first time the three-dimensional orbit of IRAS 07299-1651, according to the Radio Nuevo Mundo account. That reconstruction was built from eight years of ALMA and VLA data plus images from James Webb and the VLT.

The three-dimensional picture was central because distinguishing formation routes required direct measurement of the three-dimensional motions of the bodies, a technical challenge described as exceptionally difficult, that account said. The Nature Astronomy articles listing likewise describes the study as revealing three-dimensional orbital motion.

A stretched orbit and tilted disks

Instead of a neat circular path, the two stars are moving on a highly elongated orbit. The University of Virginia account says the stars themselves were following a highly elongated orbit, while the Nature Astronomy listing describes orbits that are highly eccentric and close to parabolic.

The disks feeding the stars are sharply out of alignment. The University of Virginia account says the disks surrounding the two stars were strongly misaligned. The Chilean accounts describe marked misalignment between the respective disks of gas and dust.

That combination — stretched motion plus tilted disks — became the key evidence that the pair did not begin as a companion formed inside one shared structure. Together, those observations provided evidence that the stars had not formed as a pair, the University of Virginia account said.

Why an earlier idea was set aside

In 2019, observations had suggested that the stars might have formed through disk fragmentation, in which a portion of the disk surrounding a young star becomes unstable and forms a companion, according to the University of Virginia account.

Continued observations changed that picture. The later finding represented a significant change from the team’s earlier understanding of the system, that account said.

If the pair had come from the same disk, the two stars would not be so out of sync with each other, the interpretation reported in popular summaries of the work holds. The formal evidence cited by the University of Virginia is the misalignment of the disks and the highly elongated orbit.

The core-merger explanation

The evidence points to what astronomers call a “core merger,” a scenario in which two independently formed stars are pulled into orbit around one another by immense gravity, that University of Virginia account said. In that picture, the two stars appear to have formed separately before gravity brought them together.

The Chilean account describes the same proposed route as independently formed stars later undergoing a near-parabolic encounter and becoming gravitationally coupled. The paper title itself refers to assembly through a core-merger parabolic encounter.

Under that interpretation, the two protostars originated separately in their own pockets of gas and dust before a chance encounter dragged them together while both remained embedded in disks. The University of Virginia phrasing is that the stars formed independently and recently came together through a close gravitational encounter.

Closest pass came about 60 years ago

The researchers estimate that the stars made their closest approach only about 60 years ago, the University of Virginia account said. On a cosmic timescale, that interval is described as barely an instant.

The lives of stars unfold over millions of years, making changes in their life cycles difficult to observe directly, that account noted. Against that background, a meeting about 60 years ago is extremely recent.

The encounter did not destroy the surrounding structures. Popular summaries of the study note that both compact star-forming disks survived, preserving rotation structure, but the University-verified core facts are the 60-year estimate, the elongated orbit and the misaligned disks.

How chance meetings can bind giants

Tan said the basic mechanism is straightforward: “In dense regions, of course, stars are orbiting around and in principle, they could come together by chance.”

The more massive the stars, he explained, the stronger their gravitational pull and the greater the possibility that they will interact with nearby objects, according to the University of Virginia account.

That description fits the core-merger picture of two unbound cores later passing close enough for gravity to couple their motion. The Nature Astronomy listing characterizes the motion as close to parabolic, the type of path expected for such a grazing capture.

Why massive stars matter for planets and life’s elements

Massive stars are defined in the University of Virginia account as those more than about eight times the mass of the sun. Such stars can eventually explode as supernovae, producing and dispersing many of the elements found throughout the universe that support life and the formation of new planets.

They also frequently exist in binary or multiple-star systems, where interactions between the stars can dramatically alter their evolutionary path, that account said. At least 90% of massive stars are estimated to exist in binary or multiple systems, although the formation routes of those systems are not clear, according to the Radio Nuevo Mundo account quoting Guido Garay.

Those later interactions can extend to compact remnants. Massive binary systems can eventually produce neutron stars or black holes, and binary black holes can generate powerful gravitational waves that reverberate throughout the universe, the University of Virginia account said.

What the result means for models

For Tan, the value of a discovery of this kind lies in having an observation that can better test the theories and simulations astronomers use to explain how massive stars form, that University of Virginia account said.

He added: “These new observations offer guidelines our simulations and theoretical models will have to match.” He also said: “It gives us a powerful constraint that will help us understand how that process begins.”

The Virginia Initiative on Cosmic Origins, or VICO, which Tan helped to establish in 2018, supported part of the environment behind the work. It is described as an interdisciplinary program that has supported undergraduate and graduate students and postdoctoral researchers, with several VICO researchers participating in the study and the team’s broader survey of massive stars.

What remains to be watched

The observations establish assembly in progress, not a finished and settled binary. The eight-year baseline from 2016 to 2024 allowed the relative-motion parameters to be measured, but continued monitoring would extend the arc that defines the eccentric path.

The multi-wavelength method itself is part of the message. ALMA tracked positions and movements at high resolution, while JWST, the VLA, the VLT and other instruments added the surrounding material, outflows and disk orientations needed to distinguish separate birth from birth in one disk, according to the two university accounts.

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