Most massive stars in the universe are not loners. Decades of surveys of massive main-sequence stars have shown that the overwhelming majority are found in binary or higher-order multiple systems, and that these partnerships profoundly shape how the stars live and die. Binaries strip each other’s outer layers, exchange mass, spin each other up and, in the most violent cases, merge — producing some of the most energetic events in astronomy, including gravitational-wave sources detected by observatories such as LIGO and Virgo. Yet despite their ubiquity, the question of how massive binaries form in the first place has remained one of the most stubborn puzzles in star formation research.

Three main pathways have been proposed. In the disk fragmentation scenario, a single massive protostar surrounded by a massive accretion disk becomes gravitationally unstable, and the disk breaks apart to form a companion. In core fragmentation, a single dense cloud core splits early on into two separate condensations that each collapse into a star while still bound together. The third possibility, stellar capture, involves two stars formed independently in the same crowded region that happen to pass close enough for their mutual gravity to bind them. Each scenario leaves a distinct fingerprint in the geometry and dynamics of the resulting binary, particularly in the eccentricity of the orbit and the alignment between the orbital plane and the disks of gas feeding the young stars.

The trouble has always been observation. Measuring the three-dimensional orbit of a binary system requires detecting the tiny apparent shift of one component relative to the other across multiple observing epochs — the orbital proper motion. For massive protostars, which are deeply embedded in dusty cocoons that block visible light and which orbit slowly because of their large masses and wide separations, this measurement demands exquisite angular resolution, sensitivity and patience. As a result, direct orbital reconstructions of massive binaries in their infancy have been essentially nonexistent, leaving theorists to argue over plausible scenarios without decisive observational constraints.

The team behind the new work, led by Yao Wang and Yichen Zhang of Shanghai Jiao Tong University together with an international collaboration, overcame this obstacle by combining years of high-resolution observations from several of the world’s most powerful telescopes. They used the Atacama Large Millimeter/submillimeter Array (ALMA) and the Jansky Very Large Array (JVLA) to image the system at wavelengths ranging from submillimetre to centimetre across multiple epochs, allowing them to track the relative positions of the two protostellar components with extraordinary precision. Complementary infrared imaging from the James Webb Space Telescope and the European Southern Observatory’s Very Large Telescope provided an additional view of the embedded pair and its surroundings, piercing the dust that hides the nursery.

The multi-epoch data revealed the unmistakable signature of orbital motion: the two components, separated by roughly 200 astronomical units — about 200 times the Earth–Sun distance — are visibly moving around each other. That detection opened the door to a full three-dimensional orbital reconstruction. But the team did not stop at astrometry. They also modeled the multi-wavelength continuum emission from each component to estimate the masses and the properties of the circumstellar disks, analyzed the kinematics of hydrogen recombination lines — spectral lines emitted by ionized gas close to each protostar that trace Keplerian motion — and examined radio jet observations that helped pin down the orientation of the disks relative to the orbit.

When all of these pieces were fitted together using Bayesian orbital modeling, a striking picture emerged. The preferred orbital solutions are highly eccentric, with the pair’s current trajectory lying close to a parabola — the razor’s edge between a bound elliptical orbit and an unbound hyperbolic escape. In other words, the two protostars appear to have been caught at the delicate moment when their first gravitational encounter is only barely holding them together. Even more tellingly, the circumstellar disks around both components are strongly misaligned with the orbital plane, tilted far from the flat configuration expected if the two stars had formed together from a single rotating structure.

Both properties — the near-parabolic eccentricity and the severe disk–orbit misalignment — are difficult to produce in the standard disk-fragmentation or core-fragmentation pictures, which tend to yield binaries on more circular orbits with disks broadly aligned with the orbital plane. Instead, they fall out naturally from the core-merger scenario. In this picture, the two protostars began their lives in separate, initially unbound cloud cores within the same star-forming region. As the cores drifted through the dense cluster environment, they converged on a near-parabolic encounter. During this close passage, dynamical friction and gravitational torques from their gas envelopes dissipated enough orbital energy to leave them barely bound, while the encounter itself scrambled the orientations of their disks, tilting them out of any common plane.

The idea that stellar encounters in young clusters can sculpt binary architectures is not new — astronomers have long studied how flybys perturb protoplanetary disks and how dynamics in cluster cores can harden or soften binaries. What makes this result remarkable is that it offers direct, quantitative evidence that such a process has operated on a massive protobinary caught in the act. IRAS 07299−1651, with its pair of massive twin disks and its razor-thin orbital binding, looks less like a binary that was born together and more like one that was assembled after the fact, stitched together by gravity during a single fateful pass. It is a snapshot of binary assembly in real time, preserved in the geometry of the system for observers to decode.

The implications ripple outward across stellar astrophysics. If core-merger encounters represent an important channel for forming eccentric massive binaries, then the population of massive binaries on eccentric orbits — including many of those that will eventually interact, exchange mass or merge — may owe their existence not to shared birth but to later dynamical matchmaking. This affects predictions for the rates of massive binary interactions, the formation of gravitational-wave progenitors and the statistics of stripped stars and X-ray binaries. More broadly, the study demonstrates that with multi-epoch, multi-wavelength observations and modern orbital fitting techniques, astronomers can now reconstruct the three-dimensional choreography of forming massive star systems, turning what was once pure theory into a measurable, evolving picture. The team has also released the reduced ALMA and JVLA continuum images and hydrogen recombination line data cubes through the Zenodo repository, ensuring that other researchers can scrutinize and build upon this rare glimpse of two young giants caught mid-encounter, their futures still hanging on a gravitational knife’s edge.

Subject of Research: The three-dimensional orbital architecture and core-merger formation pathway of the massive protobinary IRAS 07299−1651.

Article Title: An eccentric massive protobinary assembled via a core-merger parabolic encounter

Article References: Wang, Y., Zhang, Y., Fedriani, R., Tanaka, K. E. I., Rosero, V., Yang, K., Andersen, M., Beltrán, M. T., Bonfand, M., Cheng, Y., De Buizer, J. M., Di, Y., Garay, G., Gorai, P., Li, Z.-Y., Yang, Y.-L., & Tan, J. C. (2026). An eccentric massive protobinary assembled via a core-merger parabolic encounter. Nature Astronomy. https://doi.org/10.1038/s41550-026-02953-z

Image Credits: AI Generated

DOI: 10.1038/s41550-026-02953-z

Keywords: massive star formation, protobinary, orbital eccentricity, IRAS 07299−1651, ALMA, JVLA, core merger, disk misalignment, hydrogen recombination lines, protostellar disks, stellar dynamics, Nature Astronomy

The observational foundation of this ecosystem view rests on decades of precision astrometry. Near-infrared monitoring of the innermost parsec, most notably by the GRAVITY instrument on the European Southern Observatory’s Very Large Telescope Interferometer, has tracked stars on bound orbits around the compact radio source Sagittarius A star, delivering the most convincing dynamical evidence that this object is indeed a supermassive black hole as described by general relativity. The S-cluster of stars, dominated by the luminous star S2 on its sixteen-year elliptical orbit, has been used to test relativistic effects including gravitational redshift and the Schwarzschild precession, while also pinning down the distance to the Galactic Centre to within a few percent. These measurements transform the central parsec into a precision laboratory where the interplay between stellar dynamics, black hole mass and relativistic gravity can be examined against exact predictions rather than qualitative expectations.

Yet the S-cluster presents one of the field’s most persistent puzzles: the so-called paradox of youth. The central parsec contains massive, luminous B-type and Wolf-Rayet stars whose lifetimes are measured in millions of years, far too short for them to have migrated from their presumed birth sites in the surrounding disk through standard dynamical friction. At the Brno symposium, participants revisited candidate solutions, including in-situ star formation within the dense accretion disk fragments of the past, tidal disruption of binary stars passing close to the black hole which leaves one captured star in a tight orbit, and exchange interactions in which the black hole swaps into a hard massive binary. Each mechanism leaves a different fingerprint in the distribution of orbital eccentricities and inclinations, and the latest modeling of the S-cluster’s phase-space structure suggests that no single channel explains every observed orbit, hinting at a layered formation history stretching back many millions of years.

Beyond the S-cluster lies the nuclear star cluster, a dense concentration of roughly ten million solar masses packed within about ten parsecs of the black hole. New analyses presented at the meeting explored how this cluster’s complex structure, with young stars preferentially rotating in a disk-like configuration at larger radii and older populations dominating closer in, encodes the history of gas inflow episodes triggered by the galactic bar. The nuclear cluster also serves as a gravitational anchor for less massive objects, and speakers examined the fate of stellar-mass black holes, neutron stars and white dwarfs that segregate toward the centre through mass segregation. Their predicted number densities, potentially tens of thousands of stellar remnants within the central parsec, carry consequences for gravitational-wave predictions and for the rates of tidal disruption events, in which stars venturing too close to Sagittarius A star are torn apart and briefly outshine much of the surrounding galaxy.

Gas dynamics formed the second pillar of the symposium. The circumnuclear disk, a ring of dense molecular material at radii of a few parsecs, regulates the flow of gas toward the central engine and episodically feeds, or starves, both accretion onto the black hole and star formation in the central molecular zone. Recent work on the ionized and neutral gas kinematics within the central parsec, including the mini-spiral structures that thread hot plasma through the cavity inside the circumnuclear disk, illustrated how inflow is inefficient and turbulent, with only a small fraction of the supplied material ever reaching the accretion flow. Sagittarius A star today accretes at a rate roughly a billion times below the Eddington limit, producing the faint, radiatively inefficient glow observed by near-infrared polarimetry and by the Event Horizon Telescope, which resolved the ring-like shadow of the black hole’s photon emission region in 2022.

That quiescent present contrasts dramatically with a far more active past, and the evidence is written in the largest structures of the Galactic Centre region. The Fermi bubbles, two gamma-ray-emitting lobes extending tens of thousands of light-years above and below the Galactic plane, together with the softer X-ray counterpart bubbles detected by the eROSITA instrument, testify to an energetic event several million years ago, plausibly a phase of rapid accretion onto Sagittarius A star or an intense nuclear starburst. Meeting discussions emphasized how the spectral gradients across these bubbles constrain the timing, energy budget and particle acceleration mechanisms of the outburst, with estimates of total injected energy in the range of tens of millions of supernova equivalents, sufficient to reshape the thermal history of the entire Galactic halo.

On smaller scales, X-ray observations of light echoes provide a time-lapse record of more recent activity. Reflections of X-ray photons from past flares, scattered by dense molecular clouds such as the famous MC2 complex near the Sagittarius B region, have been mapped as moving fronts of fluorescing iron, indicating that the central engine brightened substantially within the last few centuries. Modeling of these reverberation signals, combined with polarization measurements that can distinguish forward-scattered from back-scattered radiation, suggests flare luminosities that transiently approached a significant fraction of the Eddington limit, a striking reversal of the present-day faintness. Speakers noted that continuous monitoring of these echoes offers an essentially forensic technique for reconstructing the accretion history of a low-luminosity galactic nucleus, a technique now being extended to nearby external galaxies where analogous echoes betray dormant black holes in action.

The Galactic Centre also functions as the Rosetta stone for galactic nuclei everywhere. Because the Milky Way’s central black hole is roughly ten thousand times less massive than those powering the most luminous quasars, phenomena observed at different scales and timescales in active galaxies can be scaled and tested locally. Symposium sessions drew explicit connections between the scaling of accretion flows, the physics of relativistic jets, the coupling between black hole feedback and star formation, and the secular evolution of gas driven inward by galactic bars. Comparative studies of nearby low-luminosity nuclei, combined with the Milky Way’s uniquely resolvable environment, allow astronomers to trace how a galactic nucleus transitions between quiescence and activity, and how feedback from the central engine regulates the fuel supply in a self-limiting loop that shapes the growth of galaxies over cosmic time.

The meeting also spotlighted the computational and observational infrastructure driving the field forward. General relativistic magnetohydrodynamic simulations of the accretion flow around Sagittarius A star, radiative transfer modeling of its polarization variability, and N-body models of the nuclear star cluster’s formation now operate at resolutions and statistical sophistication that can be directly confronted with GRAVITY, the Event Horizon Telescope, ALMA, Chandra, XRISM and the upcoming ELT datasets. High-cadence monitoring campaigns of the black hole’s infrared and submillimeter flares are converging on a picture in which magnetic reconnection and orbiting plasma instabilities generate the observed variability, linking microphysical processes near the horizon to macroscopic structures visible across the electromagnetic spectrum. As these instruments mature over the coming decade, the Galactic Centre ecosystem that the Brno symposium so comprehensively surveyed will be tested not as a collection of separate puzzles, but as a single evolutionary system whose past activity, present faintness and future episodes of renewed feeding can be read directly from the stars, gas and echoes it leaves behind.

Subject of Research: The Milky Way Galactic Centre as a laboratory for studying supermassive black holes, stellar dynamics and multiphase gas in galactic nuclei.

Article Title: Traversing the Galactic Centre in space and time

Article References: Zajaček, M., Czerny, B., Mondek, M., Mitra, S., Labaj, M., Ondro, T., Janík, J., & Dušek, J. (2026). Traversing the Galactic Centre in space and time. Nature Astronomy. https://doi.org/10.1038/s41550-026-02958-8

Image Credits: AI Generated

DOI: 10.1038/s41550-026-02958-8

Keywords: Galactic Centre, Sagittarius A*, supermassive black hole, nuclear star cluster, GRAVITY, stellar dynamics, circumnuclear disk, Fermi bubbles, X-ray light echoes, IAU Symposium 405, black hole accretion, paradox of youth