A star’s mass is the single most consequential property of its life: it sets the luminosity, the temperature, the lifespan, the kind of nucleosynthesis it performs, and the architecture of any planetary system that may form around it. For stars on the main sequence, mass can be inferred relatively well from spectral and photometric observables. For young stars still embedded in their natal clouds — pre-main-sequence systems shrouded in dust, with their light reprocessed and reddened — that fundamental measurement has historically depended on theoretical models. And until those models can be tested against direct, dynamical determinations of mass, a quiet circularity persists at the foundation of stellar astrophysics.

A new study, led by Sergio A. Dzib at the Max Planck Institute for Radio Astronomy and co-led by postdoctoral Orquídeas fellow Jazmín Ordoñez-Toro at the Astronomical Observatory of the Universidad de Nariño in Colombia, addresses precisely that gap. The DYNAMO-VLBA program, published in Astronomy & Astrophysics, uses the U.S. National Science Foundation Very Long Baseline Array to measure the dynamical masses of young binary stars in the Orion star-forming complex with a precision that, until recently, was unattainable for embedded systems.

Why Orion, and why radio

Orion is the closest large-scale stellar nursery, a textbook laboratory for understanding how stars and planetary systems take shape. Its proximity, however, does not exempt it from the universal challenge of star formation regions: the same dense gas and dust that builds the next generation of stars also blocks visible and infrared light from escaping the cradle. Most pre-main-sequence systems in Orion are therefore difficult or impossible to study at the wavelengths at which they would be brightest in unobscured environments.

Radio observations at 5 GHz cut through this veil. At centimetre wavelengths, dust becomes effectively transparent, and the synchrotron and gyrosynchrotron emission produced by magnetically active young stars provides compact, point-like sources whose positions can be determined with extraordinary accuracy. The VLBA, a continent-spanning array of ten 25-metre antennas reaching from Hawaii to the U.S. Virgin Islands, achieves angular resolutions on the order of sub-milliarcsecond — fine enough, in the words of NRAO, to resolve motions on the sky smaller than the apparent width of a human hair viewed from thousands of kilometres away.

How the masses are read

The technique is, in principle, classical celestial mechanics. Two stars in a binary system orbit a common centre of mass; the geometry of their orbits, traced over many epochs, encodes their individual masses through Kepler’s third law generalised to two-body systems. What is novel is the precision. By repeating VLBA observations over months and years, the DYNAMO-VLBA team reconstructs the stars’ apparent positions on the sky to milliarcsecond accuracy and fits orbital solutions that yield individual stellar masses without any assumptions about the stars’ evolutionary state. No models of pre-main-sequence tracks, no calibrations of bolometric luminosity, no extinction corrections that propagate through to mass estimates — just the gravitational dance, observed.

This is the cleanest possible measurement of stellar mass: dynamical, model-independent, and applicable to systems still embedded in the dust where most of the action of stellar formation actually takes place.

Three results worth dwelling on

The DYNAMO-VLBA results are not a triumphalist confirmation of theory. They are, more interestingly, a mixed verdict.

First, when the directly measured masses are compared with standard pre-main-sequence evolutionary tracks, some systems agree well — but at least one shows a clear mismatch, suggesting that current models still need refinement in regions of the mass-age plane that are observationally poorly constrained. This is exactly the kind of empirical pressure that drives theoretical progress, and it underscores why dynamical mass measurements in young systems remain so valuable.

Second, the high-resolution observations uncovered previously hidden close companions in some of the targets. This is a recurring lesson in modern stellar astrophysics: a substantial fraction of what looks like a single young star at conventional resolutions turns out to be a multiple system once observed with sufficient angular precision. Each newly detected companion changes the inferred mass and dynamical state of the system, and by extension, every conclusion about its formation pathway and the planetary architectures it might support.

Third, the radio emission itself encodes magnetic activity, and the team finds evidence that strong magnetic activity persists in relatively massive young stars — a regime where some models would predict that radiative envelopes suppress the convective dynamo responsible for non-thermal radio emission in lower-mass T Tauri stars. The persistence of activity at higher masses is a clue that the magnetic transition between fully convective and radiative young stars is more gradual or more complex than simple stellar models suggest.

Why this matters beyond Orion

Orion is not merely a special case. It is the nearest fully populated example of the kind of clustered star formation in which most stars in the Galaxy were born, including, very likely, the Sun. Calibrating the mass scale of young stars in Orion therefore propagates outward: it improves the reliability of mass estimates for embedded young stars throughout the Milky Way, refines the initial mass function in star-forming regions, and ultimately tightens the connection between observed stellar populations and the physical processes that produced them.

For exoplanet science, the implication is equally direct. The mass of a host star sets the dynamics, formation timescale, and migration history of any planetary system around it. A young-star mass scale that is off by even ten or twenty percent at the relevant masses translates into systematic errors in inferred planet masses, orbital periods, and habitable-zone boundaries. DYNAMO-VLBA’s precision measurements help anchor that chain of inferences at its origin.

What lies ahead

The DYNAMO-VLBA team intends to extend its programme to additional young systems and to push the technique to lower-mass regimes where evolutionary models are even more uncertain. As Ordoñez-Toro emphasises in the project’s framing, the accurate mass measurements now turn Orion into a precision laboratory for testing how young stars form and evolve — vastly expanding our understanding of how stellar neighbourhoods like our own are built.

The methodology is also a quiet vindication of long-baseline radio interferometry as a tool for fundamental stellar astrophysics. Decades of investment in the VLBA, and in the geodetic and astrometric calibration techniques that allow milliarcsecond positional fidelity, are paying scientific dividends in regimes that optical and infrared observatories cannot reach. With the next generation of radio facilities — the ngVLA in particular — the kind of measurements DYNAMO-VLBA pioneers in Orion will be extensible to more distant and more deeply embedded star-forming regions, multiplying the laboratory.

The full study, «Dynamical masses of young stellar objects with the VLBA: DYNAMO-VLBA,» by Sergio A. Dzib et al., is published in Astronomy & Astrophysics (2026). DOI: 10.1051/0004-6361/202558171.

© 2026 SKYCR.ORG | Homer Dávila Gutiérrez, FRAS. All rights reserved. Total or partial reproduction prohibited without express authorization. Original source: National Radio Astronomy Observatory (NRAO).