A star’s mass determines its entire life story, from how it shines to how it dies. For young stars shrouded in dust, getting an accurate mass has long been difficult…but new radio measurements are beginning to change that. Astronomers are helping unravel the mass mystery of young stars in the Orion star-forming complex by measuring their masses with unprecedented precision.
Lightweight Sun-like stars burn steadily for 10 billion years, while massive ones blaze briefly before exploding as supernovae in mere millions of years. Mass also determines what heavy elements they forge, such as carbon, oxygen, and iron, which form the building blocks of planets and life. In addition, it influences what types of planets can form around them.
Using the U.S. National Science Foundation Very Long Baseline Array (NSF VLBA), a network of radio telescopes spread across the United States that work together as one giant instrument, the team tracked the orbital motions of a sample of young binary star systems in Orion. Binary stars are pairs that orbit a shared center of mass, like dance partners spinning each other around. By watching these “dances” with extraordinary precision at radio wavelengths, researchers were able to calculate the stars’ true masses without relying on theoretical models. As lead researcher Dr. Sergio Abraham Dzib Quijano, from the Max Planck Institute for Radio Astronomy explains, “Stellar mass is the most fundamental property of a star, yet it is notoriously difficult to measure for young, embedded systems.”
Young stars in Orion are shrouded in dense clouds of gas and dust, blocking visible and even infrared light from reaching most telescopes. The NSF VLBA overcomes this by observing at radio wavelengths (5 GHz), where dust is transparent and the array’s extreme resolution (sub-milliarcsecond) resolves tight binaries that blur together at other wavelengths.
The NSF VLBA can also detect motions on the sky smaller than the width of a human hair seen from thousands of kilometers away, showcasing the remarkable technical achievement behind these mass measurements. In practice, this means measuring tiny shifts in a star’s apparent position on the sky over months and years, using repeated observations to trace out its path. Each NSF VLBA radio telescope in the array records the incoming radio waves with exquisite timing. By combining the signals from antennas spread across the country from Hawaii to the Virgin Islands, astronomers can pinpoint a star’s position with milliarcsecond accuracy, far finer than what is possible with a single dish. By comparing how that position changes from epoch to epoch, they can see the subtle orbital motion caused by the gravity of a companion star and use that motion to infer the mass of each star in the system.
In the systems where the measured masses could be compared with standard models of young-star evolution, the results were mixed: some were reproduced well, while at least one showed a clear mismatch, suggesting that the models may still need refinement. The observations also uncovered previously hidden close companions and evidence that strong magnetic activity can persist in relatively massive young stars.
Young stars in Orion are the building blocks of future planetary systems, much like our own Solar System. “These accurate mass measurements now turn Orion into a precision laboratory for testing how young stars form and evolve,” says Dr. Jazmin Ordonez-Toro, postdoctoral Orquídeas fellow at the Astronomical Observatory at the University of Nariño, who co-led the study, “These measurements vastly expand our understanding of how stellar neighborhoods like our own are built.”
