For decades, astronomers have known that neutron stars—ultra-dense remnants of massive stars—should be scattered across the Milky Way in staggering numbers. Yet, despite their abundance, most remain invisible to even the most powerful telescopes. Now, a new study published in Astronomy & Astrophysics suggests NASA’s upcoming Nancy Grace Roman Space Telescope could change that, offering the first large-scale glimpse of these elusive cosmic objects through a phenomenon called gravitational microlensing.
The study, led by Zofia Kaczmarek of Heidelberg University and involving researchers from Lawrence Livermore National Laboratory and the Georgia Institute of Technology, demonstrates how Roman’s unprecedented precision could detect dozens of isolated neutron stars—objects that are otherwise nearly impossible to observe without a companion star or extreme emission. Using detailed simulations of the Milky Way and Roman’s future observations, the team found that the telescope’s ability to measure both the brightening of background stars (photometry) and their tiny positional shifts (astrometry) could reveal the presence of these hidden stars.
Neutron stars are among the most extreme objects in the universe, packing the mass of the sun into a sphere just a few miles wide. They are born in supernova explosions and can race through the galaxy at hundreds of miles per second, but without the telltale beams of pulsars or X-ray emissions, they are effectively invisible. Roman’s microlensing technique could turn that invisibility into discovery, allowing scientists to directly measure the masses of isolated neutron stars for the first time—a feat nearly impossible with current technology.
The Hunt for the Invisible
When a neutron star drifts in front of a distant star, its immense gravity warps spacetime, bending and magnifying the background star’s light. This effect, called gravitational microlensing, causes the star to briefly brighten and shift position. While many telescopes can detect the brightening, Roman’s advanced instruments can also measure the tiny positional shift with extraordinary precision. Because neutron stars are so massive, their gravitational pull produces a larger astrometric signal than lighter objects, making them stand out even among the galaxy’s crowded stellar populations.
“What’s really cool about using microlensing is that you can get direct mass measurements,” says Peter McGill, a research scientist at Lawrence Livermore National Laboratory and co-author of the study. “Photometry tells us that something passed in front of the star, but it’s the amount the star’s position shifts that tells us how massive that object is. By measuring that tiny deflection on the sky, we can directly weigh something that is otherwise unseen.”
This capability could help astronomers answer fundamental questions about the nature of neutron stars, such as whether there is a true mass gap between neutron stars and black holes, and how rapid these remnants move through space after their violent births.
Why It Matters
Neutron stars are cosmic laboratories for studying extreme physics. They provide insights into the behavior of matter under conditions that cannot be replicated on Earth, and they play a crucial role in distributing heavy elements throughout the universe. However, current observations are limited to a handful of thousands of neutron stars, mostly detected as pulsars in binary systems. The vast majority—potentially tens of millions—remain hidden, their properties and distribution largely unknown.
“We’re seeing a small sample that’s not representative of the big picture,” Kaczmarek says. “Even a single mass measurement of an isolated neutron star would be incredibly powerful. If we found just one, it would already be incredibly stimulating to our research.”
A New Era for Microlensing
The study highlights an unexpected but powerful application of Roman’s capabilities. Originally designed to search for exoplanets using photometric microlensing, the telescope’s astrometric precision opens the door to entirely new discoveries. “This wasn’t part of the original plan,” McGill notes. “But it turns out Roman’s astrometric capability is really good at detecting neutron stars and black holes, so we can add a whole new kind of science to Roman’s surveys.”
The team plans to utilize Roman’s Galactic Bulge Time Domain Survey, which will monitor millions of stars simultaneously in vast images of the sky, taken at high frequency. Even in the first months after the telescope’s commissioning, researchers expect to start identifying promising microlensing events. If the predictions hold true, Roman could provide the first large sample of isolated neutron stars discovered through their gravity alone, revealing a hidden population that has remained out of reach until now.
Looking Ahead
The Nancy Grace Roman Space Telescope, managed at NASA’s Goddard Space Flight Center, is poised to transform our understanding of the universe’s hidden populations. With a primary mirror the same size as Hubble’s and a field of view 100 times larger, Roman will conduct surveys that cover thousands of square degrees of the sky, enabling discoveries that were previously unimaginable.
The telescope is expected to launch in the near future, with its science operations set to begin shortly thereafter. Scientists are eagerly awaiting the first data, which could unlock new insights into the nature of neutron stars, black holes, and the extreme environments in which they form.
For more information about the Nancy Grace Roman Space Telescope and its mission, visit the NASA Roman Space Telescope page.
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