Our universe is full of large structures many times the size of our planet, such as a star that only pulsates on one side and a galaxy that looks like a sombrero. However, ultra-dense neutron stars, the collapsed cores of giant stars after a supernova explosion, generally have more modest sizes. A new study by an international research team has narrowed down the radius range of typical neutron stars, which has provided astronomers with the most precise measurements to date.

“We find that the typical neutron star, which is about 1.4 times as heavy as our Sun has a radius of about 11 kilometers (6.8 miles),” Badri Krishnan, who led the research team at the Albert Einstein Institute (AEI), Hannover, said in a statement. “Our results limit the radius to likely be somewhere between 10.4 and 11.9 kilometers (6.5 to 7.4 miles). This is a factor of two more stringent than previous results.”

Published in Nature Astronomy, the team’s work used a combination of knowledge from a general first-principles description of the unknown behavior of neutron star matter and the first-ever observations of a binary neutron star merger, GW170817.

“Binary neutron star mergers are a gold mine of information!” Collin Capano, a researcher at the AEI Hannover and lead author of the study, exclaimed. “Neutron stars contain the densest matter in the observable universe... By measuring these objects’ properties, we learn about the fundamental physics that governs matter at the sub-atomic level.”

“It’s a bit mind boggling,” Capano added. “GW170817 was caused by the collision of two city-sized objects 120 million years ago, when dinosaurs were walking around here on Earth. This happened in a galaxy a billion trillion kilometers away. From that, we have gained insight into sub-atomic physics.”

The GW170817 merger was observed in gravitational waves and throughout the entire electromagnetic spectrum in August 2017. By understanding the underlying nuclear physics of the astrophysical event, the researchers were able to determine physical properties, like the mass and radii, of the neutron stars.

The calculated constraints yielded further information for the team about the fate of neutron stars merging with a black hole in a binary system. In most cases, they predict that the neutron star is likely to be swallowed whole by the black hole as opposed to being torn apart. This could have implications for future observations of such events, as they may only be seen as gravitational-wave sources and become invisible in the electromagnetic spectrum.

“These results are exciting, not just because we have been able to vastly improve neutron star radii measurements, but because it gives us a window into the ultimate fate of neutron stars in merging binaries,” Stephanie Brown, co-author of the publication and a PhD student at the AEI Hannover, explained.