Neutron stars are the stellar corpses left behind when a massive star goes supernova. They’re unimaginably dense: A tablespoonful of neutron star placed on Earth's surface would weigh roughly as much as Mount Everest (whereas a tablespoonful of the sun would weigh as little as about 5 pounds).

And while the mass range of neutron stars has been relatively well constrained over the years, it’s been harder to pin down precisely how wide they are. Most astronomers, however, think that mass is packed into a sphere about as big as a city.

Now, a new study has combined gravitational-wave measurements with other techniques to place the best constraints yet on their size. The estimate suggests that a typical neutron star is about 13.7 miles across. That size has interesting implications for what happens when they get too close to another of the cosmos’ most mysterious objects: black holes. The new size results indicate a black hole can swallow a neutron star whole in many circumstances — leaving behind little evidence that Earth-based astronomers can uncover with conventional telescopes.

How Neutron Stars Form

Massive stars explode when they exhaust their gases used for nuclear fusion. As a violent outburst of material erupts in all directions, what’s left behind condenses into a neutron star. If a star is massive enough, the remnant can further condense into a black hole.

But solitary stars like our sun are in the minority in our universe. Most stars exist in multiple systems. And when two large stars evolve side by side, these alien solar systems can end with two neutron stars, two black holes, or one of each. In recent years, astronomers have started detecting these systems thanks to the gravitational waves thrown out when they death-spiral into one another. That’s how astronomers recently made an extremely accurate measurement of a neutron star’s size.

In 2017, the Laser Interferometer Gravitational-wave Observatory (LIGO) in the U.S. and the Virgo detector in Italy picked up a gravitational-wave signal that implied two neutron stars had collided some 120 million light-years away. Soon after, traditional observatories started seeing the collision in electromagnetic wavelengths. Those detections carried unprecedented insights into the mass and spin of the objects.

Neutron Star Size

A team led by researchers at the Albert Einstein Institute (AEI) in Germany took those observations and then combined them with models of how subatomic particles behave in the extremely dense conditions inside neutron stars. While it’s impossible to recreate such conditions in labs on Earth, the physicists showed that they could use existing theory to extrapolate their calculations from the tiniest scales out to what’s happening in distant neutron stars.

Their results suggest that neutron stars must be between 13 and 15 miles across. And a typical neutron star should be about 13.7 miles wide. The estimates place tighter constraints on neutron star size than previous studies.

“Neutron stars contain the densest matter in the observable universe,” AEI researcher and study author Collin Capano said in a media release. “In fact, they are so dense and compact that you can think of the entire star as a single atomic nucleus, scaled up to the size of a city. By measuring these objects’ properties, we learn about the fundamental physics that governs matter at the subatomic level.”

Swallowed by a Black Hole

That diminutive diameter is small enough that a neutron star orbiting in tandem with a black hole could even be swallowed entirely when it gets too close. Astronomers have been eagerly watching for black hole-neutron star collisions. They expected these mergers would emit strong electromagnetic radiation — the kind of light visible by typical observatories back on Earth.

However, if the neutron star isn’t shredded when the two merge, then no light would be emitted that Earth-based telescopes could detect, according to the new study. At the same time, gravitational-wave detectors also likely wouldn’t be able to tell the difference between merging black holes and a mixed merger.

“We have shown that in almost all cases, the neutron star will not be torn apart by the black hole and rather swallowed whole,” Capano said. “Only when the black hole is very small or rapidly spinning can it disrupt the neutron star before swallowing it; and only then can we expect to see anything besides gravitational waves.”

Astronomers shouldn’t have to wait too long to find out if this idea is right. The world’s gravitational detectors will grow increasingly powerful in the coming years. If neutron star-black hole collisions prove rarer than expected, at least they’ll know why.