A Typical Neutron Star Is Only 22 Kilometres Wide, According To New Ultra-Precise Measurement

A Typical Neutron Star Is Only 22 Kilometres Wide, According To New Ultra-Precise Measurement
A typical neutron star is no wider than a city, as this model shows. (Image: NASA’s Goddard Space Flight Centre)

A typical neutron star measures 22 kilometres wide, according to new research. It’s the most accurate measurement yet of these highly compact, super-dense objects.

If black holes are the most extreme phenomena in the universe, then neutron stars have to be a close second (unless quark stars exist, which has yet to be confirmed). Formed in the wake of a supernova explosion (when a giant star collapses in on itself), neutron stars don’t have the required mass to become a black hole, but they’re freakishly dense nonetheless.

Typical neutron stars contain as much mass as half a million Earths, yet they’re no larger than a mid-sized city. Astronomers use solar masses to describe the weight of neutron stars, in which one solar mass is equal to the weight of our Sun. A standard neutron star mass is typically given as 1.4 solar masses (the minimum weight required for an object to become a neutron star), but recent discoveries have scaled this range up to to 2.3 solar masses. Any heavier, and you start to get into black hole country.

With all this stuff packed into such a small space, the overall size of neutron stars become tightly constrained; there’s not much wriggle room, so to speak, inside these ultra-compact spheres. As a consequence, neutron stars are exceptionally round.

That being said, the precise radius of a typical neutron star of 1.4 solar masses remains in doubt, with estimates varying from 10 to 14 kilometres. This variability is a problem, Thankful Cromartie, a PhD student from the Department of Astronomy at the University of Virginia, told Gizmodo.

“Well constrained measurements of neutron star radii are really important in our effort to understand the way matter behaves at extremely high densities,” said Cromartie, who wasn’t involved in the new research.

Using a new technique, astrophysicists from the Albert Einstein Institute (AEI) Hannover at the Max Planck Institute for Gravitational Physics have provided a new estimate for the radius of 1.4 solar mass neutron stars: 11 kilometres, with a plus-minus range of -0.6 km to +0.9 km. With these error bars attached, that’s an estimated diameter between 20.8 and 23.8 kilometres. The new paper was published this week in Nature Astronomy.

That’s an exceptionally tight estimate, especially when you consider where the data for this measurement came from: a neutron star merger, called GW170817. Astronomers from the LIGO and Virgo collaborations observed the collision of these two neutron stars on August 17, 2017, from a distance of 130 million light-years. Not too shabby.

The new measurement is roughly in the same ballpark as previous estimates, but the error bars are now considerably narrower. It’s “a factor of two improvement in the uncertainty on the neutron star radius over previous estimates,” Capano told Gizmodo.

To reach their new size estimate, the AEI researchers, led by astrophysicist Collin Capano, looked at the entire electromagnetic spectrum and gravitational waves produced by GW170817 and applied this data to equations derived from particle physics. This allowed them to determine various physical properties, such as radius and mass.

“In synthesising what we learned from observations of GW170817 with a more thorough theoretical description of the equation of state [an equation describing the state of matter given a set of physical conditions], this work presents an excitingly stringent limit on typical neutron star radii,” Cromartie told Gizmodo.

“This looks like an interesting result: new constraints on the radius of a canonical neutron star from a combination of nuclear physics with observations of gravitational and electromagnetic waves from the binary neutron star merger GW170817,” wrote Manu Linares, an astrophysicist from Polytechnic University of Catalonia in Spain who wasn’t involved in the new research, in an email to Gizmodo.

Indeed, the incredible thing about neutron stars is that they’re basically gigantic particle physics experiments floating in space, and they tend to spew out a tremendous amount of useful information. The challenge for scientists is to figure out what’s actually happening inside them.

“What’s remarkable about neutron stars is that they they are so dense and compact, that you can think of them as a single nuclear atom scaled up to the size of a city,” Capano told Gizmodo. “This means that subatomic physics are manifested in the macroscopic properties of the star, such as the star’s mass, radius, and how easily it is deformed when exposed to an external gravitational field.”

In this case, the researchers were able to predict how subatomic particles were interacting at the tremendous densities presumed to exist inside neutron stars.

“GW170817 was caused by the collision of two objects with radii about the size of Manhattan and masses about one and half times that of the Sun,” said Capano. “This happened when dinosaurs were walking around on Earth, in a galaxy a billion trillion kilometres away. From that, we’ve gained insight into subatomic physics, on length scales that are less than a trillionth of a millimetre. It’s mind boggling and a testament to the incredible sensitivity that the thousands of scientists who planned, built, and maintain our gravitational-wave detectors and telescopes have achieved.”

Linares said he liked the new paper but that GW 170817 is the only event for which this new method has been applied, which means “the systematic uncertainties of the method are still unclear,” he said.

“The radius constraints might become less stringent with different model assumptions, with new observations of binary neutron star mergers or when accommodating super-massive neutron stars,” Linares told Gizmodo, reminding us of those colossal 2.3 solar mass neutron stars. Regardless, the 11 kilometre estimate aligns with previous measurements, he said, and the study “paves the road to a near-future where we expect many more detections of binary neutron star mergers.”

The paper also presents some predictions for astronomers, in terms of what they should expect not to observe.

Specifically, the authors say mergers involving black holes and neutron stars, in which neutron stars are torn apart, will rarely be seen by astronomers. More often, neutron stars will be swallowed up whole. For astronomers, this means they shouldn’t expect to detect many of these events in the electromagnetic spectrum but instead as gravitational-wave sources.

“From the perspective of an observer, it’s disappointing to hear that black hole-neutron star mergers will rarely be glimpsed in the electromagnetic spectrum,” said Cromartie.

Ah, well. Sometimes science gotta be that way.