Most Massive Neutron Star Ever Discovered Is at 'Tipping Point' of What Can Exist, Scientists Say

Scientists have discovered one of the most massive neutron stars ever, having identified an 18 mile-wide pulsar with a mass over two times that of the sun. The pulsar, named J0740+6620, could provide an insight into the "tipping point" of neutron stars—where these extremely dense objects collapse and become black holes.

Neutron stars are densest objects in the known universe, with a huge amount of matter packed into a sphere the size of a city. A sugar cube-sized piece neutron star would weigh around 100 million tons on Earth. These objects are created from the collapsed cores of giant stars that have died as supernovae. Generally, neutron stars contain between 1.3 and two times the mass of the sun, all packed into a space between 12 and 18 miles wide.

Over 2,000 neutron stars have been discovered and most belong to a subclass of fast-spinning, young objects known as pulsars. J0740+6620 is a millisecond pulsar (meaning it rotates extremely fast) that sits 4,600 lightyears from Earth and was detected using data from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) and the Green Bank Telescope. Combining observations of the pulsar from these two telescopes allowed scientists to calculate the mass of the neutron star.

Findings, published in Nature Astronomy, show that J0740+6620 has a solar mass of 2.14. One solar mass is equivalent to the mass of the sun. Millisecond pulsars like this one are extremely useful to scientists as they can be used to test fundamental physics. The speed at which they rotate means researchers can test out a range of physical phenomena, the team explained. It also means they are able to precisely measure the neutron star's mass.

"Neutron stars are as mysterious as they are fascinating," study author Thankful Cromartie, from the University of Virginia, said in a statement. "These city-sized objects are essentially ginormous atomic nuclei. They are so massive that their interiors take on weird properties. Finding the maximum mass that physics and nature will allow can teach us a great deal about this otherwise inaccessible realm in astrophysics."

Previously, scientists have identified neutron stars that appear to have a mass larger than J0740+6620. In 2018, researchers announced the discovery of PSR J2215+5135, which appeared to be 2.3 solar masses. Cromartie told Newsweek the mass of this neutron star was calculated in a different way and that a large amount of modeling was needed to work out its mass—modeling that is subject to more systematic errors.

"While the result is interesting, it is generally accepted that mass determinations through techniques such as pulsar timing are more reliable," she said, referring to the way the mass of J0740+6620 was worked out.

The researchers say 2.14 solar masses is approaching the level of how massive and compact an object can get before it breaks down and turns into a black hole—a region of space where the gravitational pull is so strong nothing can escape, not even light.

"Neutron stars have this tipping point where their interior densities get so extreme that the force of gravity overwhelms even the ability of neutrons to resist further collapse," study co-author Scott Ransom, an astronomer at NRAO, said in a statement. "Each 'most massive' neutron star we find brings us closer to identifying that tipping point and helping us to understand the physics of matter at these mind-boggling densities."

Cromartie said J0740+6620 will not become a black hole: "It would have to accrete a lot more mass or merge with another neutron star, and it's far enough along in its evolution with a binary white dwarf companion that that won't happen," she said, adding they are now planning to observe the neutron star regularly to refine the measurement of its mass. They also hope to try to determine its radius and further constrain its properties.

"We will be able to measure its mass—and therefore probe its interior physics—with even more precision, and its long-term observation will form part of a collection of the most precisely timed pulsars; we will soon be able to use these to detect the aforementioned gravitational waves from supermassive black holes in the early Universe," study co-author Robert Ferdman, from the U.K.'s University of East Anglia, told Newsweek.

There are many unanswered questions about neutron stars. It is thought the outer layers freeze to form a crust. Below this, there is thought to be a liquid core, with protons and neutrons form shapes known as "nuclear pasta." This is thought to be the strongest material in the universe.

"We don't know what they're made of and one really important question is, 'how massive can you make one of these stars?' It has implications for very exotic material that we simply can't create in a laboratory on Earth," Maura McLaughlin, another study author, said in a statement.

This article has been updated to include comments from Robert Ferdman.