When a giant star dies it turns into a neutron star or a black hole, depending on just how giant it is. There is still debate about the largest mass a neutron star can have without becoming a black hole. A new discovery, the most massive neutron star ever detected, may well be nudging up against that limit.
J0740+6620 is a millisecond pulsar, a type of neutron star that spins so rapidly the beam of radiation it emits returns every 2.8 milliseconds. It's like a lighthouse, but one the size of a city and turning so fast we see its beam every 350th of a second.
If J0740+6620 was voyaging through the cosmos solo that would be about all the information available. However, in February a white dwarf companion was announced. During its orbit, the white dwarf passes through the path of the beam on its way towards Earth, and the warping of spacetime it induces creates a delay in the signals reaching us. The extent of the delay allows us to measure the white dwarf’s mass, and from the speed of its orbit, to calculate J0740+6620's mass.
In Nature Astronomy the discoverers calculate this at 2.17 times the mass on the Sun. For an ordinary star, that would not be all that impressive – it’s slightly greater than nearby Sirius, and stars of more than 100 solar masses are known. However, stellar giants shed most of their mass before they become neutron stars, so this is a record for the class.
Moreover, J0740+6620 is just 30 kilometers (20 miles) across, with as much mass as all of humanity squeezed into a space the size of a sugar cube. The principal author of the paper, University of Virginia graduate student Thankful Cromartie, said in a statement: "These city-sized objects are essentially ginormous atomic nuclei. They are so massive that their interiors take on weird properties.”
Calculating J0740+6620's mass so precisely was only possible because the orbits of the white dwarf and neutron star almost edge on to Earth. Other known pulsars may be similarly massive, but without the same orbital alignment, we can’t tell.
However, astrophysicists have put considerable effort into calculating the point where a neutron star collapses into a black hole. Some models of the behavior of subatomic particles, that put this at less than two solar masses, had to be abandoned when a neutron star of 2.01 solar masses was discovered.
The recent observations of neutron star collisions have produced new models, with some estimates putting the maximum not far above 2.17 solar masses. Co-author Dr Scott Ransom of the National Radio Astronomy Observatory said: “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 mindboggling densities."