In a new paper appearing in the Monthly Notices of the Royal Astronomical Society by Sebastiano Bernuzzi, researchers have begun modeling what happens when two very different-massed neutron stars collide. 

Neutron stars have a very small range of masses they can exist at. Anything that is a dead star – no longer undergoing nuclear reactions – that is under about 1.2 solar masses is going to form a white dwarf star that is supported by electron degeneracy pressure. Once you get over that 1.2, those electrons no longer exert enough pressure, and the object collapses down, protons and electrons combining to form neutrons in a massive blast of energy. 

If you take that neutron star and you keep piling mass on until it’s somewhere around 2.5 or three solar masses, the neutron degeneracy pressure that is supporting that dead star can’t do it anymore, and it collapses further down to form a black hole. And we don’t know what black holes are. 

When two neutron stars merge, which is a thing that happens and produces a whole lot of gold, it’s thought that, in general, they form a black hole exceeding that three solar mass limit. When this occurs, no light escapes; the black hole just consumes everything except for the gravitational waves, which is what we’re detecting here on Earth. You can have a 1.3 solar mass neutron star and a twice-as-large 2.5-2.6 (we’re not really sure) solar mass neutron star. 

In new models, looking at what happens when these two objects come together, it appears that the more massive neutron star will shred the smaller neutron star, causing light to be released, marking what is happening with dramatic sky events that maybe someday we’ll get to see. Showing us sometimes, in the process of forming a black hole, if things are just different enough, you can get light. 

More Information

Penn State Eberly College of Science press release 

 “Accretion-Induced Prompt Black Hole Formation in Asymmetric Neutron Star Mergers, Dynamical Ejecta and Kilonova Signals,” Sebastiano Bernuzzi et al., 2020, Monthly Notices of the Royal Astronomical Society (Preprint on arxiv.org)