WHAT LIES BETWEEN NEUTRON STARS AND BLACK HOLES?

There has long been thought to be a mass gap between these two cosmic heavyweights, but does the theory need to be revised?

- David Crookes

Scientists know about neutron stars. They’re very much aware of black holes, too. But what if there’s something in between? This is what astronomers are pondering having discovered a cosmic event called GW190814. It was found through the detection of gravitational waves on 14 August 2019, and involved an object of a mass that had never been seen before.

Up to that point, the heaviest known neutron star was found to have a solar mass of 2.3, with the lightest known black hole being about five solar masses. Yet GW190814 pointed to an object of 2.6 solar masses, making it either the heaviest neutron star ever seen, the lightest black hole or perhaps even something else entirely. It’s so significant that it’s set to change how we look at neutron stars and black holes, not least because it poses a great challenge to current theoretical models.

For years astronomers have worked on the assumption of a ‘mass gap’ to explain why there are no objects with a solar mass greater than 2.3 or less than five. But now that an object involved in the GW190814 event sits firmly within the gap, albeit at the lower end, that assumption must be revised. “The mass spectrum is not thought to be continuous – there are gaps in it,” explains Dr Laura Nuttall, senior lecturer in gravitational waves at the University of Portsmouth’s Institute of Cosmology and Gravitation.