Black Neutron Star Discovered by LIGO

Ceres dwarf planet
Image Credit: Caltech/MIT/LIGO Lab)

Astronomy is very much a work in progress and our understanding of it is still lacking. Several topics in astronomy are well understood, others are approximated by complex models and simulations. Up to very recently we thought that the death of stars was part of the former, but it turns out that not everything is quite understood yet.

What is a neutron star?

If a star is between 10 and 29 solar masses it will become a super nova, and what is left after the explosion is what we call a neutron star. A typical neutron star will be about 20 km in diameter and have a mass of roughly 1.4 times that of our Sun. They can get heavier, but are capped off by the catchily named ‘Tolman–Oppenheimer–Volkoff limit’, which puts the maximum mass of a neutron star at about 2.1 solar masses. Anything above that and the degeneracy pressure pushing out loses from the gravity pushing in, and the whole thing collapses into a black hole…or so we thought!

Latest LIGO discovery baffles astronomers

On June 24th the LIGO (Laser Interferometer Gravitational-wave Observatory) collaboration published an article in which they described a gravitational wave detection unlike any other measured before. The detection was of two heavy objects merging, one of them was 23 solar masses and the other was 2.6 M⊙. What makes this remarkable is that the smaller object is too heavy to be a neutron star, but also seems to be too light to be a black hole. (The lightest of which is known to date is over 5 solar masses.) This is the first object that has ever been found in the so called ‘mass gap’ and astronomers are unsure what to make of this so-named “black neutron star”.

The easiest solution would be that it is a very light black hole. However, the problem with that is that there are no agreed upon theories that could form a black hole that is so light. A potential solution for that is a controversial theory by Prof Fabio Antonioni, that involves solar systems with three stars. These kind of systems are notoriously unstable and by many thought to be impossible, but the recent discovery has raised the theory out of obscurity.

Another possibility is that our understanding of neutron stars is not as complete as we thought. We do not fully understand the physics of neutron stars, especially what is going on inside of them. Theoreticians like Prof Bernard Schutz see this as an opportunity to rid themselves of the earlier limitations of 2.1 solar masses, stating that maybe this is evidence that we can get much heavier neutron stars.

Whatever this object was, it is gone now due to the merger. But that does not mean that there are no others. Now that evidence of such objects exists, surveys can be designed to look for them and theories can be made to explain them.

In the words of Charlie Hoy, a PhD student involved in the study; “We don’t know what it is and this is why it is so exciting because it really does change our field.”

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