Trust me, this is cool. Astronomers have discovered a stellar remnant with 2.6 solar masses, which is within a range of mass called the “mass gap” because of the almost complete lack of such objects in that range.  This is both an astronomy mystery (how do such objects form) and a physics mystery (what forces dominate at this size). Any new data points give us clues to solve the mystery of the mass gap, so this is exciting news.

Even still, yet again I find the headlines and even the popular reporting hyping the find. The BBC headline reads, “‘Black neutron star’ discovery changes astronomy.” No, this is not going to “change astronomy,” unless you count every incremental addition of new information as changing the entire field. Also, calling it a “black neutron star”, while a possibility, is assuming only one possible conclusion. But let’s get into the interesting details.

For quick background, when stars die they leave behind a stellar remnant. When stars run out of fuel they are able to burn (which is partly determined by their mass) they no longer produce the outward pressure of fusion and so gravity takes over and they collapse. If they are large enough (8-15 solar masses) the core collapse results in a supernova. Either way, what’s left behind is a stellar remnant. Small remnants become a white dwarf, a glowing hot ember but without fusion. If the remnant is at least 1.4 solar masses the force of gravity will overcome the repulsive force among the positive proton and negative electrons and the white dwarf will collapse down to a neutron star – in simplistic terms, the electrons and protons will merge into neutrons, so the entire thing is made of neutrons.

This is where things get even more interesting. Neutron stars are held up by degeneracy pressure, a quantum effect I won’t get into here. At some mass, however, the gravitational force is greater even than this outward force and the neutron star collapses all the way down to a singularity, becoming a black hole. At what precise mass does this happen? That is the question of the mass gap – because this gap is the mass between the most massive neutron star, about 2.3 solar masses, and the least massive black hole, about 5 solar masses. What happens in that mass gap? We don’t know.

There are two general ways to approach this question. One is theoretical, doing the math to see what should happen at different masses. However, we don’t have the physics for this. We lack well developed theories that merge both quantum effects and relativity, something like quantum gravity. We simply don’t know how physics works at such masses and densities. So we cannot yet really answer the question theoretically.

The second approach is astronomy – to look at the universe and see what is out there, and hopefully this will inform our theories. This is why the mass gap is so frustrating – we cannot find objects in the mass range of interest. So we are left with a multilayered mystery. Why are there so few objects in the mass gap? What happens to stellar remnants that are 3 or 4 stellar masses? Is there a sharp mass line when neutron stars become black holes? There are reasons to think neutron stars cannot be 3 or more solar masses, but are they true black holes? Maybe there is a third class of stellar object that is neither a neutron star nor a black hole – a black neutron star. But what physics governs such objects?

The potential for discovery here is immense, including the possibility of new physics, and maybe even clues to that elusive theory of quantum gravity.

So the discovery of a 2.6 solar mass stellar remnant is exciting. It is above the upper limit of previously discovered neutron stars, and maybe the absolute upper limit. So what was it? I say was because it no longer exists, and this gets to how it was discovered. This is also a cool story because it was discovered using gravitational astronomy.

Here is the quicky on this – if you have two strong lasers at right angles to each other, with the beam length really long (2.5 miles for LIGO), where they intersect there will be wave interference between the two beams. Anything that causes the lasers to shift slightly will change the interference. This instrument is so sensitive it can detect movements at the subatomic scale – 1/10,000th the width of a proton! Let that fry your brain for a while. This is sensitive enough to detect the ripples in space-time generated by massive objects colliding. Because LIGO does not have to look in any direction, it can detect such ripples from anywhere. So theoretically LIGO can detect massive objects colliding with each other anywhere in the visible universe.

Now that LIGO is up and running it is detecting lots of such collisions. One recent one was a collision between two objects, one 15 solar masses (so definitely a black hole) and one 2.6 solar masses – the object of interest. Of course the collision created a new 17.6 solar mass object, and therefore the 2.6 solar mass object is gone. But we were able to detect the screams of its violent death in the form of ripples in space-time.

The expectation is that LIGO, and potentially other and even better gravitational wave detectors, will start detecting more objects in the mass gap. We can then star answering question about how common they are, where do they exist, and when do they exist (remember, the farther away an object is the further back in time it is). Now we just wait for the next such collision. Meanwhile physicist can try to sort out how such objects exist.