Neutron stars are among the densest objects in the Universe, the result of supernovae explosions. The only thing denser is black holes, and if a neutron star were to acquire enough mass, it is expected to collapse into one. Thanks to new research, astronomers believe they know how big that neutron star has to be – and they have found one that is literally at that limit.

As reported in The Astrophysical Journal Letters, the object in question is called PSR J0952-060 and is a black widow pulsar. It’s a pulsar and not just a regular neutron star because it spins very fast around its axis with a jet of material pointing toward us, so it pulsates. That pulsation is fast – 707 times per second.

The black widow part is because it stole material from and destroyed its original companion which had become a red giant. This thievery helps the pulsar spin faster, leading to more energetic particles coming off the pulsars. This wind of particles strips the other star, reducing it to the size of a planet and then to nothing. In this system, the companion now weighs just 20 times the mass of Jupiter.

At the same time, the pulsar can gain more mass – but up to a point. By studying a dozen black widow pulsars, the researchers found a reasonable number for the highest mass one can be before the neutron star collapse under its weight. PSR J0952-060 is there, weighing 2.35 times the mass of our Sun.

“By combining this measurement with those of several other black widows, we show that neutron stars must reach at least this mass, 2.35 plus or minus 0.17 solar masses,” lead author Roger Romani, who is a professor of physics at Stanford’s School of Humanities and Sciences and member of the Kavli Institute for Particle Astrophysics and Cosmology, said in a statement. “In turn, this provides some of the strongest constraints on the property of matter at several times the density seen in atomic nuclei. Indeed, many otherwise popular models of dense-matter physics are excluded by this result.”

Given that we can’t see black holes as they are shrouded by the event horizon, this object is the densest visible object we know of. By studying these and similar objects, physicists can discover new behaviors of fundamental particles that could not be recreated on Earth.

“We know roughly how matter behaves at nuclear densities, like in the nucleus of a uranium atom,” said Alex Filippenko, Distinguished Professor of Astronomy at the University of California, Berkeley. “A neutron star is like one giant nucleus, but when you have one-and-a-half solar masses of this stuff, which is about 500,000 Earth masses of nuclei all clinging together, it’s not at all clear how they will behave.”

The team will keep looking for this kind of star hoping to refine this mass measurement and understand even better the threshold between neutron stars and black holes.