Neutron stars, the incredibly dense remnants of massive stars that have exploded as supernovae, have long fascinated astronomers and physicists alike. These celestial objects pack the mass of two suns into a sphere the size of a city, with each teaspoon of their material weighing billions of tons. Despite the intense interest in these extreme objects, we are still actively learning about them, and one of the most pertinent outstanding questions is where the line is between becoming a neutron star and becoming a black hole when a star dies. A new paper by researchers at the HUN-REN Wigner Research Centre for Physics in Hungary provides a definitive answer to this question, suggesting that the maximum mass of a neutron star is between 2.2 and 2.3 solar masses. However, this raises a deeper question about the actual size of these behemoths, as their physical dimensions vary based on the starting model used. This leaves a few strange objects in the lurch, as they are too big to qualify as neutron stars by this metric, but also don't seem to be black holes either. For example, object GW190814 weighs in at 2.59 solar masses, and if this object is assumed to be a neutron star, it would break the DD2 model entirely. The results strongly imply that GW190814, as well as a fellow "size gap" object HESS J1731-347, are in fact black holes rather than neutron stars. This new paper provides plenty of insight into the inner workings of some of the universe's strange objects, even if we never get to physically see those inner workings ourselves. Personally, I find it fascinating that we can use mathematical models and observations from telescopes to understand the extreme physics of neutron stars and black holes. It's a testament to the power of human curiosity and ingenuity that we can explore the universe's most extreme objects without ever having to physically visit them. However, I also think that there is still much to learn about these objects, and I am excited to see what future research will reveal about the mysteries of neutron stars and black holes.
