Neutron stars have a wide range of possible masses, but they typically fall within a narrow range. Most observed neutron stars have masses between about 1.1 and 2.2 times the mass of the Sun. This range is known as the "mass gap" because no black holes of similar mass have been detected, making it a distinct population of compact objects.
However, there is ongoing research and discovery in this area, and recent observations have provided evidence for neutron stars with masses that push the boundaries of our previous understanding. In 2019, the discovery of a neutron star called J0740+6620 was announced with a mass of about 2.17 times that of the Sun, which is very close to the upper limit of the previously observed range.
It is theoretically possible for neutron stars to have even higher masses, but the exact upper limit is not well-constrained. There are models that suggest the maximum mass of a neutron star could be around 2.5 to 3 times the mass of the Sun. However, above this range, the gravitational collapse of the neutron star is expected to result in the formation of a black hole.
Determining the precise upper mass limit of neutron stars is a topic of active research, and future observations and theoretical advancements will help refine our understanding of these extreme objects.