Neutron stars, the incredibly dense remnants of massive stars, have captivated scientists for decades. These celestial objects, formed when a star collapses under its own gravity, are so dense that a teaspoon of their material would weigh billions of tons. But just how heavy can a neutron star get before it transforms into a black hole? A recent study by researchers at the HUN-REN Wigner Research Centre for Physics in Hungary provides some intriguing answers.
The key to understanding neutron stars lies in the Equation of State, a set of rules describing how matter behaves under extreme pressures. However, since we can't directly observe neutron stars, scientists rely on models to define this rulebook. The researchers used two models, SFHo and DD2, each with distinct properties, to estimate the maximum mass of a neutron star.
SFHo models neutron stars with "softer" and more compressible nuclear matter, while DD2 represents them as tougher and more resistant. To ensure the speed of sound in these materials didn't exceed the speed of light, the authors manually adjusted the models to align with perturbative Quantum Chromodynamics (pQCD) results.
The study's findings are fascinating. When the models were tested against data from the Neutron Star Interior Composition ExploreR (NICER) telescope and the gravitational wave detection of GW170817, they converged on a similar conclusion: neutron stars with masses between 2.2 and 2.3 solar masses.
However, this raises intriguing questions about the physical dimensions of these massive neutron stars. Their radii are estimated to be around 12 km, but objects like GW190814, weighing 2.59 solar masses, challenge this classification. If assumed to be neutron stars, they would violate the DD2 model, as the material supporting such a mass wouldn't be deformable enough.
This leads to a fascinating conclusion: GW190814 and similar "size gap" objects, like HESS J1731-347, are likely black holes rather than neutron stars. This study also provides a definitive answer to the Tolman-Oppenheimer-Volkoff (TOV) equations, originally formulated in 1939 to describe neutron stars.
The implications are profound. With a clear weight and estimated size, we can gain valuable insights into the inner workings of these extraordinary cosmic phenomena, even if we never physically observe them. This research not only advances our understanding of neutron stars but also highlights the ongoing quest to unravel the mysteries of the universe.
