Intro
Stars die in roughly two ways that leave a dense remnant: a neutron star, or, if the leftover core is heavy enough, a black hole. For a while it looked like there was a suspicious gap in between, a range of masses where remnants were rare or absent.
What We Know
Gravitational-wave astronomy has been steadily filling in the census. The event GW190814 involved a compact object squarely in the contested range. We measured its mass precisely. We are far less sure what it is.
What We Think
The object could be an unusually heavy neutron star, an unusually light black hole, or a member of a class we have not properly characterized. The maximum mass a neutron star can reach before collapsing is tied, once again, to the equation of state we do not fully know.
What We Do Not Know
Whether the mass gap is a genuine feature of how stars die, or an artifact of small numbers and detection biases, is still being worked out. Each new detection nudges the answer.
Why It Matters
The boundary between “star” and “black hole” is where stellar death physics, nuclear physics, and gravity all meet. Naming what lives there would tell us how the heaviest collapses actually end.