7 Black holes

Although this course is about white dwarfs and neutron stars, we cannot end without a brief mention of black holes. Black holes are the end point of a star’s evolution if the mass of the degenerate stellar core exceeds the TOV limit. They have infinite density and are characterised by their Schwarzschild radius. This radius marks the event horizon of the black hole, within which the escape velocity exceeds the speed of light, meaning that nothing can escape from it or be observed within it. For a non-rotating black hole of mass M_BH, the Schwarzschild radius , which is ~ 3 km per solar mass.

If the main-sequence mass of a star is in the range , it may be that neutron degeneracy pressure is insufficient to halt the core collapse, and a black hole forms directly (instead of a neutron star) with no accompanying supernova explosion. Such an event may result in the formation of a black hole of mass . The apparent absence of compact objects between the upper mass limit observed for a neutron star (~ 2.2 M_☉) and the lower mass limit observed for a black hole (~ 5 M_☉) is referred to as the mass gap. However, recent observations of gravitational wave sources have found hints of objects in this mass range, thereby challenging our understanding of their nature and formation.

Single stars with main-sequence masses will generally undergo a core-collapse supernova and produce a black hole with a mass of . At the lower end of the mass range, the black hole will form by fallback of material onto an initially created neutron star, while for stellar masses of , the black hole forms directly.