Work by Subrahmanyan Chandrasekhar suggested that collapsing stars beyond a certain mass cannot be supported by degeneracy pressure, but this result was challenged by the more prestigious Arthur Stanley Eddington, and was not fully accepted for several decades.

In consultation with theoretical physicists, it became apparent that the only sensible explanation for these sources were extremely large black holes residing in the cores of galaxies, producing intense radiation as they fed and, in the case of quasars, blasting out incredibly powerful jets of material in opposite directions, heating the surrounding galactic gas until it glowed in radio frequencies.

[1] Toward the end of the text, Thorne deals with the much more speculative question of the nature of the core of a black hole; the so-called gravitational singularity predicted by Einstein's field equations.

By introducing quantum behavior to curved spacetime, several physicists have suggested that black holes do not possess a true mathematical singularity, but rather a region of chaotic space, in which time does not exist.

In the final chapter, Thorne delves into even more speculative matters relating to black hole physics, including the existence and nature of wormholes and time machines.