Editing A Brief History of Time (section)

=== Chapter 6: Black Holes ===
[[File:Black hole lensing web.gif|thumb|A [[black hole]], showing how it distorts its background image through [[gravitational lens]]ing]]
Hawking discusses [[black hole]]s, regions of [[spacetime]] where extremely strong gravity prevents everything, including light, from escaping them. The term black hole was coined by [[John Archibald Wheeler]] in 1969, although the idea is older. The Cambridge clergyman [[John Michell]] imagined stars so massive that light could not escape their gravitational pull. Hawking explains [[stellar evolution]]: how main sequence stars shine by fusing [[hydrogen]] into [[helium]], staving off gravitational collapse. A collapsed star may form a [[white dwarf]], supported by electron degeneracy, or a [[neutron star]], supported by the exclusion principle. [[Subrahmanyan Chandrasekhar]] found that for a collapsed star of more than 1.4 solar masses, there would be nothing to halt complete gravitational collapse. He was dissuaded from this thinking by [[Arthur Eddington]], though it later won him the [[Nobel Prize in Physics]]. The critical mass is known as the [[Chandrasekhar limit]]. 

He describes the [[event horizon]], the black hole's boundary from which no particle can escape. He writes: "One could well say of the event horizon what the poet [[Dante]] said of the entrance to Hell: 'All hope abandon, ye who enter here.'"{{Sfnp| Hawking| 1996|p=177}} Hawking discusses non-rotating black holes with [[spherical symmetry]] and rotating ones with [[axisymmetry]]. The discovery of [[quasars]] by [[Maarten Schmidt]] in 1963 and [[pulsars]] by [[Jocelyn Bell-Burnell]] in 1967 gave hope that black holes might be detected. Even though black holes (by definition) do not emit light, astronomers can observe them through their interactions with visible matter. A star falling into a black hole would be a powerful source of [[X-rays]]. [[Cygnus X-1]], a powerful source of X-rays, was the earliest plausible candidate for a black hole. Hawking concludes by mentioning his [[Thorne–Hawking–Preskill bet#Earlier Thorne–Hawking bet| 1974 bet]] with American physicist [[Kip Thorne]]. Hawking argued that Cygnus X-1 does not contain a black hole. Hawking conceded the bet as evidence for black holes proved overwhelming.