sentence: Black holes are stars that have collapsed into one very small point.
sentences_before: Black holes are talked about in this chapter.
sentences_after: This small point is called a 'singularity'. Black holes suck things into its center because its gravity is very strong. Some of the things it can suck in are light and stars. Only very large stars, called 'supergiants', are big enough to become a black hole. The star must be one and a half times the mass of the sun or larger to turn into a black hole.
concept_name: black hole
headings: A Brief History of Time Summary Chapter 6
categories: 1988 books Science books