A black hole is a region of space-time where gravity is so strong nothing—no particles or even electromagnetic radiation such as light—can escape from it.
The matter that falls onto a black hole can form an external accretion disk heated by friction, forming quasars, some of the brightest objects in the universe.
By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may form.
The boundary of the region from which no escape is possible is called the event horizon.
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