How the Universe Works
As the name suggests, supermassive black holes contain between a million and a billion times more mass than a typical stellar black hole. Although there are only a handful of confirmed supermassive black holes (most are too far away to be observed), they are thought to exist at the centre of most large galaxies, including the centre of our own galaxy, the Milky Way.
For many years, astronomers had only indirect evidence for supermassive black holes, the most compelling of which was the existence of quasars in remote active galaxies. Observations of the energy output and variability timescales of quasars revealed that they radiate over a trillion times as much energy as our Sun from a region about the size of the Solar System. The only mechanism capable of producing such enormous amounts of energy is the conversion of gravitational energy into light by a massive black hole.
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