Teach Astronomy – Properties of Supermassive Black Holes

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Stellar mass black holes are a natural anticipated consequence of stellar evolution. Evidence for their existence is strong but not beyond doubt, so most people are surprised when they hear astronomers routinely talking about the existence of supermassive black holes millions or billions of times more massive than the Sun. Yet the existence of supermassive black holes is also anticipated theoretically. A dense star cluster will naturally evolve to form a black hole with perhaps a seed mass of a hundred times the mass of the Sun. Over a billion years or so this black hole can grow by accretion and by devouring stars whole to a mass of millions of times the mass of the Sun. The dense center parts of galaxies are good environments for the growth of supermassive black holes. The Schwarzschild radius of a supermassive black hole like that in M87 is about forty astronomical units. Imagine three billion times the mass of the Sun crushed into a region the size of the solar system. Yet the density of material inside a supermassive black hole is not extraordinary. The density is only about a hundred times less than that of water, so the physical state in a supermassive black hole is not that extraordinary.