Material, mostly gas and dust, is falling into it, and as it does it forms a flat disk called an accretion disk that starts just outside the event horizon and extends for many billions of kilometers. It’s also an active galaxy: Unlike the supermassive black hole in the Milky Way, the one in the center of M87 is actively gobbling down matter. ![]() It’s a huge galaxy, bright enough to be seen using just binoculars, even though it’s 55 million light years away. M87 is an elliptical galaxy in the heart of the Virgo Cluster, a collection of hundreds of galaxies spread across the sky between the constellations of Leo and Virgo. The Milky Way has one that’s more than 4 million times the Sun’s mass, for example. We now think that every big galaxy in the Universe has a supermassive black hole in its center, with millions or even billions of times the mass of the Sun. ![]() Mind you, the Sun is 1.4 million km across now! So you have to make objects incredibly small and dense for them to become black holes. If you do the math - first calculated by Einstein in the early 1900s - you’ll find that if you compress the Sun down to a black hole it would be 6 kilometers in diameter. The size of the event horizon depends on the mass of the black hole. A photon, a particle of light, passing near that limit but still outside it will have its path bent considerably, but it can escape. But just outside it, gravity is intensely strong but not impossibly so. The surface of this sphere, if you think of it that way, is called the event horizon (because any event that happens inside it is beyond your horizon, and cannot be seen). What you’re left with is a spherical region in space where inside it nothing can get out.
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