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The biggest astronomy story of 2019 arguably was the first-ever image of a black hole, captured by a world-spanning observatory made up of dozens of telescopes. One big reason this achievement was so astounding is because black holes are relatively tiny compared to their mass: this black hole is 6.5 billion times the mass of our sun, but in overall size, it’s comparable to the size of the solar system. So what sets the size of a black hole, and how big — or small — can they get? And what does the size of a black hole even mean?