At the center of our galaxy lies Sgr A — a supermassive black hole. With over 4 million times the Sun’s mass, you can see why it gets that moniker.
One reason we know its mass is that there’s a cluster of young, luminous stars orbiting around it. These are called S stars, and they form a group around the black hole about a quarter of a light year across — a few trillion kilometers. One of these stars, S2, has an elliptical orbit that takes it to a distance of just 16 billion kilometers from the black hole as it travels on its elliptical orbit. Until recently, that star had the closest encounter we knew of.
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