How Do You Measure the Mass of Stars Nearby?

Black holes are so far away, we've barely seen one directly. One way we can see them is by seeing their gravitational effect stuff near by. We measure the mass of stars whizzing very close by to it using light they emit. And from that you can deduce the mass of the black hole. That's based on models and some observations. There are uncertainties, but we can trust those because there are a lot of minary star systems out there in the universe - more than you might expect.