Black holes often have stars or gas orbiting around them. It is then possible to measure the mass of the black hole, just by measuring the speed of the orbiting
Consider the case in which a star and a black hole orbit around their mutual center of gravity. Although we can't see the black hole, we can see the star.
With accurate observations, we can measure the speed of the star as well as the size of the orbit. Once these have been measured, the laws of gravity tell us
exactly what the black hole mass is.
For example, let's assume that a star like our Sun orbits a black hole. Suppose that we measure the speed of the star to be 117 miles per second, and that
we measure the diameter of its orbit to be similar to the distance of the planet Mercury from our Sun. This implies that the star orbits the black hole once every
12 days. The laws of gravity then tell us that the black hole must be 10 times more massive than our Sun.
The supermassive black holes in the centers of galaxies can often be measured using this method. For example, the mass of the black hole in the center of
our Milky Way galaxy was calculated by measuring the speeds of individual stars that orbit around it. This showed that the black hole is three million times more
massive than our Sun. And the mass of the black hole in the center of the nearby Andromeda galaxy has been calculated by measuring the average speeds of all the
stars that orbit around it. This showed that Andromeda's black hole is 30 million times more massive than our Sun. The following pages provide more details about
these galaxies and their supermassive black holes: