Weighing a black hole

Studies of x-ray emissions from the center of the galaxy have allowed an estimate of the mass of the black hole at the center of our galaxy.

When black holes devour matter, they "burp" x-rays, and observations of these x-rays can tell us a lot about the black hole. In this case, measurements of the Doppler shift were consistent with a clump of matter orbiting the black hole about once every seven days.

If we compare matter swirling around a black hole with water swirling down a drain, we find the fastest movement is just before the final drop down the hole. This is also the point when matter is stressed the most, and emitting the most x-rays. This gives us an upper limit on the size of the black hole.

Since the size is a function of the mass, estimating the mass is pretty straightfoward. It's not a very precise estimate – somewhere between 10 and 50 million solar masses, but it's a start. That mass corresponds to a diameter of about one astronomical unit (AU), or the diameter of the Earth's orbit.

More observations, and better detectors, should refine this estimate further.