Even the most supermassive of the supermassive black holes aren't very large, making it extremely difficult to measure their sizes. However, astronomers have recently developed a new technique that can estimate the mass of a black hole based on the movement of hot gas around them—even when the black hole itself it smaller than a single pixel.
Supermassive black holes are surrounded by tons of superheated plasma. That plasma swirls around the back hole, forming a torus and an accretion disk that continually feeds material into the black hole. Because of the extreme gravity, that gas moves incredibly quickly and shines fiercely. It's that light that we identify as a quasar, which can be seen from across the universe.
While the quasars are relatively easy to spot, it's much more challenging to quantify the properties of the central black hole. Now Felix Bosco, in close collaboration with Jörg-Uwe Pott, both from the Max Planck Institute for Astronomy (MPIA) in Heidelberg, and former MPIA researchers Jonathan Stern (now Tel Aviv University, Israel) and Joseph Hennawi (now UC Santa Barbara; U.S. and Leiden University, the Netherlands), has succeeded for the first time in demonstrating the feasibility of directly determining the mass of a quasar using a technique called spectroastrometry.