How Do You Weigh a Black Hole?

"In a recent piece for my column The Universe, I wrote about the biggest black holes in the cosmos. These can tip the scales at many billions of times the sun's mass, outweighing even entire galaxies. But how do we know that? Black holes are rather famous for being, well, black because they can gobble down even light itself. So how can we figure out how massive they are? There are several ways, actually, mostly depending on the kind of black hole we're examining."

"The most common kind of black hole we know of is a stellar-mass black hole; one with a few to a few dozen times the mass of the sun. This type usually forms when a massive star explodes at the ends of its short life and its core collapses. The infalling material becomes so dense that the gravity skyrockets, becoming so strong that nothing, not even photons, can escape its grasp after getting too close."

Black holes range from stellar-mass objects a few to a few dozen times the Sun's mass to supermassive ones weighing billions of solar masses. Stellar-mass black holes form when massive stars explode and their cores collapse, producing densities so extreme that gravity prevents even light from escaping. Theoretical work indicates a minimum black hole mass near three solar masses, and astronomers classify objects under about 100 solar masses as stellar-mass. Measuring mass requires observing gravitational influences on nearby companions, accretion-driven radiation, or other signals, with binary systems often providing the clearest mass constraints.

Read at www.scientificamerican.com