The Blandford–Znajek process is a mechanism for the extraction of energy

Energy

In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 from a rotating black hole

Rotating black hole

A rotating black hole is a black hole that possesses spin angular momentum.-Types of black holes:There are four known, exact, black hole solutions to Einstein's equations, which describe gravity in General Relativity. Two of these rotate...

, discovered by Roger Blandford

Roger Blandford

Roger David Blandford FRS is an astronomer and astrophysicist. He is a Fellow of the Royal Society and Fellow of the American Academy of Arts and Sciences...

 and Roman Znajek in 1977. It is one of the best explanations for the way quasar

Quasar

A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...

s are powered. It requires an accretion disc

Accretion disc

An accretion disc is a structure formed by diffuse material in orbital motion around a central body. The central body is typically a star. Gravity causes material in the disc to spiral inward towards the central body. Gravitational forces compress the material causing the emission of...

with a strong polar magnetic field around a spinning black hole. The magnetic field extracts spin energy and the power can be estimated as the energy density at the speed of light cylinder times area:

where B is the magnetic field strength, the speed of light radius and Ω the angular velocity.