The quasar

Gas gathers into a disk as it circles a supermassive black hole. As it moves inward, gravitational energy is released and the gas heats up. The disk becomes so luminous that a small region at the center of a galaxy can outshine the galaxy’s stars.

This helps explain the early puzzle of quasars. A telescope could record a point of light that looked like a star, while the object’s distance implied an extraordinary energy output. The small bright disk in this reconstruction shows where much of that energy is released.

A quasar’s central black hole can contain millions or billions of times the Sun’s mass. Material around it gives off the light we receive before crossing the event horizon. Once light passes inside that boundary, it cannot escape to an outside observer.

The dark mark here indicates the black hole’s location. Its size has been adjusted for the illustration. It is not an observed shadow, and it gives us no view of the interior.

Some quasars launch narrow streams of particles from the region around their black holes. Strong magnetic fields help organize these jets, which can travel close to the speed of light. Their launch region lies outside the event horizon. The particles have not escaped from inside the hole.

Jets from supermassive black holes can extend hundreds of thousands of light-years. This painting shows only a short stretch. Many quasars have no powerful jets; how the flow of gas, magnetic fields and black-hole spin produce one remains an active subject of research.

The same system can send material in opposite directions and still appear strongly one-sided. When a jet points partly toward us and moves very fast, relativity increases the brightness we receive. The jet traveling away appears dimmer.

This effect is called Doppler beaming. The unequal brightness in the watercolor represents that viewing effect. A faint counterjet does not by itself mean the black hole is sending less material that way.

Farther from the hot inner disk, dust can survive. A thick, uneven ring of dusty gas surrounds the engine in this reconstruction. Depending on our viewing angle, it can obscure the bright center and change the kind of active galaxy we appear to see.

ESO’s infrared observations of Messier 77 traced heated dust around a hidden central engine. Observations of another active galaxy, NGC 3783, also found substantial dust above and below the expected ring. The torus is a useful model; real dust distributions can be more complicated than a neat doughnut.