From the April 2016 issue

A black hole’s gravity is so strong that light can’t escape. But every time I see a photo or an illustration there is something streaming in or out of it. How is this possible?

Dane Allen, Weston, Wisconsin
By | Published: April 25, 2016 | Last updated on May 18, 2023
Black holes
Most illustrations show black holes feeding on debris. But black holes themselves are completely black.
Black holes really are black. Many photos show bright black holes because matter is falling into the hole.

As the accreting matter approaches the event horizon, it gets accelerated to near light speed by the black hole’s gravitational pull.

If the inflow rate of matter is high and collisions occur within the flow, then the matter is heated and radiates.

This radiation is what you see in the illustrations. Lots of black holes without this radiation must exist that astronomers can’t see.

The radiation is most intense within the innermost regions, with about half the radiation originating from within five to 15 times the event horizon’s radius, depending on how the black hole is spinning.

The feeding black hole remains completely black while lying at the center of the brightest persistent emission in the universe.

Andy Fabian
University of Cambridge, England