Near a black hole, gravity is rather intense, but light still can muster an escape by climbing out of the black hole's gravitational "well," losing energy during the climb. Using a spectrogram scientists can analyze the rate of loss. It has been observed that light from iron atoms emitted at a specific frequency creates a bright line in the spectrogram. As the light loses energy while it climbs out of a gravitational well, the line broadens, or stretches to lower energy levels on the spectrogram. The broad iron K line refers to this shape of a spike generated on the spectrogram and is typically observed with light emitted close to a black hole. It is also used as a measure to calculate a black hole's gravitational force.

Scientists can study black hole activity with X-ray telescopes like the Rossi Explorer.