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This is a simplified illustration of two hot chunks of matter orbiting a black hole, showing how scientists tracked the blobs by observing their Doppler shift. First, we see one blob. Note how the energy emitted from this orbiting material rises to about 6.5 kilo-electron volt (an energy unit) as it moves
towards us, and then falls to about 5.8 kilo-electron volt as it moves away.

This is the 'Doppler effect' and a similar phenomenon happens with the changing pitch of a police siren. If it is approaching, the frequency of the sound is higher, but if it is receding the frequency is lower. Matter goes round and
round; energy goes up and down. About 14 seconds into the animation, a second blob is added, which also displays a rise and fall in energy during its orbit.