Black hole. Hubble Space Telescope (HST) data that reveals the presence of a black hole in the centre of the M84 galaxy. The image shows the velocity of gas seen along a long, narrow slit. Gas moving towards Earth appears left of centre, whilst gas moving away from us is right of centre. The light produced by approaching gas appears blue, whilst that from receding gas appears red; this change in frequency is known as the Doppler Effect. The material swirling around the black hole is travelling at up to 400 kilometres per second. This indicates that the black hole is at least 300 million times more massive than our Sun.