29 March 2001 Ref.
Many astronomers now believe that most galaxies have massive black holes at their centres.
It is also possible that the presence of these black holes may be necessary to spark the formation of the first stars and thus the complexity that led to intelligent life. Black holes cannot be observed directly, but by studying the X-rays produced by material falling into a black hole, astronomers can measure their mass and distance, and even how fast they are rotating. This allows X-rays characteristic of iron atoms with a sharply defined energy to be identified. When seen from afar, this energy is altered by the conditions near the black hole as the material swirls rapidly around in an accretion disk. At any moment, some of the iron atoms are rushing away from us, so their apparent energy is decreased, while others are moving towards us, and their apparent energy is increased due to the well-known Doppler effect.