Black hole. Computer model of the accretion disc around a non-rotating (Schwarzschild) black hole (centre). The colours relate to the amount of red shifting (doppler shifting) of radiation emitted from the disc, rotating anticlockwise around the hole, ranging from red (most) to blue (least). Red shift occurs when a radiation source moves away from an observer, increasing its apparent wavelength. The orbits of particles in the disc become unstable half way into this disc, and particles begin spiralling inwards. A black hole is a region where the gravitational field is so strong that light cannot escape. Work done at the University of Utah, USA.