Hairy black holes, horizon mass and solitons

Properties of the horizon mass of hairy black holes are discussed with emph asis on certain subtle and initially unexpected features. A key property su ggests that hairy black holes may be regarded as 'bound states' of ordinary black holes (without hair) and coloured solitons. This model is then used to predict the qualitative behaviour of the horizon properties of hairy bla ck holes, to provide a physical 'explanation' of their instability and to p ut qualitative constraints on the end-point configurations that result from this instability. The available numerical calculations support these predi ctions. Furthermore, the physical arguments are robust and should also be a pplicable in more complicated situations where detailed numerical work is y et to be carried out.