Properties of helium under high pressure are of great importance for a variety of astrophysical objects. This work presents results of extensive ab initio calculations for high-pressure helium regarding the equation of state, the melting line, the electrical conductivity, the reflectivity, and the band gap. The calculations determined a higher-order phase transition from an insulating to a conducting phase which has been discussed for decades. This work finally proposes an update of the phase diagram of helium and discusses its implications on dwarf stars and giant planets.<eng>
