Plasma is a state of matter that results from one of the other three classical states having undergone an appreciable degree of ionization. It thus consists of a significant portion of charged particles. While rarely encountered on Earth, it is estimated that 99.9% of all ordinary matter in the universe is plasma. Stars are almost pure balls of plasma, and plasma dominates the rarefied intracluster medium and intergalactic medium. Plasma can be artificially generated, for example, by heating a neutral gas or subjecting it to a strong electromagnetic field.