Plasma

Plasma, in physics, almost completely ionized gas containing equal numbers of free electrons and positive ions. Stars and interstellar gas consist of plasma. Unlike un-ionized gas, plasma conducts electricity and is affected magnetically. Plasma may be a 4th state of matter (besides gases, liquids, and solids). Because the temperature of a plasma is theoretically high enough to support a controlled nuclear fusion reaction, plasmas are being widely studied as a means to contain such a reaction to create energy. Plasmas are formed by heating low-pressure gases until the atoms have sufficient energy to ionize each other. Unless the plasma can be successfully contained by electric or magnetic fields, rapid cooling and recombination occur.

See also: Physics.