When a gas is heated by many thousands of degrees, the individual atoms collide with enough violence to knock electrons free, resulting in a collection of positively charged ions and free, negatively charged electrons. The gas is said to be ionized, and when a sizable number of the atoms become ionized, the gas is called a plasma. Because the free electrons tend to recombine with the ions to once again form a neutral gas, however, a plasma can be maintained only if the energy is continuously applied. Scientists have estimated that more than 99 percent of the matter in the universe exists in the plasma state.
Plasma exists in and around the stars, including the sun, and throughout interstellar space.
Because the free electrons are extremely mobile, for example, plasmas are excellent conductors of heat and electricity. Plasma is unique in the way in which it interacts with itself and with electric and magnetic fields. The motions of the free electrons set up electric currents in the plasma. A magnetic field is usually associated with the plasma and interacts with these electric currents. On a small scale the magnetic field causes the individual ions and electrons to move in circles around the magnetic field.