Fusion reactions are inhibited by the repulsive electric force acting between two positively charged nuclei. For fusion to occur, the two nuclei must approach each other at high speed to overcome electrical repulsion and achieve a separation small enough (less than a trillionth of a centimeter) to dominate the short-range strong nuclear force. To produce useful quantities of energy it is necessary for a large number of nuclei to undergo fusion: that is, a gas of melting nuclei must be produced. In an extremely high temperature gas, the middle nucleus contains sufficient kinetic energy to undergo fusion. Such a medium can be produced by heating an ordinary gas of neutral atoms above the temperature at which electrons are ejected from the atoms. The result is an ionized gas made up of free negative electrons and positive nuclei. This gas constitutes a plasma. Plasma, in physics, is an electrically conductive medium in which there are a more or less equal number of positively and negatively charged particles, produced when the atoms of a gas are ionized. It is sometimes called the fourth state of matter, distinct from the solid, liquid, and gaseous states. When energy is continuously applied to a solid, it first melts, then vaporizes, and finally electrons are removed from some atoms and molecules of the neutral gas to produce a mixture of positively charged ions and negatively charged electrons, while the neutral charge density overall is maintained. When a significant portion of the gas has been ionized, its properties will be altered so substantially that little resemblance to solids, liquids, and gases remains. A plasma is unique in the way it interacts with itself, with electric and magnetic fields, and with its environment. A plasma can be thought of as a collection of ions, electrons, neutral atoms and molecules, photons in which some atoms are ionized simultaneously with other electrons recombining with the ions to form neutral particles, while photons are continuously produced and absorbed. they estimated that more than 99% of the matter in the universe exists in the state of plasma. All observed stars, including the Sun, are made up of plasma, as are interstellar and interplanetary media and the outer atmospheres of planets. Although most of Earth's matter exists in solid, liquid, or gaseous states, plasma is found in lightning and aurorae, in gas discharge lamps (neon lights), and in the crystalline structure of metallic solids..