Helium nucleus. Computer artwork of the nucleus of a helium atom, or an alpha particle given off during radioactive decay. The nucleus consists of two positively-charged protons (red) and two neutral neutrons (green) surrounded by a quantum cloud of gluons, a type of subatomic particle. Gluons mediate the strong nuclear force which holds the nucleus together by overcoming the repulsive electrostatic forces between the protons. The branch of physics that describes the behaviour of gluons is known as quantum chromodynamics (QCD).