A constant that relates the kinetic energy of a particle in a gas to the temperature of that gas. It has the value 1.380 6485 × 10−23 joules per kelvin. The particles can be molecules, atoms, ions, or electrons. The Boltzmann constant relates pressure, p, and temperature, T, by the equation p = nkT, where n is the number of particles per unit volume. In astrophysics, this equation is of importance in understanding the interiors and surface layers of stars, and the atmosphere of planets. The constant is named after the Austrian physicist Ludwig Edward Boltzmann (1844–1906).