Discovered by Karl Schwarzschild, the Schwarzschild criterion is a criterion in astrophysics where a stellar medium is stable against convection where where is gravity and is the heat capacity at constant pressure. In order to keep rising or sinking in the star the displaced element must not be able to become the same density as the gas surrounding it. In other words, it must respond adiabatically to its surroundings. In order for this to be true it must move fast enough for there to be insufficient time for the element to exchange heat with its surroundings.