Under conditions of very high pressure, gases can behave in a quite different way. These gases are called degenerate gases and their properties are governed by the principles of quantum mechanics.
One very important property of degenerate gases is that the pressure is independent of the temperature. Thus, contrary to our usual experience, if we could heat a balloon of degenerate gas it would not expand. This has very large consequences for thermonuclear reactions, such as those that occur in a nova outburst.