Classical novae are runaway thermonuclear reactions (essentially, hydrogen bombs) that occur on the surfaces of white dwarf stars - the cinders that remain after low-mass stars have spent their nuclear fuel. When a white dwarf in a binary system gains mass by accreting matter from its companion star (Fig. 1), the immense pressure at the base of the added hydrogen layer forces the gas into a state in which its pressure is set by the quantum properties of its electrons, rather than by the thermal pressure. A thermonuclear reaction occurs when this layer becomes hot enough to trigger fusion reactions. Because fusion rates increase extremely rapidly with temperature, and because increasing the temperature does not relieve the pressure, the reaction quickly gets out of control.