Comparison of relevant timescales as a function of the initial density. Solid line: expansion timescale, computed as the free-fall or hydrodynamic timescale,  s; dotted line: rise time in a deflagration wave, computed as the nuclear timescale at the critical temperature at which the nuclear timescale matches the heat diffusion timescale; dashed line: rise time in a detonation wave, computed as the nuclear timescale at the temperature of shocked matter.