The distance between one part of the wave profile, (e.g. the peak of the wave) at a particular instant in time, and the next identical part of the wave profile at the same instant in time. Wavelength λ is related to frequency f and speed v of a wave by v = fλ.

A small hot star, left behind when a red giant throws off its outer layers as a planetary nebula. A white dwarf has a mass similar to that of the Sun, but a radius similar to that of the Earth. A white dwarf is supported against gravity by an effect of quantum physics known as electron degeneracy pressure.