Transcript: The H-R diagram is a plot of spectral class, or equivalently effective temperature, against stellar luminosity. The Stephan-Boltzmann Law tells us that luminosity goes as a high power of the temperature, the fourth power, and this is seen in the H-R diagram where the full range of the diagram is only about a factor of 20 in temperature but a factor of 108 or a hundred million in luminosity. Lines of constant radius can also be represented on the H-R diagram according the Stephan-Boltzmann Law. They are diagonal lines on the diagram which tell us the difference between giant stars, main sequence stars, and dwarf stars.