<P> Russell's early (1913) versions of the diagram included Maury's giant stars identified by Hertzsprung, those nearby stars with parallaxes measured at the time, stars from the Hyades (a nearby open cluster), and several moving groups, for which the moving cluster method could be used to derive distances and thereby obtain absolute magnitudes for those stars . </P> <P> There are several forms of the Hertzsprung--Russell diagram, and the nomenclature is not very well defined . All forms share the same general layout: stars of greater luminosity are toward the top of the diagram, and stars with higher surface temperature are toward the left side of the diagram . </P> <P> The original diagram displayed the spectral type of stars on the horizontal axis and the absolute visual magnitude on the vertical axis . The spectral type is not a numerical quantity, but the sequence of spectral types is a monotonic series that reflects the stellar surface temperature . Modern observational versions of the chart replace spectral type by a color index (in diagrams made in the middle of the 20th Century, most often the B-V color) of the stars . This type of diagram is what is often called an observational Hertzsprung--Russell diagram, or specifically a color - magnitude diagram (CMD), and it is often used by observers . In cases where the stars are known to be at identical distances such as within a star cluster, a color - magnitude diagram is often used to describe the stars of the cluster with a plot in which the vertical axis is the apparent magnitude of the stars . For cluster members, by assumption there is a single additive constant difference between their apparent and absolute magnitudes, called the distance modulus, for all of that cluster of stars . Early studies of nearby open clusters (like the Hyades and Pleiades) by Hertzsprung and Rosenberg produced the first CMDs, antedating by a few years Russell's influential synthesis of the diagram collecting data for all stars for which absolute magnitudes could be determined . </P> <P> Another form of the diagram plots the effective surface temperature of the star on one axis and the luminosity of the star on the other, almost invariably in a log - log plot . Theoretical calculations of stellar structure and the evolution of stars produce plots that match those from observations . This type of diagram could be called temperature - luminosity diagram, but this term is hardly ever used; when the distinction is made, this form is called the theoretical Hertzsprung--Russell diagram instead . A peculiar characteristic of this form of the H--R diagram is that the temperatures are plotted from high temperature to low temperature, which aids in comparing this form of the H--R diagram with the observational form . </P>

If you plot luminosity against temperature for the stars in the universe what do you find