The Hertzsprung–Russell diagram is a stars scatter graph that shows the relationship between the stars' absolute magnitudes or luminosities versus their spectral types and effective temperatures. Hertzprung-Russell diagrams are not pictures or maps of the locations of the stars. Rather, they plot each star on a graph measuring the star's absolute brightness against its temperature and color. The diagram was created circa 1910 by Ejnar Hertzsprung and Henry Norris Russell and represents a major step towards an understanding of stellar evolution or "the lives of stars".
The Hertzsprung-Russell (H-R) diagram is simple. It plots two basic properties of stars: their luminosity (intrinsic brightness) and their surface temperature (as revealed by their color). In doing so, it anchors stellar astronomy just as the periodic table anchors chemistry. Whereas the periodic table groups together similar chemical elements—for example, placing all noble gases, such as helium, neon and argon, into one column—the H-R diagram groups together stars passing through similar stages of life. When astronomers invented the diagram, no one knew why the sun and other stars shine. No one knew how stars are born or how they die. No one could even assure the public that the sun would never explode. Nor did anyone know that the stars had forged most of the elements that make up Earth and our bodies.
Contemplation of the diagram led astronomers to speculate that it might demonstrate stellar evolution, the main suggestion being that stars collapsed from red giants to dwarf stars, then moving down along the line of the main sequence in the course of their lifetimes. Stars were thought therefore to radiate energy by converting gravitational energy into radiation through the Kelvin-Helmholtz mechanism. This mechanism resulted in an age for the sun of only tens of millions of years, creating a conflict over the age of the solar system between astronomers, and biologists and geologists who had evidence that the earth was far older than that. This conflict was only resolved in the 1930s when nuclear fusion was identified as the source of stellar energy.