The Hertzsprung–Russell diagram is a scatter graph of stars which shows the relationship between the stars' absolute magnitudes and their spectral types or classifications 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 magnitude or brightness against its temperature and color. The Hertzsprung–Russell diagram is also referred to by the abbreviation H-R diagram or HRD. They are also known as colour-magnitude diagrams or CMD. 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".
There are several forms of the Hertzsprung–Russell diagram. The original diagram displayed the spectral type of stars on the horizontal axis and the absolute magnitude on the vertical axis. The first quantity is difficult to plot as it is not a numerical quantity and in modern versions of the chart it is replaced by the B-V color index of the stars. This type of diagram is what is often called a Hertzsprung–Russell diagram, or specifically a color-magnitude diagram, and it is often used by observers. In cases where the stars are known to be at identical distances such as with a star cluster, a color-magnitude diagram is often used to describe a plot of the stars in the cluster in which the vertical axis is the apparent magnitude.