The Hertzsprung - Russell diagram

The Hertzsprung - Russell diagram is a useful graphic depiction of the different types of Stars arranged according to their Spectral Classification and Luminosity. Named in honor of the Danish astronomer Ejnar Hertzsprung (1873–1967) and the American astronomer Henry Norris Russell (1877–1957), who developed the diagram independently of each other.

In the early 1900s Ejnar Hertzsprung in Denmark and Henry Norris Russell in the United States independently began to consider how the brightness of stars might be related to their spectra. In 1911 Hertzsprung plotted the apparent magnitudes of stars in several clusters against their spectral class. In 1913 Russell plotted the absolute magnitudes of stars in the solar neighborhood against their spectral class. Both astronomers emphasized the non-random patterns they saw in the arrangement of the data on these graphs. Such graphs have become extremely important in modern astronomy

It is a Plot between the absolute magnitude (Luminosity) of stars against their spectral class. It is equivalent to a plot of the luminosity of stars against their surface temperature. The Hertzsprung–Russell diagram is an extremely valuable tool wherein observational and theoretical information is blended to produce a deep understanding of stellar evolution. It is a very powerful diagram, able to represent almost the entire evolution of any star. There are a finite number of equations defining a star’s structure at every point in its life. These equations are interpreted on an HR diagram, so that determining the position of any star on an HR diagram will give information on its structure and evolutionary phase.

The diagram consists of a graph on which a parameter measuring stellar brightness is plotted against a parameter related to a star’s surface temperature. For example, apparent magnitude, absolute magnitude, absolute bolometric magnitude or luminosity may be plotted along the horizontal axis, while spectral class, color index or effective temperature may be plotted along the vertical axis.

These HR diagrams show that the total range in stellar brightness is around 27 magnitudes (corresponding to a factor of 10¹¹ in luminosity), and the range in the surface temperature of stars is from 2200 K to 50,000 K. The size of a star is related to both its luminosity and its surface temperature, as indicated by the dashed lines. Most main sequence stars are roughly the same size as the Sun. The three main groupings on the HR diagram correspond to the three very different stages through which a typical star passes during its life. Computer models reveal how the luminosity and surface temperature of a star change as it evolves. With this information, the path followed by an evolving star can be plotted on an HR diagram.

Stellar evolution calculations relate the HR diagram to time, and thus these graphs can provide information about the ages of stars. The HR diagram is particularly useful for estimating the age of star clusters.

Questions to Ponder

• How can we tell that Red Giants are very large stars, from just their position in the HR diagram?
• What are the three main gropus of stars identified in the HR diagram?