H-R Diagram: Definition & Purpose

Instructor: Tashna Richards

Tashna has taught all four disciplines of science to K-2 tudents and is pursuing a master's degree in STEM education.

The Hertzsprung-Russell provides important information about stars such as color, temperature and luminosity. Check out which stars are blue, hot and red, and which ones are so cool they're in a class by themselves.

Who Were Hertzsprung and Russell?

Ever notice how theories and models in science are commonly named after the men and women who discovered them? From Bohr's example of the atom to Darwin's theory of evolution, those who made great contributions and innovations in science have long been honored by this eponymous tradition. This same naming system also holds true as with the Hertzsprung-Russell diagram.

Hertzsprung and Russell were two astronomers who noticed significant patterns between a star's luminosity, or energy output of light, and its surface temperature. Ejnar Hertzsprung was from Denmark, while Henry N. Russell was from the United States. They placed these findings in a graph called, none other than, the Hertzsprung-Russel, or HR Diagram, for short.

Hertzsprung-Russell Diagram

The Hertzsprung-Russell diagram is generally depicted as a scatter plot graph where each individual star is represented by a single point along the graph. In the diagram above, you may notice that there are four main types of stars: main sequence, white dwarfs, giants and supergiants. The central diagonal spanning across from the upper left corner to the bottom right comprises the main sequence, or approximately 90% of all stars. The supergiants fall within the lower temperature ranges, but due to their large masses, they possess higher luminosities. The white dwarfs are small, hot and dim with relatively low luminosities, whereas the giants fall somewhere in between the supergiants and main sequence within spectral classes G-M.

Spectral class refers to the categorization of stars based upon their color and temperatures. Stars can be classified within any of the seven spectral classes: O, B, A, F, G, K and M.

On the HR diagram (Figure 1), temperature and spectral class are shown on the x-axis. Contrary to most graphs, which typically display the temperature scale from the least to greatest values, the scaled temperatures on the HR diagram decrease from left to right.

The y-axis denotes the absolute magnitude, or luminosity of stars, ranging from the dimmest to the brightest in solar units.

While looking for more patterns, you may notice that if you divide the diagram into quadrants, the top left quadrant stars are hot and bright, while the top right quadrant stars are cool and bright. You may also notice that both the lower left stars and the lower right quadrants are hot and dim. The sun has relatively average temperatures and lies somewhere in the middle amongst the main sequence.

Luminosity vs. Temperature

The two primary characteristics of the HR diagram are temperature and luminosity. Luminosity is defined as the brightness / absolute magnitude or output of energy; whereas, temperature is defined as the average speed of moving particles. The particles moving within a star determine the star's temperature, which in turn, causes the star to emit energy in the form of light. A star's temperature directly correlates to its luminosity, meaning that, as the temperature of a star increases, its luminosity also increases.

Specifically, luminosity is the amount of energy being distributed over one square meter. Since this is true, the larger that a star is, the greater its luminosity or brightness will be for the same temperatures. For example, Betelgeuse is a supergiant star with a spectral range of M and a temperature of approximately 2,900 degrees. Gliese 725 B, a much smaller star, falls within the same categories. However, when comparing luminosities, Betelgeuse's solar units are far greater than Gliese 725 B's, as shown in Figure 1.

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