The Hertzsprung-Russell Diagram and the Lives of Stars

hertzsprung-russell diagram
This version of the Hertzprung-Russell diagram plots the temperatures of stars against their luminosities. The position of a star in the diagram provides information about what stage it is in, as well as its mass and brightness. European Southern Observatory

Have you ever wondered how astronomers sort stars into different types? When you look up into the night sky, you see thousands of stars., and, like astronomers do, you can see that some are brighter than others. There are whitish-colored stars, while some look slightly red or blue. If you take the next step and graph them onto an x-y axis by their color and brightness, you start to see some interesting patterns develop in the graph.

Astronomers call this chart the Hertzsprung-Russell Diagram, or the H-R Diagram, for short. It may look simple and colorful, but it's a powerful analytical tool that helps them not only classify stars into various types, but reveals information about how they change over time. 

The Basic H-R Diagram

Generally, the H-R diagram is a "plot" of temperature vs. luminosity. Think of "luminosity" as a way to define the brightness of an object. Temperature helps define something called a star's spectral class, which astronomers figure out by studying the wavelengths of light that come from the star. So, in a standard H-R diagram, spectral classes are labeled from hottest to coolest stars, with the letters O, B, A, F, G, K, M (and out to L, N, and R). Those classes also represent specific colors. In some H-R diagrams, the letters are arranged across the top line of the chart. Hot blue-white stars lie to the left and the cooler ones tend to be more toward the right side of the chart.

The basic H-R diagram is labeled like the one shown here. The nearly diagonal line is called the main sequence and nearly 90 percent of the stars in the universe lie along that line or did at one time. They do this while they are still fusing hydrogen to helium in their cores. When that changes, then they evolve to become giants and supergiants.

On the chart, they end up in the upper right corner. Stars like the Sun may take this path, and ultimately shrink down to become white dwarfs, which appear in the lower left part of the chart.

The Scientists and Science Behind the H-R Diagram

The H-R diagram was developed in 1910 by the astronomers Ejnar Hertzsprung and Henry Norris Russell. Both men were working with spectra of stars — that is, they were studying the light from stars using spectrographs. These instruments break down the light into its component wavelengths. The way the stellar wavelengths appear gives clues to the chemical elements in the star, as well as its temperature, its motion, and its magnetic field strength. By plotting the stars on the H-R diagram according to their temperatures, spectral classes, and luminosity, it gave astronomers a way to classify the stars.

Today, there are different versions of the chart, depending on what specific characteristics astronomers want to chart. They all have the same layout, however, with the brightest stars stretching up toward the top and veering off to the top left, and a few in the lower corners.

The H-R diagram uses terms that are familiar to all astronomers, so it's worth learning the "language" of the chart.

You have probably heard the term "magnitude" when applied to stars. It's a measure of a star's brightness. However, a star might appear bright for a couple of reasons: 1) it could be fairly close and thus look brighter than one farther away; and 2) it could be brighter because it's hotter. For the H-R diagram, astronomers are mainly interested in a star's "intrinsic" brightness — that is, its brightness due to how hot it is. That's why you often see luminosity (mentioned earlier) plotted along the y-axis. The more massive the star is, the more luminous it is. That's why the hottest, brightest stars are plotted among the giants and supergiants in the H-R Diagram.

Temperature and/or spectral class is, as mentioned above, derived by looking at the star's light very carefully. Hidden within its wavelengths are clues about the elements are in the star.

Hydrogen is the most common element, as shown by the work of astronomer Cecelia Payne-Gaposchkin in the early 1900s. Hydrogen is fused to make helium in the core, so you would expect to see helium in a star's spectrum, too. The spectral class is very closely related to a star's temperature, which is why the brightest stars are in classes O and B. The coolest stars are in classes K and M. The very coolest objects are also dim and small, and even include brown dwarfs.

One thing to keep in mind is that the H-R diagram is not an evolutionary chart. At its heart, the diagram is simply a chart of stellar characteristics at a given time in their lives (and when we observed them). It can show us what stellar type a star can become, but it doesn't necessarily predict the changes in a star. That's why we have astrophysics — which applies the laws of physics to the lives of the stars.

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Petersen, Carolyn Collins. "The Hertzsprung-Russell Diagram and the Lives of Stars." ThoughtCo, Mar. 22, 2017, Petersen, Carolyn Collins. (2017, March 22). The Hertzsprung-Russell Diagram and the Lives of Stars. Retrieved from Petersen, Carolyn Collins. "The Hertzsprung-Russell Diagram and the Lives of Stars." ThoughtCo. (accessed March 21, 2018).