Diagramming the Lives of Stars

The stars are the most amazing physical engines in the universe. They radiate light and heat, and they create chemical elements in their cores. However, when observers look at them in the night sky, all they see are thousands of pinpoints of light. Some appear reddish, others yellow or white, or even blue. Those colors actually give clues to the temperatures and ages of the stars and where they are in their life-spans. Astronomers "sort" stars by their colors and temperatures, and the result is a famous graph called the Hertzsprung-Russell Diagram. The H-R diagram is a chart that every astronomy student learns early on.

Learning 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 is something we're all familiar with, generally as the heat of an object. It helps define something called a star's spectral class, which astronomers also 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 as the one shown here. The nearly diagonal line is called the main sequence. Nearly 90 percent of the stars in the universe exist along that line at one time in their lives. They do this while they are still fusing hydrogen to helium in their cores. Eventually, they run out of hydrogen and start to fuse helium. That's when they evolve to become giants and supergiants. On the chart, such "advanced" stars end up in the upper right corner. Stars like the Sun may take this path, and then 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 by using spectrographs. Those instruments break down the light into its component wavelengths. The way the stellar wavelengths appear gives clues to the chemical elements in the star. They can also reveal information about its temperature, motion through space, and its magnetic field strength. By plotting the stars on the H-R diagram according to their temperatures, spectral classes, and luminosity, astronomers can classify stars into their different types.

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

The Language of the H-R Diagram

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

•  It could be fairly close and thus look brighter than one farther away
•  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 actually is. That's why luminosity (mentioned earlier) is 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 are, as mentioned above, derived by looking at the star's light very carefully. Hidden within its wavelengths are clues about the elements that 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 that's why astronomers 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 can show us what stellar type a star can become, but it doesn't necessarily predict any changes in a star. That's why we have astrophysics — which applies the laws of physics to the lives of the stars.