Hertzsprung-Russell Diagram

The nineteenth century saw the development of a powerful technique called spectroscopy. This technique involves the use of an instrument called a spectrograph, which disperses light passing through it into its component colors in the same way that an ordinary prism does. Indeed, many spectrographs in use today have prisms as one or more of their components.

When sunlight or starlight passes through a spectrograph and is dispersed, the resulting spectrum has many narrow, dark lines in it. These lines are called absorption lines. A line occurs only at a certain wavelength and is caused by the presence of a specific element in the star's atmosphere. They are called absorption lines because they are caused when elements in the star's atmosphere absorb some of the light radiating outward from the star's surface. Less light escapes from the star's atmosphere where there is a line than in other portions of the spectrum, so the line looks dark.

Different stars have different patterns of absorption lines, and the pattern present in a particular star depends on the star's surface temperature. For example, hydrogen, the most common element in stars, produces several very strong absorption lines in the visual part of the spectrum—but only if the star's temperature is about Figure 2. Note how the upper end of the main sequence has a hook, with stars seeming to peel off to the right. This is because massive, bright stars burn their fuel faster and die earlier than less massive, cooler ones. Since all the stars in the Pleiades got their start in life at the same time, the fast-burning O stars have evolved into giants while the more sedate F and G stars are still on the main sequence. Because we know about how long the main sequence phase for different stars lasts, the location of the end of the main sequence, or turn-off point, tells how old the Pleiades are: about 100,000,000 years. Illustration by Hans & Cassidy. Courtesy of Gale Group. 10,000K (17,541°F [9,727°C]). If the star is much hotter, say 20,000K (35,541°F [19,727°C]), the hydrogen atoms can no longer absorb as much light in the visual spectrum, so the lines are weaker. Very cool stars also have weaker hydrogen lines.

In the early 1900s, a group of astronomers led by Annie Jump Cannon at the Harvard Observatory began to classify stellar spectra. They grouped stars into spectral classes, with all the stars in a given spectral class having similar patterns of lines. This is just like the way that biologists classify animals into groups such as families and species. Spectral classes are denoted by letters, and the main ones, in order of decreasing surface temperature, are O, B, A, F, G, K, and M. You can remember this by the mnemonic "Oh Be A Fine Girl (or Guy), Kiss Me!" Because stars have many elements in their atmospheres (hydrogen, helium, calcium, sodium, and iron, to name only a few), their spectra can have thousands of lines. To accommodate this complexity, the spectral classes are each divided into 10 subclasses, denoted Figure 3. The cluster M67 however, is much older than the Pleiades. Its O, B, A, and F stars are already well into gianthood. It takes F stars several billion years to burn all their hydrogen, so M67 must be around five billion years old. Illustration by Hans & Cassidy. Courtesy of Gale Group. by numbers. For example, there are F0 stars, F1 stars, and so on until F9; the next class is G0. The Sun, with a surface temperature of 5,800K (9,981°F [5,527°C]), is a G2 star.

The first H-R diagrams were created independently in the early 1900s by the astronomers Ejnar Hertzsprung and Henry Norris Russell. Russell's graph had spectral class plotted along the x-axis and a quantity related to luminosity (or brightness) plotted along the y-axis. Figure 1 is such a graph.