The HR diagram becomes immensely valuable as a tool to determine distances to far away stars when the additional information provided by classification of spectral line widths is considered. Luminosity classes Ia through V have been calibrated as a function of spectral type and may be added to the HR diagram (see Figure 1).
HR diagram with luminosity classes added.
From the spectrum of a star, astronomers can obtain both the spectral type (OBAFGKM) and luminosity class (Ia‐V). Therefore, where the star is placed in the HR diagram is known, but most importantly, the absolute magnitude of the star can be read off the vertical scale on the diagram. A comparison of the absolute brightness with the star's measured apparent brightness gives the distance to the star. This technique of spectroscopic parallax will give the distance to any star for which a spectrum can be obtained. There are other techniques for getting distances to very distant objects, but this is the most important one and permits distance determination even into nearby galaxies several million parsecs away.