 Star spectra has one more characteristic called luminosity class that enable us to determine whether a star is on the main sequence or not. This is the key to using the HR diagram to determine a star's distance. If you recall, the evolution of a star off the main sequence involves the expansion of the outer layer to gargantuan proportions. This makes the density of the gas in the outer layer of a giant much less than the density in the outer layer of a star on the main sequence. It turns out that the photon absorption characteristics of closely packed atoms makes the spectral lines fuzzier. For a given spectral classification, the fuzzier the spectral line, the smaller the star. Roman numerals are used to identify luminosity classes. Our sun is class 5, a main sequence star. We'll use Betelgeuse to illustrate how star spectra works with the HR diagram to determine a star's distance. First, we use the star's color, temperature and spectra to find its point on the horizontal axis. Looking up the vertical luminosity axis, we see Betelgeuse could either be a main sequence star or a giant. Examining the luminosity class, we see that it is very sharp, implying that Betelgeuse is a supergiant. Now drawing the line to the vertical axis, we see that the star's intrinsic luminosity is 120,000 times greater than our sun's luminosity. Using the apparent luminosity and using the inverse square law, we get the distance. If stars everywhere behave like the stars in our neighborhood, then the HR diagram can show us how far away they are. Astronomers call this technique spectroscopic parallax, but we'll just stick with HR diagram. Thank you. Thank you. Bye. Bye. Bye. Bye. Bye. Bye. Bye. Bye.