Transcript: Stars of a particular photospheric temperature can have vastly different sizes and luminosities in an HR diagram. For example at a temperature of about three thousand Kelvin there is Proxima Centauri, a low mass main sequence star only three percent the size of the Sun, Aldebaran, that’s twenty times larger than the Sun, and Betelgeuse, a red supergiant a thousand times the Sun’s size. Yet spectroscopy alone can distinguish between these situations. This is because spectral lines reveal the diffuse nature of the atmosphere. In the more diffuse atmospheres of the larger stars there are fewer collisions, and the lines are narrower. In the denser atmospheres of the smaller stars there are more collisions, and the lines are broader so line broadening gives an information about luminosity class and astronomers can distinguish between main sequence stars, giants, and supergiants.