Deep Inside a Star
The great majority of astronomers, both amateur and professional, would probably agree that the most interesting part of the Hertzsprung–Russell diagram is the bit that lies above the main sequence. Here we find most of the intrinsically variable single stars, and what’s more, the light variations of these stars are largely due to things that are going on beneath their surface layers. In some cases, for example, with Cepheid variable stars, the process is now fairly well understood, but with the red giant variables, such as Mira-type stars and semi-regular variables, the situation is much less clear. Professionals are using supercomputers to model the kinds of things that go on in the vast envelopes of these stars, but here, perhaps, more than anywhere else, the role of amateur variable-star observers, who make regular observations of these stars, is pivotal in furthering our understanding of what goes on in the upper right-hand corner of the HR diagram.