A stellar perspective on chromospheres

The Sun is not alone in hosting a chromosphere: virtually all convective stars (Ftypes and later) possess them. Properties can vary wildly from object to object. Historically, three key systemic behaviors were recognized. First is the strong preference of chromospheres for the cool half of the H–R diagram. Second is the so-called rotation-age-activity connection (“Skumanich law”). Third is the Ca II H & K emission width–luminosity relation (“Wilson-Bappu effect”). In the modern era of ultraviolet and X-ray telescopes in space, additional phenomenology has been recognized; mainly concerning energetic relationships between chromosphere and corona, importance of atmospheric dynamics, and the curious “buried coronae” of red giant stars. Collectively, these pieces of evidence hint that “relentlessly dynamic” stellar chromospheres are the rule, not the exception.