This swarm of facts can be rearranged into a crude but somewhat more succinct picture: the structure of the chromosphere is profoundly affected by the interior structure of the star as well as its gross parameters (e.g., mass, rotation rate), and the chromosphere in turn profoundly influences the nature and variability of the emergent spectrum, particularly in the ultraviolet. Understanding chromospheric activity and variability is therefore essential to a complete understanding of the physics and evolution of a star and, by virtue of the ionizing ability of the short wavelength radiation it emits, to an understanding of the variability of the heliosphere and Earth’s atmosphere.
Studies of the chromospheric activity of stars have benefited greatly from complementary studies of the Sun, to the point that a review of stellar chromospheric activity would be woefully incomplete without devoting significant space to solar studies. While the stars provide a laboratory for deducing chromospheric properties across a range of masses and ages, the Sun’s proximity allows high-resolution observation of one (hopefully typical) chromosphere, which we can use as a springboard to test hypotheses about stellar chromospheres in general, and as a vital picture of what the unresolved stellar structures may look like. Therefore, although I use the terms solar and stellar to mean “Sun” and “other than Sun”, I will take care not to consider solar and stellar studies in isolation.
These considerations open a number of portals into an enormous literature, so this review could be of deadly proportions. To keep things manageable, I have condensed the material into three sections, each addressing one of three broad questions.
In Section 2, I ask the deceptively simple question: What is a chromosphere? This encompasses the definition of a few essential terms.
In Section 3, I ask: What does a chromosphere look like? Here I summarize the development of chromospheric activity research, both theoretical and observational, since about 1960, with the general purpose of showing how our understanding of chromospheric structure has evolved from the classical, time- and space-averaged view to today’s dynamic picture.
In Section 4, I address the question: How does a chromosphere vary with time? With an emphasis on long-term, synoptic observations of Sun-like stars, I review how activity is manifested in the chromosphere, how it evolves, and the astrophysical as well as practical implications of this variability.
Each of these sections has several subsections that deal with a particular aspect of the question under consideration, covering the historical context as well as the present state of our understanding. Throughout, I attempt to point readers to recent work as well as the fundamental references, given the relative ease today of searching both backward and forward through the literature using online resources. Anecdotes and further entry points into the literature are provided in the Going further subsections that conclude each of Sections 2, 3, and 4.
Finally, in Section 5, I revisit the question of what constitutes a chromosphere with the more complete perspective that the previous sections provide, and offer a few thoughts about productive lines of work in the next decade.
Per the guidelines for a Living Review, I assume throughout that the reader has at least a beginning graduate training in astrophysics, but is not as afflicted by an interest in chromospheres as the author or those cited herein. The sections hopefully are both useful summaries of the literature for stellar workers and readable introductions for non-specialists and students. (For each interesting study and result cited, I must apologize to the authors of the 20 or 30 complementary references omitted or overlooked.) If by the end one has a general idea of what a chromosphere looks like, how it varies with time, and what researchers are working on today, I have achieved the intended result.
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