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However, although stars are generally in hydrostatic equilibrium, all stars are also variable. We can distinguish broad families of variation. The most spectacular variation involves catastrophic changes such as supernova explosions. Some stellar variability arises from external influences, such as accretion of mass from a disk or a companion.
In addition, even modest rotation rates can cause significant alterations to the internal structure of stars. Finally, knowing how stars are spun up and spun down is important if we are to study a star’s complete history, from a slowly whirling gas cloud to a swiftly spinning white dwarf or a millisecond pulsar.
Since this was about 50 times the Kelvin–Helmholtz time for the Sun, a non-gravitational source of energy was obviously required to keep the Sun shining over the age of the solar system. A hint of what that energy source could be was provided by the physicist Francis Aston in 1920.
Up to this point, we have mainly been treating stars as if they exist in splendid isolation. However, stars are frequently found in binary systems, with two stars orbiting their barycenter. If the stars are sufficiently close to each other, then they will be tidally distorted, destroying the spherical symmetry of the standard equations of stellar structure.
Since all stars other than the Sun are at a distance that is large compared to their diameter, discerning their detailed structure is challenging. In this chapter, we start with the Hoyle-ish assumption that a star is a pretty simple structure: a static, isolated sphere.
This new graduate textbook adopts a pedagogical approach to contemporary cosmology that enables readers to build an intuitive understanding of theory and data, and of how they interact, which is where the greatest advances in the field are currently being made. Using analogies, intuitive explanations of complex topics, worked examples and computational problems, the book begins with the physics of the early universe, and goes on to cover key concepts such as inflation, dark matter and dark energy, large‑scale structure, and cosmic microwave background. Computational and data analysis techniques, and statistics, are integrated throughout the text, particularly in the chapters on late-universe cosmology, while another chapter is entirely devoted to the basics of statistical methods. A solutions manual for end-of-chapter problems is available to instructors, and suggested syllabi, based on different course lengths and emphasis, can be found in the Preface. Online computer code and datasets enhance the student learning experience.