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Spiral galaxies are named for the spiral structures that extend from the center into the galactic disk. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disk because of the young, hot OB stars that inhabit them. Roughly two-thirds of all spirals are observed to have an additional component in the form of a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin. The proportion of barred spirals relative to their barless cousins has changed over the history of the Universe, with only about 10% containing bars about 8 billion years ago, to roughly a quarter 2.5 billion years ago, until present, where over two-thirds of the galaxies in the visible universe have bars.
This book focuses on why these disk-shaped (or spiral) galaxies have spiral arms. Why do these structures exist? Why are they so stable? And what is the connection between the spiral arms and the star formation that is seen within them? In this book you will find the answers to these questions and more.