A spiral galaxy is a type of galaxy originally described by Edwin Hubble in his 1936 work The Realm of Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are surrounded by a much fainter halo of stars, many of which reside in globular clusters.

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.

Marc S. Seigar, University of Minnesota Duluth
Marc Seigar is a Professor of Astrophysics and Associate Dean of the Swenson College of Science and Engineering at the University of Minnesota Duluth (UMD). He is also the current director of the Marshall W. Alworth Planetarium at UMD. Prior to his arrival at UMD, he worked as a Professor of Astrophysics at the University of Arkansas at Little Rock, a Project Scientist at the University of California, Irvine, and a Staff Astronomer at the United Kingdom Infrared Telescope. Professor Seigar has published numerous papers and conference proceedings articles in the field of galaxy dynamics, spiral structure, and dark matter. He is the author of Dark Matter in the Universe, IOP Concise Physics, A Morgan & Claypool Publication, 2015.

Table of Contents
1. The Discovery of Spiral Galaxies in the Universe
2. The Classification of Galaxies
3. Density Wave Theories of Spiral Structure
4. Other Theories of Spiral Structure
5. Star Formation in Spiral Galaxies
6. Spiral Structure and Its Connection with Black Holes and Dark Matter
7. Concluding Remarks