‘Galaxy formation’ is the title of this book and so we should summarise what is known about the properties of galaxies. This is not a trivial business because normal galaxies are complex, many-body systems. Typically, a galaxy can consist of hundreds of millions or billions of stars, it can contain considerable quantities of interstellar gas and dust and can be subject to environmental influences through interactions with other galaxies and with the intergalactic gas. Star formation takes place in dense regions of the interstellar gas. To complicate matters further, it is certain that dark matter is present in galaxies and in clusters of galaxies and the dynamics of galaxies are largely dominated by this invisible dark component. Its nature is, however, unknown.
Galaxy Formation. (2008). Galaxy Formation. Springer Berlin Heidelberg. https://doi.org/10.1007/978-3-540-73478-9