is dark matter, whose nature we do not understand. And at the center of most or all galaxies lies a supermassive black hole. Thus something as common as a galaxy is both exotic and mysterious. The stars in spiral galaxies like ours are arrayed in two main components: a nearly spherical and slowly rotating “bulge” and a thin and rapidly rotating “disk” (which also contains the gas clouds that can be used to form new stars). Galaxies exhibit a bewildering array of shapes and sizes that are determined largely by the mass of the halo of dark matter surrounding them. Besides spirals, there are ellipticals, three-dimensional balls that formed most of their stars early on, and so have no gas/star disks and little star formation today; and irregulars, tiny galaxies with an abundance of gas and plenty of star formation today.

The lives of galaxies are determined by both nature and nurture, that is, by processes internal to the galaxies as well as through the influence of the surrounding environment. The most massive galaxies today would have begun forming in the early universe in the regions of the highest density of dark matter and gas. They later merged with other galaxies of comparable mass (major mergers), scrambling the disks of the merging galaxies into a single nearly spherical bulge component. The collision would also send material raining into the center of the bulge where it could be used to form and grow a supermassive black hole. In contrast, the life story of low-mass galaxies is more sedate. Originating in regions of lower density, they were only slowly supplied with gas, formed their stars gradually over the history of the universe, suffered fewer major mergers, and retained their disk-like form to the present day. These different life stories explain the strong dichotomy in the observed properties of the high- and low-mass galaxies.

Internal processes in galaxies are complex and affect their ability to make new stars. Supernovae from the explosive deaths of short-lived massive stars violently heat the surrounding gas (see Box 2.3). If the rate of such supernova explosions is high enough, they can act together to expel much of the galaxy’s gas supply (see in Box 2.4 Figure 2.4.4). This will have a more severe impact on low-mass galaxies: their gravity is so weak that material can be easily ejected from them. This may explain why dark matter halos with low mass contain so few stars and so little gas today. The role played by the supermassive black hole is instead important for the lives of the most massive galaxies (which contain the most massive black holes). The energy released by the black hole during periods of intense eruptions can prevent new gas from being captured by the galaxy, explaining why the most massive galaxies are no longer forming stars.

Understanding the details of galaxies and their interstellar gas, dust, and stars requires a community of astronomers to study stellar populations, the dynamics of galaxies and clusters, interstellar and intergalactic gas, stars with a range of properties such as high and low metallicities, and stellar streams resulting from tidal interactions of galaxies, as well as studies of the wide range of galaxies around

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