us, from the smallest dwarf galaxies to the largest spirals and ellipticals. From the analysis of stellar populations, we can study how the Milky Way assembled.

While we have a rather good description of the properties of galaxies in the present-day universe, we have far less information about how these properties have changed over the 13.7-billion-year history of the universe. The galaxies we can observe in detail teach us of the complex interplay among the components of normal and dark matter, constrained by the physical laws of the cosmos. A high priority in the coming decade will be to undertake large and detailed surveys of galaxies as they evolve across the wide interval of cosmic time—to have a movie of the lives of galaxies rather than a snapshot. See Box 2.4.

As described above, the lives of galaxies and the supermassive black holes at their centers seem to be inextricably linked. Two of the major goals of the coming decade are to understand the cosmic evolution of black hole ecosystems—the intense interplay between the black holes and their environments—and to figure out how these extremely powerful “engines” function. Black hole masses will be measured by JWST and ground-based optical and radio telescopes. Observations of black holes in the X-ray and gamma-ray regimes offer uniquely powerful insights. For example, the Fermi Gamma-ray Space Telescope as well as the ground-based atmospheric Čerenkov telescopes such as VERITAS are constantly reporting new and powerful variations of emission, in both the energy and the time domains, from large numbers of these systems over the whole sky and from cosmological distances. The Chandra and XMM-Newton X-ray observatories are being used to measure the environmental impact of energy injection from the black hole and also to give us a glimpse of matter as it swirls inexorably inward toward the event horizon at the very edge of the black hole. Future more powerful X-ray observatories will provide detailed maps of these processes, so that we can directly witness the accretion of matter (by which black holes grow) and can also understand the impact they have on the lives of their “host” galaxy.

Stars

Stars are the most observable form of normal matter in the cosmos. They have produced about 90 percent of all the radiant energy emitted since the big bang (with black holes accounting for most of the balance). Through the nuclear reactions that power them, they have taken the primordial hydrogen and helium produced during the big bang, converted this into heavier elements like carbon, oxygen, and iron, and then dispersed this material so that it can be incorporated in subsequent generations of stars and of the planets that accompany them (see in Box 2.4 Figure 2.4.3). Such recycling is proceeding continuously within galaxies like our own.

We now have a mature theory for the structure and evolution of stars. This theory is based on a synthesis of known physical processes (nuclear reactions; the



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