galaxies suggest that a black hole is present before the formation of a massive galactic halo. ALMA and the EVLA may provide more such examples.

But we cannot answer these questions definitively yet, because we do not have a robust theory for how supermassive black holes form. In the coming decade we expect a major breakthrough in our understanding. A space-based observatory to detect gravitational radiation will allow us to measure the rate at which mergers between less-massive black holes contributed to the formation process. Are the supermassive black holes we can now detect only the tip of the iceberg (the biggest members of a vast unseen population)? Deep imaging surveys in the near-infrared and X-ray, with follow-up spectroscopy with JWST and ground-based extremely large telescopes, will detect and study the growth of the less massive objects through the capture of gas and accompanying emission of electromagnetic radiation. These surveys will also allow us to search for such black holes at even earlier eras: back to the end of the dark ages.

The Origin of Stars and Planets

Looking up on a clear night from a dark location, we see that the sky is full of stars. Telescopic observations by Galileo revealed that the Milky Way’s white band traversing high across the summer and fall sky can be resolved into countless stars. Gazing upon the winter constellation of Orion, the sharp eye will note the fuzzy Orion Nebula (see in Box 2.4 Figure 2.4.3) with its nursery of stars born “yesterday” in cosmic time—not long after the first humans walked. Nearby is the famed Pleiades star cluster—formed when dinosaurs still roamed Earth. In contrast, some stars of our galaxy are nearly as old as the universe itself. The story of how successive generations of stars form out of the gas and dust in the interstellar medium in both benign and exotic environments is fundamental to our understanding of, on the larger scale, the galaxies in which stars reside and, on the smaller scale, the planetary systems they might host.

What was it about the Sun’s birth environment or its star formation process that determined the final properties of our solar system versus that of other planetary systems? (See Box 2.2.) How and on what timescale did the solar mass build up, and how much gas and dust were left over for planet formation? How rapidly did the high-energy radiation of young stars disperse their gas disks, ending the phase of major planet formation? Do all environments yield the same mass distribution of stars, and what determines the lower and upper mass limits in the distribution (Figure 2.8)? What is the star formation history of our galaxy in particular, and of galaxies in general? Does star formation regulate itself, or are there external factors at work?

A key aim of studies in the next decade is to understand, through both observations and theory, the process of star formation over cosmic time. Beginning near



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