The disk formation process of the previous chapter forms the basis for the “Nebular Model” for the formation of planetary systems, including our own solar system. As a proto-stellar cloud collapses under the pull of its own gravity, conservation of its initial angular momentum leads naturally to formation of an orbiting disk, which surrounds the central core mass that forms the developing star. We then explore the “ice line” between inner rocky dwarf planets and outer gas giants.

There is little doubt that the climate around the world is changing, and changing at a disturbing rate. Summers are now warmer on average than a few decades ago and wildfires are more common and more severe. The United States is experiencing more extreme weather events these days – more flash flooding in some parts of the country, more dry spells in others, and recurring bone-chilling temperatures in winter, which are said to stem from warming trends in the Arctic. Polls show that the vast majority of Americans see climate change as a pressing problem. Yet there is no consensus on the next steps to be taken. Some policymakers favor applying taxes to industries that produce greenhouse gasses that contribute to global warming. Others call for the government to set an overall cap on greenhouse gas emissions, and then let industries come up with an arrangement among themselves to stay under cap limits. Still others call for government policies to counteract the disruptive effects of floods, droughts, and extreme temperatures, rather than regulate industries. Advocates for these positions, among others, often marshal sophisticated evidence for their position and dismiss the views of opponents as not only wrong-headed, but morally suspect.

Conflict is at the heart of the national policymaking process in Madison’s Republic, especially conflict within each of the two legislative branches, House and Senate; and conflict between the president and Congress. As every schoolchild learns, a bill cannot become law unless it passes both houses of Congress and is signed by the president. As Madison’s theory in Federalist 51 makes plain, these three institutions are designed to have different interests and different conceptions of the national interest. This idea of dividing institutions from one another by establishing them with different interests, while at the same time making them dependent on one another in order to make national policy is known as the separation of powers. As a result of the separation of powers, conflict and stalemate are regular, often frustrating, features of American national politics.

To understand ways we might infer stellar distances, we first consider how we intuitively estimate distance in our everyday world, through apparent angular size, and/or using our stereoscopic vision. We explain a practical, quite direct way to infer distances to relatively nearby stars, namely through the method of trigonometric parallax. This leads to the definition of the astronomical unit and parsec, and the concept of solid angles on the sky, measured in steradians or square degrees.

Following directly the from the previous chapter, we see that in addition to a shift toward shorter peak wavelength, a higher temperature also increases the overall brightness of blackbody emission at all wavelengths. This suggests that the total energy emitted over all wavelengths should increase quite sharply with temperature. We introduce the Stefan–Boltzmann law, one of the linchpins of stellar astronomy.

Our initial introduction of surface brightness characterized it as a flux confined within an observed solid angle. But actually the surface brightness is directly related to a more general and fundamental quantity known as the “specific intensity.” The light we see from a star is the result of competition between thermal emission and absorption by material within the star.

There is a long and celebrated history of skepticism about political parties in the United States, stretching back to George Washington’s farewell address in which he warned against partisan factions and, of course, to Madison’s Republic, in which majority factions are equated with tyranny. The skepticism of people like Madison and Washington at the founding is echoed by contemporary critics who lament current manifestations of partisanship in national politics ranging from partisan attacks (and defenses) of Supreme Court nominees, to obviously biased information spread by presidential spokespeople (and leaders of the opposing party to the current administration, of whichever party), to the antics on display at national nominating conventions or campaign rallies. Many close observers of American politics worry about the effects of partisan polarization on gerrymandering, policymaking, gridlock, and extremism.

Compared with stars, the region between them, called the interstellar medium or “ISM,” is very low density; but it is not a completely empty vacuum. A key theme in this chapter is that stars are themselves formed out of this ISM material through gravitational contraction, making for a star–gas–star cycle. We explore the characteristics of cold and warm regions of the ISM and their roles in star formation.

We now consider why stars shine with such extreme brightness. Over the long-term (i.e., millions of years), the enormous energy emitted comes from the energy generated (by nuclear fusion) in the stellar core, as discussed further in Chapter 18. But the more immediate reason stars shine is more direct, namely because their surfaces are so very hot. We explore the key physical laws governing such thermal radiation and how it depends on temperature.