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.

This chapter explores observations and properties of quasars, which were first observed in the 1960s as point-like sources that emit over a wide range of energies from the radio through the IR, visible, UV and even extending to the X-ray and gamma-rays. They are now known to be a type of active galactic nucleus thought to be the result of matter accreting onto a super-massive black hole (SMBH) at the center of the host galaxy.

It turns out that stellar binary (and even triple and quadruple) systems are quite common. In Chapter 10 we show how we can infer the masses of stars through the study of stellar binary systems. For some systems, where the inclination of orbits can be determined unambiguously, we can infer the masses of the stellar components, as well as the distance to the system. Together with the observed apparent magnitudes, this also gives the associated luminosities of their component stars.

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.

Mass is clearly a physically important parameter for a star, as it will determine the strength of the gravity that tries to pull the star’s matter together. We discuss one basic way we can determine mass, from orbits of stars in stellar binaries, and see the range of stellar masses. This leads us to the virial theorem, which describes a stably bound gravitational system.

We conclude our discussion of stellar properties by considering ways to infer the rotation of stars. All stars rotate, but in cool, low-mass stars like the Sun the rotation is quite slow. In hotter, more-massive stars, the rotation can be more rapid, with some cases (e.g., the Berillium stars) near the "critical" rotation speed at the star’s surface.

This chapter considers stellar ages. Just how old are stars like the Sun? What provides the energy that keeps them shining? And what will happen to them as they exhaust various available energy sources? We show that the ages and lifetimes of stars like the Sun are set by long nuclear burning timescales and the implications that high-mass stars should have much shorter lifetimes than low-mass stars.

The timescale analyses in Chapter 8 show that nuclear fusion provides a long-lasting energy source that we can associate with main sequence stars in the H-R diagram. This chapter addresses the following questions: What are the requirements for H to He fusion to occur in the stellar core? And how is this to be related to the luminosity vs. surface temperature scaling for main sequence stars? In particular, how might this determine the relation between mass and radius? What does it imply about the lower mass limit for stars to undergo hydrogen fusion?

Much as stars within galaxies tend to form within stellar clusters, the galaxies in the universe also tend to collect in groups, clusters, or even in a greater hierarchy of clusters of clusters, known as "super-clusters." Plots of galaxy positions versus redshift distance reveal the large-scale structure of the universe as a "cosmic web," with galaxies lying along extended, thin "walls" and densely clustered intersections, surrounded by huge voids with few or no galaxies in between.

Observations of binary systems indicate that main sequence stars follow an empirical mass-luminosity relation L ~ M^3. The physical basis for this can be understood by considering the two basic relations of stellar structure, namely hydrostatic equilibrium and radiative diffusion. In practice, the transport of energy from the stellar interior toward the surface sometimes occurs through convection instead of radiative diffusion; this has important consequences for stellar structure and thus for the scaling of luminosity.