The post-main-sequence evolution of stars with higher initial mass (>8 solar masses) has some distinct differences from those of solar and intermediate-mass stars. We show how multiple-shell burning can lead to core-collapse supernovae, which are important in generating elements heavier than iron. Some supernovae can lead to the curious stellar endpoints of neutron stars and black holes.

What are the key physical properties we can aspire to know about a star? In this chapter we consider the properties of stars, identifying first what we can directly observe about a given star: position on the sky, apparent brightness, color/spectrum. When these observations are combined with a clear understanding of some basic physical principles, we can infer many of the key physical properties of stars. We also make a brief aside to discuss ways to get our heads around the enormous distances and timescales we encounter in astrophysics.

Radiation generated in the deep interior of a star undergoes a diffusion between multiple encounters with the stellar material before it can escape freely into space from the stellar surface. We define the optical depth by the number of mean free paths a photon takes from the center to the surface. This picture of photons undergoing a random walk through the stellar interior can be formalized in terms of a diffusion model for radiation transport in the interior.

Exoplanets are planets orbiting stars other than our sun. While some have now been detected (or confirmed) by direct imaging, most exoplanet detections have been made via two other more-indirect techniques, known as the radial-velocity and transit methods. These methods have analogs in the study of stellar binary systems, as outlined in Chapter 10. We explore the population of known exoplanets and how we must compensate for observational biases inherent in each of these techniques.

We start with some of the historical work on measuring distances to galaxies, leading to the Hubble (or Hubble–Lemaîe) law, a linear proportionality between recession velocity and and a galaxy’s distance, with a proportionality constant known as the Hubble constant. For more distant galaxies, it becomes increasingly difficult to detect and resolve even giant stars like Cepheid variables as individual objects, limiting their utility in testing the Hubble law to larger distances and redshifts. For much larger distances, an important alternative method is the Tully–Fisher relation.

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 “superclusters.” 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.

The close proximity of the Sun, and its extreme apparent brightness, makes it by far the most important star for lives here on Earth. In modern times we have access to powerful telescopes, both on the ground and in space, that observe and monitor the Sun over a wide range of wavelength bands. These vividly demonstrate that the Sun is, in fact, highly structured and variable over a wide range of spatial and temporal scales.

The tendency for conservation of angular momentum of a gravitationally collapsing cloud to form a disk gives rise to the disk in our own Galaxy, the Milky Way. We explore the main components, including the disk, bulge, and halo. Studies of Galaxy rotation curves lead us to the existence of “dark matter,” the nature of which is unknown but is detectable through its gravitational interactions with normal, baryonic matter. We finish by exploring the supermassive black hole at the Milky Way’s center.

Stars generally form in clusters from the gravitational contraction of a dense, cold giant molecular cloud. We explore the critical requirement for such contraction, known as the Jeans criterion, and the factors that affect the star formation rates and the initial mass function in star clusters and galaxies. We finish by looking at how the conservation of angular momentum can lead to proto-stellar disks, with important implications for forming planets.

To test which of these models applies to our universe, one needs to extend redshift measurements to large distances, out to several Giga-light years. The most successful approach has been to use white-dwarf supernovae (SN type Ia) as very luminous standard candles. One of the greatest surprises of modern astronomy is that the expansion of the universe must be accelerating! This implies there must be a positive, repulsive force that pushes galaxies apart, in opposition to gravity. We dub this force “dark energy.”

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.

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.

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.