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.

Social relationships are crucial to healthy human development. We first learn to relate to others as young children, and the ways in which we learn to attach and relate to others have life-long consequences. In this chapter, we further explore our understanding of social bonds as a key aspect of human development and provide an overview of the different ways that sociologists define social relationships. Then, we discuss the relationship between social support and mental health and consider ways that it is influenced by social statuses such as socioeconomic status, race and ethnicity, and gender identity. As you read, we encourage you to consider these questions: Can you differentiate the perceived from the structural aspects of your relationships? Why do you suppose that the perception of social support matters more than “actual” support received in predicting psychological well-being? Can you give an example of a main effect of social support you have experienced, as well as a buffering effect?

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.

In this chapter we examine the relationship between mental health and terrorism in the social science literature. We expand the study of terrorism to areas such as domestic terrorism and other forms of hate-motivated violence. Both broad theoretical advances and focused conceptual refinements are identified and discussed. There has been a dramatic increase in research focusing on the topics of terrorisms and mental health in the years after the four coordinated September 11 attacks in the United States. Since then, a focus on other forms of hate-motivated intergroup violence has emerged and continues to slowly grow, encompassing domestic terrorism (in various forms) and its impact on others in targeted groups (ethnic, racial, religious, or ideological), across generations, (legacy and historical trauma) or within their communities (community trauma). There has also been a growing focus on positive outcomes, especially resilience. Many of the questions raised by the research remain unanswered. Do the effects of trauma that occurred during the peak in terrorism persist among its victims? Will the emerging crisis in domestic terrorism in Western nations cause researchers to again turn inward to focus on domestic problems or broaden their scope of investigation to examine the trend more globally? Will the recent events of another war in Europe (now focused, at the time of writing, on the escalating Russian invasion of Ukraine) provide a framework to again examine how people respond to heinous attacks on civilians, and whether they succumb to the effects of trauma or remain resilient?

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.

We walk through the different epochs and eras of the universe, going forward in time from the Hot Big Bang. In the earliest universe, radiation (photons) dominated over matter. As the universe cools, electrons are able to recombine with protons, then helium and other light elements were formed in the first few minutes. Cosmic inflation is posited to overcome several problems, but investigations to probe and perhaps confirm inflation are ongoing.

In our everyday experience, there is another way we sometimes infer distance, namely by the change in apparent brightness for objects that emit their own light, with some known power or luminosity. For example, a hundred watt light bulb at close distance appears a lot brighter than the same bulb from far away. Similarly, for a star, what we observe as apparent brightness is really a measure of the flux of light, i.e. energy emitted per unit time per unit area.

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 di usion model for radiation transport in the interior.

The average adult spends a large portion of their time in paid employment. Depending on their circumstances, these people most likely hold other social roles, too; they may be parents, spouses, children, or grandchildren, for example. Expectations from these additional roles can impact their work schedules, impede paid employment demands, or conflict with the identity of being a “worker” more generally. This is particularly the case in situations where individuals have caregiving obligations, either to young children or elderly family members. The space where work and nonwork roles interact is referred to as the work–family interface. When these roles are experienced as competing or conflictual, stress may result, subsequently impacting one’s mental health. However, these stressful experiences are not created equal. For example, domestic responsibilities tend to fall more often on women, creating an unequal distribution of labor and more vulnerability to the stressors created by competing work and family roles for women compared to men. These experiences can be made even worse by intersecting disadvantages such as low socioeconomic status or racial inequalities. Broader contexts may also impact work–family interface situations, like the COVID-19 pandemic, which dramatically changed the lived realities of work and family for a large portion of the population. Many employed parents were forced to work from home. Childcare services for young children were halted and schools temporarily closed. These transitions placed a greater burden on parents to figure out childcare and at-home schooling while maintaining full-time work schedules. These changes had implications for the stressors and mental health experiences of the work–family interface. Students can discuss the various ways that social circumstances may interact with the work–family interface to either amplify or reduce the mental health consequences associated with stressors like work–family conflict. They may also discuss how workplaces, organizations, and/or employers could implement policies to assist workers in balancing competing work and family responsibilities.

Compared to 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.

Although many disciplines study mental illness, not all disciplines think of mental illness in the same way. This chapter considers two things: (1) how different disciplines (e.g., sociology versus psychiatry) and actors (e.g., clinicians versus the public) approach the concept of mental illness, and (2) how decisions regarding what mental illness is affect conclusions regarding its consequences, determinants, and treatability. Although decisions regarding what mental illness is are consequential, they are often made on an ad hoc basis, on narrow psychometric grounds, or on the practical interests of a profession, discipline, or person. This chapter outlines some of these decisions and the interests that inform them, evaluating categorical versus dimensional approaches and lay diagnostic instruments. In addition, it outlines more recent alternatives to conventional measures, including the network approach to symptoms. Long-standing controversies regarding outcomes in mental health research are unlikely to be resolved anytime soon, but the topic of mental illness is no less significant for being unsettled. For discussion: What is the optimal approach to studying mental health problems? What is the best way to think about an “optimal” approach, given many competing conceptions of mental disorders?