In reality., stars are not perfect blackbodies, and so their emitted spectra don’t depend solely on temperature, but instead contain detailed signatures of key physical properties like elemental composition. For atoms in a gas, the ability to absorb, scatter, and emit light can likewise depend on the wavelength, sometimes quite sharply. We find that the discrete energies levels associated with atoms of different elements are quite distinct. We introduce the stellar spectral classes (OBAFGKM).

As a star ages, more and more of the hydrogen in its core becomes consumed by fusion into helium. Once this core hydrogen is used up, how does the star react and adjust? Stars at this post-main-sequence stage of life actually start to expand, eventually becoming much brighter giant or supergiant stars, shining with a luminosity that can be thousands or even tens of thousands that of their core hydrogen-burning main sequence. We discuss how such stars reach their stellar end-points as planetary nebulae or white dwarfs.

This chapter considers stellar ages. Just how old are stars such as 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.

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.

As a basis for interpreting observations of binary systems in terms of the orbital velocity of the component stars, we review the astrometric and spectrometric techniques used to measure the motion of stars through space. Nearby stars generally exhibit some systematic motion relative to the Sun, generally with components both transverse (i.e., perpendicular to) and along (parallel to) the observed line of sight.

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 such as 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.

Hubble’s law gives us the simple and obvious interpretation that we currently live in an expanding universe. The inverse of Hubble’s constant defines the “Hubble time,” which effectively marks the time in the past since the expansion began. More realistically, one would expect the universe expansion to be slowed by the persistent inward pull of gravity from its matter. We consider how various theoretical models for the universe connect with the observable redshift that indicates its expansion.

We have seen how a star’s color or peak wavelength indicates its characteristic temperature near the stellar surface. But what about the temperature in the star’s deep interior? Intuitively, we expect this to be much higher than at the surface, but under what conditions does it become hot enough to allow for nuclear fusion to power the star’s luminosity? And how does it scale quantitatively with the overall stellar properties, such as mass, radius, and luminosity? To answer these questions, we identify two distinct considerations.

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.