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 supermassive black hole (SMBH) at the center of the host galaxy.

Earth’s Moon is quite distinct from other moons in the solar system, in being a comparable size to Earth. We explore the theory that a giant impact in the chaotic early solar system led to the Moon’s formation, and bombardment by ice-laden asteroids provided the abundant water we find on our planet. Further, we find that Earth’s magnetic field shields us from solar wind protons, that protect our atmosphere from being stripped away. The icy moons of Jupiter and Saturn are the best targets for exploring if life exists elsewhere in the solar system.

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.

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.

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.