The main feature of a spiral or S0 galaxy is its conspicuous extended stellar disk. Stars in the disk of a large spiral galaxy, like our Milky Way, follow nearly circular orbits with very little random motion. Ordered rotation accounts for almost all the energy of motion, with random speeds contributing less than ~5%: the disk is dynamically ‘cold’. In smaller galaxies, random motions are proportionally larger, but most of the disk's kinetic energy is still in rotation. Because the stars have little vertical motion perpendicular to the disk plane, the disk can be quite thin.

Spiral galaxies are distinguished from S0 systems by the multi-armed spiral pattern in the disk. The disks of spiral galaxies still retain some gas, whereas S0 systems have lost their disk gas, or converted it into stars. Both S0 and spiral galaxies can show a central linear bar; in Figure 1.11, the sequence of barred galaxies SB0, SBa, …, SBm runs parallel to the ‘unbarred’ sequence S0, Sa, … Apart from the bar and spiral arms, the stellar disks of large galaxies are usually fairly round; but many smaller systems are quite asymmetric.

We begin this chapter by discussing galaxies with an active nucleus, a compact central region from which we observe substantial radiation that is not the light of stars or emission from the gas heated by them. Active nuclei emit strongly over the whole electromagnetic spectrum, including the radio, X-ray, and γ-ray regions where most galaxies hardly radiate at all. The most powerful of them, the quasars, easily outshine their host galaxies. With luminosities exceeding 1012 L ☉, many are bright enough to be seen most of the way across the observable Universe. But the emitting region may be no bigger than the solar system; its power source is probably the energy released by gas falling into a central black hole. Very luminous active nuclei, such as the quasars, were far more common when the Universe was 20%–40% of its present age than they are today; nuclear activity seems to be characteristic of a galaxy's early life.

In many bright quasars, narrow twin jets are seen to emerge from the nucleus; they are probably launched and kept narrow by strong magnetic fields that build up in the surrounding disk of inflowing matter. In some cases, the jets appear to move outward faster than the speed of light. This is an illusion: the motion is slower than, but close to, light speed.

About half the galaxies in the Universe are found in groups and clusters, complexes where typically half the member galaxies are packed into a region ≲1 Mpc across. Groups and clusters no longer expand with the cosmic flow: mutual gravitational attraction is strong enough that the galaxies are moving inward, or have already passed through the core. Clusters are the denser and richer structures. Within the central megaparsec, they typically contain at least 50 luminous galaxies: those with L ≳ L * ~ 2 × 1010 L ☉. Poorer associations are called groups; they are generally less massive than ~ 1014 M ☉.

Galaxies and clusters are not simply concentrations of galaxies: the fact that a galaxy is a member significantly affects its development. The most common inhabitants of groups are spiral and irregular galaxies. The Local Group is typical; its three bright members are all spirals. Almost all the large galaxies of the Ursa Major group, featured in Figures 5.6, 5.8, and 5.23, are disk systems; there are no more than two ellipticals among the brightest 79 galaxies. Elliptical and S0 galaxies predominate in dense clusters. Curiously, both the most luminous giant ellipticals and the dwarfs with L ≲ 3 × 109 L ☉ are concentrated more strongly into clusters than the midsized ellipticals.

By contrast with groups, which we discuss in Section 7.1, we will see in Section 7.2 that most of the baryonic (‘normal’) matter in galaxy clusters is not in the galaxies themselves.

Our position in the Milky Way's disk gives us a detailed and close-up view of a fairly typical large spiral galaxy. We begin this chapter by looking at the Sun's immediate vicinity. Examining the closest stars gives us a sample of the disk stuff, and we can ask how many stars of each luminosity, mass, composition, and age are present. Combining this information with theories of stellar evolution, we investigate the star-forming past of the solar neighborhood.

In Section 2.2 we venture further afield. Measuring stellar distances allows us to map out the Milky Way's structure: the thin and thick disks, the metal poor halo, and the central bulge. Star clusters, where all the members were born together, with the same initial composition, are especially useful; comparing their color–magnitude diagrams with the predictions of stellar models yields the age and composition jointly with the cluster's distance. We find that the youngest stars belong to the disk, and are relatively rich in elements heavier than helium, whereas the stars of the metal-poor halo are extremely old.

Most of the Milky Way's stars and almost all of its gas lie in the disk, orbiting the center like the planets around the Sun. Section 2.3 deals with the Galaxy's rotation: how we measure it, and how we use it to find the distribution of gas within the disk.

We have drawn for our presentation on the following graduate texts:

J. Binney & S. Tremaine, 1987, Galactic Dynamics (Princeton University Press, Princeton, New Jersey), on the dynamics of galaxies and star clusters; F. Combes, P. Boissé, A. Mazure, & A. Blanchard, Galaxies and Cosmology, 2nd edition (English translation, 2002; Springer, Heidelberg, Germany), covering similar ground to our text; and J. Binney & M. Merrifield, 1998, Galactic Astronomy, 3rd edition (Princeton University Press, Princeton, New Jersey), which gives a comprehensive review of observations of our Galaxy and others.

On a similar level to this book, see S. Phillipps, 2005, The Structure and Evolution of Galaxies Wiley, Chichester, UK) and P. Schneider, Extragalactic Astronomy and Cosmology: An Introduction (English translation, 2006; Springer, Heidelberg/Berlin, Germany).

At a more elementary level, see M. H. Jones & R. J. Lambourne (eds.), 2003, An Introduction to Galaxies and Cosmology (Cambridge University Press and the Open University, Cambridge, UK).

Since the early 1980s, multi-object spectrographs, CCD detectors, and some dedicated telescopes have allowed the mass production of galaxy red shifts. These large surveys have revealed a very surprising picture of the luminous matter in the Universe. Many astronomers had imagined roughly spherical galaxy clusters floating amongst randomly scattered field galaxies, like meatballs in sauce. Instead, they saw galaxies concentrated into enormous walls and long filaments, surrounding huge voids that appear largely empty. The galaxy distribution has been compared to walls of soapy water, surrounding bubbles of air in a basinful of suds; linear filaments appear where the walls of different soap bubbles join, and rich clusters where three or more walls run into each other. A more accurate metaphor is that of a sponge; the voids are interlinked by low-density ‘holes’ in the walls. Sometimes we think of the filaments as forming a cosmic web.

For a star like the Sun in the disk of our Milky Way, the task of finding where it formed is essentially hopeless, because it has already made many orbits about the galaxy, and the memory of its birthplace is largely lost. But the large structures that we discuss in this chapter are still under construction, and the regions where mass is presently concentrated reveal where denser material was laid down in the early Universe.

This text is aimed primarily at third- and fourth-year undergraduate students of astronomy or physics, who have undertaken the first year or two of university-level studies in physics. We hope that graduate students and research workers in related areas will also find it useful as an introduction to the field. Some background knowledge of astronomy would be helpful, but we have tried to summarize the necessary facts and ideas in our introductory chapter, and we give references to books offering a fuller treatment. This book is intended to provide more than enough material for a one-semester course, since instructors will differ in their preferences for areas to emphasize and those to omit. After working through it, readers should find themselves prepared to tackle standard graduate texts such as Binney and Tremaine's Galactic Dynamics, and review articles such as those in the Annual Reviews of Astronomy and Astrophysics.

Astronomy is not an experimental science like physics; it is a natural science like geology or meteorology. We must take the Universe as we find it, and deduce how the basic properties of matter have constrained the galaxies that happened to form. Sometimes our understanding is general but not detailed. We can estimate how much water the Sun's heat can evaporate from Earth's oceans, and indeed this is roughly the amount that falls as rain each day; wind speeds are approximately what is required to dissipate the solar power absorbed by the ground and the air.

Galaxies appear on the sky as huge clouds of light, thousands of light-years across: see the illustrations in Section 1.3 below. Each contains anywhere from a million stars up to a million million (1012); gravity binds the stars together, so they do not wander freely through space. This introductory chapter gives the astronomical information that we will need to understand how galaxies are put together.

Almost all the light of galaxies comes from their stars. Our opening section attempts to summarize what we know about stars, how we think we know it, and where we might be wrong. We discuss basic observational data, and we describe the life histories of the stars according to the theory of stellar evolution. Even the nearest stars appear faint by terrestrial standards. Measuring their light accurately requires care, and often elaborate equipment and procedures. We devote the final pages of this section to the arcana of stellar photometry: the magnitude system, filter bandpasses, and colors.

In Section 1.2 we introduce our own Galaxy, the Milky Way, with its characteristic ‘flying saucer’ shape: a flat disk with a central bulge. In addition to their stars, our Galaxy and others contain gas and dust; we review the ways in which these make their presence known.

Stars travel around the Galaxy, and galaxies orbit within their groups and clusters, under the force of gravity. Stars are so much denser than the interstellar gas through which they move that neither gas pressure nor the forces from embedded magnetic fields can deflect them from their paths. If we know how mass is distributed, we can find the resulting gravitational force, and from this we can calculate how the positions and velocities of stars and galaxies will change over time.

But we can also use the stellar motions to tell us where the mass is. As we discovered in Chapter 2, much of the matter in the Milky Way cannot be seen directly. Its radiation may be absorbed, as happens for the visible light of stars in the dusty disk. Some material simply emits too weakly: dense clouds of cold gas do not show up easily in radio-telescope maps. The infamous dark matter still remains invisibly mysterious. But, since the orbits of stars take them through different regions of the galaxies they inhabit, their motions at the time we observe them have been affected by the gravitational fields through which they have travelled earlier. So we can use the equations for motion under gravity to infer from observed motions how mass is distributed in those parts of galaxies that we cannot see directly.

The Local Group contains roughly three dozen galaxies within a sphere about a megaparsec in radius, centred between the Milky Way and our nearest large neighbor, the Andromeda galaxy M31. Figure 4.1 shows the brighter members. The three most prominent are M31, the Milky Way, and M33; according to the classification of Section 1.3, these are all spiral galaxies. M31 is about 50% more luminous than the Milky Way, while M33 is only 20% as luminous. Between them, these three galaxies emit 90% of the visible light of the Local Group. The only elliptical galaxy is M32, a satellite to M31. The remaining systems are irregular galaxies, or the even less luminous dwarf irregulars, dwarf ellipticals, and dwarf spheroidals. Many of these smaller galaxies are in orbit either around the Milky Way or around M31.

Table 4.1 lists known and probable members of the Local Group within a megaparsec of the Sun. The apparent brightness of each member is generally known to within 10%, except for the Milky Way, where our location within the disk presents special problems. Distances to Local Group galaxies are derived by picking out individual stars, measuring their apparent brightness, and estimating their true luminosity, using methods such as the period–luminosity relation for Cepheid variables. In this way, distances to the ten or so brightest galaxies can be measured to within 10%.

Elliptical galaxies look like simple objects; but they are not. As their name implies, they appear round on the sky; the light is smoothly distributed, and they lack the bright clumps of young blue stars and patches of obscuring dust which are such obvious features of spiral galaxies. Ellipticals are almost devoid of cool gas, except at the very center; in contrast to S0 systems, they have no prominent disk. Their smooth appearance suggests that, like the molecules of air in a room, their stars have had time to reach a well-mixed equilibrium state. As with stars on the main sequence, we would expect the properties of elliptical galaxies to reflect the most probable state of a fairly simple system, with 'no surprises'.

Instead, detailed studies reveal a bewildering complexity. Elliptical galaxies cover a huge range of luminosity and of light concentration. Some ellipticals rotate fast, others hardly at all. Some appear to be oblate (grapefruit shaped), while others have a triaxial shape with three unequal axes, like a squashed (American or rugby) football. These properties are interlinked: luminous ellipticals are more likely to be triaxial, slowly rotating, and also strong X-ray sources, while the less luminous systems are oblate and relatively rapidly rotating, and have dense stellar cusps at their centers.