Descendants of our civilization may never become advanced enough to journey far enough beyond our Milky Way Galaxy to look back and witness the full grandeur of our extended home in space. The finite speed of light is too limiting, the Galaxy too vast. A literal picture of our resident swarm of a hundred billion stars floating proudly and silently in the great abyss of space may forever elude us. Yet, from our Earth-based vantage point in the suburbs of our Galaxy—nearly 30,000 light-years from its hub—astronomers study the variety and spread of other colossal star systems well beyond our own Milky Way. Many of these distant objects launched their light—some of the very light in tonight’s nighttime sky—long before Earth and Sun even emerged in the firmament.

Deep space harbors myriad objects looking strangely unlike stars. Photographic exposures, taken with even small, backyard telescopes, often reveal fuzzy lens-shaped images resembling disks more than the bright, round points typical of stars. The 18th-century German philosopher Immanuel Kant regarded these blurry blobs of light, like so many flattened yet luminous puffs of cotton, as individual “island universes” far outside the confines of our Milky Way. We now know that labeling each of them a universe—“the totality of all things”—presents a clear semantics problem, but he was correct in thinking that these peculiar patches of light reside way beyond the well-known stars comprising the familiar constellations.

Large, modern telescopes have since revealed these remote beacons to be entire galaxies, each a huge collection of matter comparable to our Milky Way, typically measuring ~100,000 light-years across, or ~10¹⁸ km. Replete with hundreds of billions of stars bound loosely by gravity, each galaxy harbors more stars than people who have ever lived on Earth. Silently and majestically, galaxies twirl in the deep realms of the Universe—vast hordes of radiation, matter, and perhaps life—simultaneously granting us a feeling both for the immensity of the Universe and for the minuteness of our position in it.

That position, when internalized intellectually, often resembles a boat adrift at sea. For there are as many galaxies in the Universe as there are stars in our Galaxy—all told, probably as many stars in the observable cosmos as grains of sand on all the beaches of the world— ~10²², to be numerical about it. Yet organized patterns abound—grand dynamical processions like the pinwheeling of individual galaxies and the outward recessional of all of them—provided we are willing to ponder the big and the broad.

The learning goals for this epoch are:

• to distinguish between normal and active galaxies, as well as their distribution in space
• to appreciate some properties of black holes and their central engines at the cores of galaxies
• to understand the basic problems plaguing our current knowledge of the origin of galaxies
• to gain a feeling for the roles of gas turbulence and gravitational instability in an expanding medium
• to appreciate the general outline of galaxy formation, including their quantum seeds in the early Universe
• to realize that galaxies, much like life forms, change because of both intrinsic and environmental factors
• to recognize that galactic evolution may hold keys to understand the largest structures in the Universe.