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:
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