Expansion implies a beginning in time. And we will explore where and how the expansion might have begun in Chapter 25, "What About the Big Bang?". In Chapter 26, "(How Will It End?", we'll explore the details of the expansion itself. But before we move on to some of these big questions, we turn our attention in the following chapter to some of the most energetic and unusual members of the galactic family: the quasars, black holes (again), and galactic jets, all of which keep the universe quite active, thank you very much.
The Least You Need to Know
V Galaxies were first classified by Edwin Hubble into three broad types: spiral, elliptical, and irregular.
V The majority of the mass (90 percent) of most galaxies and clusters (99 percent) is made of material we cannot observe, dark matter. We see it only by its gravitational effect.
V Distances on intergalactic scales are measured by observation of Cepheid variable stars, the Tully-Fisher relation, standard candles such as Type I supernovae, and Hubble's Law.
V Many galaxies are grouped in galactic clusters, which, in turn, are grouped into superclusters. Superclusters are found together on the edges of huge voids.
V Edwin Hubble first observed that galaxies recede from us at a rate proportional to their distance from us. The constant of proportionality, called the Hubble constant, tells us the age of the universe.
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