15.1 Islands of Stars • What are the three major types of galaxies? (1) Spiral galaxies have prominent disks and spiral arms. (2) Elliptical galaxies are rounder and redder than spiral galaxies and contain less cool gas and dust. (3) Irregular galaxies are neither disk-like nor rounded in appearance. • How are galaxies grouped together? Spiral galaxies tend to collect in groups that contain up to sev-eral dozen galaxies. Elliptical galaxies are more common in clusters of galaxies, which contain hundreds to thousands of galaxies, all bound together by gravity. 15.2 Distances of Galaxies • How do we measure the distances to galaxies? Our measurements of galaxy distances depend on a chain of methods. The chain begins with radar ranging in our own solar system and parallax measurements of distances to nearby stars, then relies on standard candles to measure greater distances. • What is Hubble’s law? Hubble’s law
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