FIGURE 3.8 Map of the solar neighborhood. This diagram projects the position of younger clusters of stars onto the galactic plane. The Sun is denoted by S in the diagram. From observations of external galaxies we know that the spiral arms are apparent largely because of the presence of bright stars in relatively young clusters of stars. In showing the position of these stars near the Sun we can see a similar pattern. Parts of three arms of our galaxy are visible from the diagram. SOURCE: Morgan et al., 1953, p. 318. Reproduced by permission from the American Astronomical Society.

parsecs). So even in our description of the far reaches of the universe, the distance scale is tied to the distance from Earth to the Sun. This distance scale is an appropriate homage to the process of spatial thinking that led from measuring the diameter of Earth to an understanding of some of the most fundamental properties of the universe.

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