Maria Cruz,
Robert Coontz
CREDIT: NASA/JPL-CALTECH
It wasn't until the 1920s that astronomers realized that there were other galaxies in the universe besides our own. Using the 100-inch telescope at Mount Wilson in California, Edwin Hubble determined the distance to Andromeda (M31) and to the Triangulum (M33) and concluded that each was an “isolated system of stars and nebulae, lying far outside the limits of the galactic system.” Before that, these and other galaxies were classified as nebulae, extended objects other than planets or comets; although their location was a matter of great debate, they were generally thought to be within our galaxy.
Nowadays there is no doubt that the universe extends well beyond the confines of the Milky Way and that our galaxy is just one among many. Telescopes much more powerful than those used by Hubble have produced ever-larger and more comprehensive surveys of galaxies. The detailed understanding of our galaxy has also evolved dramatically. As explained by Tolstoy (p. 176), the study of individual stars in the Milky Way can help us understand the history of our galaxy and the physical processes that happened when the first stars and galaxies formed. More generally, the history of star formation in galaxies tells us how the structure and average chemical composition of the universe have changed over its 14-billion-year history. On page 178, Dunlop discusses the different methods used to quantify this history back to about 500 million years after the big bang./.../
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