![]() Hubble was able to measure their periods, use Leavitt’s relation to infer their intrinsic luminosities, and, by measuring their observed brightness, find their distances. In 1912, Henrietta Leavitt, who worked at Harvard, had found a relation between a Cepheid’s period of variability and its luminosity. Based on repeated observations of the Andromeda Nebula, Hubble identified several stars that periodically brightened and dimmed, which he understood to be a category of stars called Cepheid variables. When Hubble took pictures of the Andromeda Nebula with this telescope in the 1920s, he resolved its diffuse light into individual stars, just as Galileo had discovered when he pointed his primitive telescope at the fuzzy arc of the Milky Way, 300 years earlier. Mount Wilson Observatory, where Hubble worked, overlooking the Los Angeles basin, had the largest telescope in the world at the time-100 inches (2.5 meters) in diameter. The astrophysicist who made the observations that settled the question once and for all was Edwin Hubble. ![]() Shapley found this notion to be completely implausible. Given Shapley’s determination of the extent of the Milky Way and the small angular size of spiral nebulae on the sky, if Kant was right, this meant that they must be astonishingly-seemingly impossibly-distant, millions or tens of millions of light-years away. Still, the German philosopher Immanuel Kant had speculated as early as 1755 that the spiral nebulae were other “island universes,” that is, objects as large as the then entire known universe-the Milky Way. However, the spiral structure of the Milky Way disk that we discussed in the last chapter was unknown 100 years ago, because, living within the disk itself, we didn’t have a good understanding of its three-dimensional structure, making it harder to detect its resemblance to that larger class of objects. The astrophysicist Harlow Shapley, working at the beginning of the twentieth century, thought spiral nebulae were gas clouds within our own galaxy. The most prominent of these, the Andromeda Nebula, is bright enough to be seen with the naked eye under dark skies, far away from city lights. There is yet another class, sensibly called spiral nebulae, because of their shape. We have already encountered a variety of nebulae in this book, including planetary nebulae, which result when red giants throw off their outer layers the Orion Nebula, a region of intense star formation in which the surrounding gas fluoresces because of light from hot young stars and even dark nebulae, the dust clouds that block light from background stars. They gave them the generic name nebula (plural nebulae), from the Latin word for cloud. ![]() But seventeenth-century astronomers noticed a number of other objects in the sky that were extended and often fuzzy looking. Stars are so far away, they appear as unresolved points of light, even through modern telescopes.
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