Advances in astronomy (in fact, in all sciences) during the twentieth century far exceed those of all previous centuries. Increasing reflection telescopes were built. Studies with these instruments revealed the structure of huge and distant clusters of stars, called galaxies, and clusters of galaxies.
When arriving at this century several of the precopernicanas beliefs had resurfaced when speaking of the galaxies, it was considered that the Sun was near the center of the Milky Way, that constituted the entire universe. Beyond the confines of the galaxy it was considered that there was nothing but an infinite void.
The study under spectroscopy of elliptical nebulae at the beginning of the century, showed that they had no characteristics of being clouds of gases but rather stellar characteristics, which indicated that at least some spiral nebulae were constituted by stars.
The study of variable stars by Harlow Shapley led him to discover Cepheid variables, pulsating stars changing brightness. The variation cycle of cepheid brightness is directly related to its intrinsic brightness, a discovery made by Henretta Swan Leavitt. This property of the Cepheids allowed to know its absolute magnitude.
Shapley when studying the variables of the globular clusters realized that their distance was much greater than was believed and that they were towards the center of the galaxy, when calculating its distance to the Sun, it should be located on the periphery of the Milky Way. in this way the Sun moved from the center of the known universe to a periphery of it.
Although several astronomers defended the theory of the Island Universes exposed by Kant and followed by Herschel, there was no confirmatory evidence of the fact. This proof would come from the observations of Edwin Hubble, who on February 19, 1924 wrote to Shapley: "You will surely be interested to know that I have found a Cepheid variable in the Andromeda Nebula." In this way, Shapley's idea of a single galaxy, ours, as a constituent of the entire universe was refuted and revealed the presence of other galaxies in space.
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