<P> In 1750 Thomas Wright, in his work An original theory or new hypothesis of the Universe, correctly speculated that the Milky Way might be a body of a huge number of stars held together by gravitational forces rotating about a Galactic Center, akin to the solar system but on a much larger scale . The resulting disk of stars can be seen as a band on the sky from our perspective inside the disk . In a treatise in 1755, Immanuel Kant elaborated on Wright's idea about the structure of the Milky Way . At the time, the existence of other galaxies had not been discovered . </P> <P> In 1917, Heber Doust Curtis observed a nova within what then was called the "Andromeda Nebula". Searching the photographic record, 11 more novae were discovered . Curtis noticed that novas in Andromeda were drastically fainter than novas in the Milky Way . Based on this, Curtis was able to estimate that Andromeda was 500,000 light - years away . As a result, Curtis became a proponent of the so - called "island Universes" hypothesis, which held that objects previously believed to be spiral nebulae within the Milky Way were actually independent galaxies . </P> <P> In 1920, the Great Debate between Harlow Shapley and Curtis took place, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the Universe . To support his claim that the Great Andromeda Nebula (M31) was an external galaxy, Curtis also noted the appearance of dark lanes resembling the dust clouds in our own Galaxy, as well as the significant Doppler shift . In 1922 Ernst Öpik presented a very elegant and simple astrophysical method to estimate the distance of M31 . His result put the Andromeda Nebula far outside our Galaxy at a distance of about 450,000 parsec, which is about 1,500,000 ly . Edwin Hubble settled the debate about whether other galaxies exist in 1925 when he identified extragalactic Cepheid variable stars for the first time on astronomical photos of M31 . These were made using the 2.5 metre (100 in) Hooker telescope, and they enabled the distance of Great Andromeda Nebula to be determined . His measurement demonstrated conclusively that this feature was not a cluster of stars and gas within our Galaxy, but an entirely separate galaxy located a significant distance from our own . This proved the existence of other galaxies . </P> <P> Hubble also demonstrated that the redshift of other galaxies is approximately proportional to their distance from the Earth (Hubble's law). This raised the appearance of our galaxy being in the center of an expanding Universe, however, Hubble rejected the findings philosophically: </P>

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