On one side of the debate stands Shapley firmly convinced that the Milky Way encompasses the Universe and that all spiral galaxies—such as what we now know as the Andromeda Galaxy (then referred to as the Andromeda Nebula)—reside within its bounds. Shapleys position is supported by Van Maneens observations, an astronomer who claimed to have measured Andromedas velocity. Such measurements would only be possible if Andromeda were positioned inside our Way since rotation faster than light speed would be required if it were located outside.
On the side we have Curtis asserting that our Milky Way is just one, among countless cosmic islands scattered throughout the universe. According to his viewpoint the Andromeda galaxy exists independently from our galaxy.
Curtis possessed some measurements of redshift along with observations of dust bands and supernovae in Andromeda and other spiral nebulae. The debate came to a standstill due to evidence to definitively answer the question. A years later Edwin Hubble successfully determined the distance, to the Andromeda galaxy by studying the variability of a Cepheid star. This confirmation established that Andromeda could not be part of the Milky Way thus supporting Curtiss thesis. Around the time it was proven that Van Maneens observations were completely inaccurate consequently disproving Shapleys thesis.
It is worth noting that not all of Curtiss theories were correct just as not all of Shapleys ideas were incorrect. Specifically Curtis asserted that the Sun resided at the center of the Milky Way whereas Shapley argued for a position—later confirmed as valid. However neither scientist accurately estimated the size of the Milky Way; Curtis proposed dimensions that were too small while Shapley proposed dimensions that were too large.
Credit; Adam Evans.
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