How can we tell that these objects are galaxies outside the Milky Way?
In the early 20th century, astronomers debated whether “spiral nebulae” were gas clouds inside one big galaxy that made up the entire universe or independent galaxies like our own. In 1920, two astronomers, Harlow Shapley and Heber Curtis, argued the opposing positions in a famous debate held at this Museum.
Inspired by the debate, astronomer Edwin Hubble used the largest, most powerful telescope of the time to observe stars and supernovae within some of the closest “spiral nebulae”. Hubble helped prove that the “spiral nebulae” were galaxies too large and distant to be part of the Milky Way.
Advances in telescope technology allow astronomers to continue to fine-tune their observations and measurements of distant objects beyond the borders of our galaxy.