If I were to leave a blank after the word “Andromeda”, then chances are that you would be filling it in with the word “galaxy”. This is because the Andromeda galaxy is the nearest major galaxy next to our own Milky Way, making the association natural. A little over 100 years ago, however, nobody would have filled it with the word “galaxy”.

That’s because even early in the 1920s there was a raging ongoing debate about such “spiral nebulae,” Andromeda being one of many such faint, fuzzy patches of light. On one side were those who believed that the Milky Way was the entire galaxy and that these nebulae were just smaller objects within, and on the other side were those who believed they were galaxies in their own right, extremely large and at rather vast distances.