Andromeda galaxy
In 1751 William Herschel estimated the distance to the Andromeda "nebula" as "no more than 2000 times the distance to Sirius," or around 17,200 light years.  The best modern estimate of Andromeda's distance is 2.54 million light years. In the 1920s astronomer Edwin Hubble proved that Andromeda was a galaxy, and not a gas cloud in the Milky Way as had been thought.