Why were some of William Herschel’s assumptions about stars later shown to be incorrect?

William Herschel was a pioneering astronomer who often used bold, simplifying assumptions to interpret his observations and advance theoretical astronomy. However, some of these assumptions were later proven incorrect as observational technology and scientific understanding evolved:

• Stellar Brightness: Herschel assumed that all stars possessed equal intrinsic brightness, meaning that any variation in their apparent brightness was solely a result of differences in distance. He maintained this assumption throughout his career despite accumulating evidence to the contrary.1
• Structure of the Milky Way: To map the Milky Way, Herschel assumed that his telescope could see all the stars in our system and that those stars were distributed regularly. He eventually abandoned this model when he realized his telescope lacked the power to penetrate to the galaxy's edge.23
• Nature of Nebulae: Herschel initially argued that all nebulae were composed of stars and that "luminous fluid" did not exist. This view was challenged when he observed a nebula in 1790 that appeared to be a central star condensing out of a surrounding cloud of fluid, forcing him to adapt his theories.14

These shifts in his thinking illustrate his willingness to refine his cosmogony as new evidence emerged.1