Why did William Herschel think star clusters could change over time?

William Herschel believed that star clusters and nebulae could change over time because he observed variations in their relative densities, ranging from tightly packed groups to widely scattered ones. He interpreted these differences as evidence of an evolutionary process driven by universal gravitation.12

Herschel maintained that, over time, attractive forces would cause widely scattered stars to condense into more tightly packed clusters. Consequently, he viewed a group of widely scattered stars as being at an earlier stage of development than one whose stars were already tightly packed. This perspective led him to develop a cosmogony—a theory of the origin of the universe—in which stars were originally scattered throughout infinite space before being organized by gravity into increasingly dense formations. Even when he later encountered evidence of luminous fluid surrounding stars, he adapted his theory to suggest that stars were actively condensing out of such material, further reinforcing his view of the heavens as a dynamic, evolving system.13