Galileo inaugurated the era of telescopic stellar astronomy by discovering vast numbers of unsuspected stars. In the Starry Messenger, on one star chart he showed 36 new stars around the original six of the Pleiades, and on another, 80 new stars near the belt and sword of Orion.
Uncountable stars might exist, located much farther away than was previously believed. On the old assumption of solid crystalline spheres, these discoveries required the outermost “sphere of stars” to be unimaginatively immense and implausibly thick. Aristotelians would now have to explain how such an immense and thick sphere of stars could rotate every 24 hours around a tiny central, stationary Earth.
“For the Galaxy is nothing else than a congeries of innumerable stars distributed in clusters. To whatever region of it you direct your spyglass, an immense number of stars immediately offer themselves to view, of which very many appear rather large and very conspicuous but the multitude of small ones is truly unfathomable.” Galileo, Sidereus Nuncius, trans. Albert Van Helden (University of Chicago, 1989).