Beginner amateur here. I see mentioned many times that Galileo was surprised to see the moons of Jupiter and all that their existence proved, i.e. the Earth not necessarily being the center of everything in the heavens. However I would think that being the ancients saw stars of various magnitudes, they would actually expect there to be stars of 7th magnitude and up, and whomever first used the telescope would not be surprised at all to actually see them. Is there any historical knowledge about this?
tl;dr: Probably yeah, Galileo was a smart guy and probably reasoned that if he could get a better view, he could see more stuff (like stars)
Consider what a night sky looks like with absolutely no light pollution; it’s pretty spectacular, with an ocean of stars. When Galileo was alive, light pollution wasn’t nearly as much of an issue as it is now, and so he would have observed this ocean of stars at its peak every new moon.
Since some of these stars are fainter than others, and that there were so many he could see, I don’t think it would’ve really surprised him to find that what once looked like a blank part of the sky had a couple stars, inasmuch as I would not be surprised if looking down on a village from a far away mountain to find more buildings or people if I used some binoculars. It does not take a lot of imagination for him to make that jump.
To give context for why Galileo was surprised, what was groundbreaking for his time was that the geocentric model of the universe was seemingly contradicted by the moons orbiting Jupiter; the notion put forward by his contemporaries was that geocentricity was a matter of not only science but of divine appointment: nothing could orbit anything but the Earth for it was the footstool of God, etc. And so, to see something orbit around something else was not only a scientific paradigm shift, but went against the popular religious ideologies of his time.
That, for Galileo, was likely much more surprising than finding the innumerable amount of stars of the night sky to be a little more innumerable than he could see.
To your final question, I doubt there would be any historical info about this because it wasn’t worth noting. Astronomers were familiar with the idea of there being stars obscured for one reason or another, like the full moon or what have you, and so I would be more surprised if they thought that the only stars out there were the ones they could see.
Hans Lippershey made the first practical telescope in 1608 and based on it, Galileo made his own telescope in the following year having 3x magnification which he later improved to 30x magnification. It was called terrestrial telescope or a spyglass and with it, he could see magnified, upright images on the Earth. He published his initial telescopic astronomical observations in March 1610 in a brief treatise entitled Sidereus Nuncius (Starry Messenger). In this treatise, he concluded that moons have mountains and pits (unlike the previous belief that moon is a smooth sphere), phases of Venus and "stars" of Jupiter which he later confirmed to be 4 of the largest moons of Jupiter (and aptly named Galilean moon). This led Galileo to believe that Earth is not the center but the sun where all these bodies revolve around.
The "stars" of Jupiter was a huge motivation for Galileo as he correctly predicted that these 4 bodies were not the only stars but the twinkling objects in the sky were stars and not just heavenly bodies. He confirmed that the Milky Way consists of multitude of stars packed so densely that they appeared from Earth to be clouds (due to refraction of Earth's atmosphere) instead of being nebulous (which he referred to as "invisible stars"). He depicted the Orion's Head nebula and the nebula of Praesepe (located in the Cancer constellation) which are shown below. The pictures are from Sidereus Nuncius on page 63.
He also located many other stars too distant to be visible with the naked eye. He observed the double star Mizar in Ursa Major in 1617. Galileo went on to prove this assertion by sketching out two "nebulae" which were indeed clusters of stars. However, he missed to prove that gas and dust surrounds these stars. Probably, the telescope was not that advanced to detect such matter but it was still a groundbreaking discovery.
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