The number of visible stars you can see with your eye on the clearest possible evening if you observed from both the north and south hemispheres on the Earth is about 9100. Astronomers measure the brightness of stars by their apparent magnitudes. The faintest stars we can see with our eyes is about an apparent magnitude of +6.5. On a night where you can just see the stars in the Big Dipper or Orion, this magnitude limit is about +5.0 and there are about 1466 stars that you could see in the north and south hemispheres combined.
Star atlases such as Norton 2000.0 show all the stars brighter than +6.5. The Uranometria Atlas, published in 1988, shows all of the 332,556 stars brighter than a magnitude of +9.5. Meanwhile, the Hubble Space Telescope Guide Star Catalog lists all of the 20 million stars brighter than a magnitude of +15.0 and the Palomar Digital Sky Survey will provide a list of all stars brighter that a magnitude of +20.0 for which there are several BILLION. The total number of stars in the Milky Way is something like a few trillion by the way.
The point is that the number of stars we can see from the Earth depends on what we use to view them with. They eye can see up to perhaps 9000-12,000 give or take, and depending on local atmospheric conditions and your vision quality. If you use a pair of binoculars, you would see probably 10 times that number.
Copyright 1997 Dr. Sten Odenwald
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