There are about 6000 stars in the night sky that are visible to a healthy adult standing in a properly dark location. Of course, you can never see more than half the sky at a time, so to see all of them you would need to live in the tropics and spend some time looking upwards regularly — it takes a full year for the entire sky to pass overhead. So at any one time, there’s something between 2500 and 3500 stars visible. But if the moon is up, or if there is any light pollution, this number drops dramatically. In most modern city centres, the number of visible stars is more like three than three thousand!
If, on the other hand, you use binoculars or a telescope, the numbers shoots upwards. My 10×50 binoculars can theoretically show me almost 500,000 stars! This is because those binoculars look through a lens which is much larger than the human eye. That big lens gathers a lot more light and concentrates it down into the eye, so that stars which would normally be much to faint to see become visible.
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