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When I was 20, I went to Naxos (Greece) with friends. I was driving my car during the night on the mount Zeus (1003 meters - 3,291 ft) listening Pink Floyd.

Up there we stopped the car to take a walk, looked above and ... wooooooowwwwww!

The sky was dense! Full of stars. We didn't know it could have been like that.

My question is:

  • How dense is the deep sky from outside the atmosphere?
    • For example, on the ISS during the night (the Earth that cover the Sun) looking at the deep sky, how many stars can you see? I saw a lot of pictures of the stars taken by the Hubble telescope), but I don't think it can be compared, right?
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  • $\begingroup$ I had a similar "wow" experience in Mexico in the 1980s. Light pollution is a factor. $\endgroup$ – userLTK Apr 22 '17 at 22:11
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Depending on your eyesight, the faintest stars you can see in a perfect environment have an apparent magnitude (brightness) of 6-6.5 (though some people have managed to see magnitude-7). According to this site, that translates into about 10,000 stars.

As for how it would look, I have seen the sky from the stratosphere (about 3 miles up, in a plane, where there is a lot less atmosphere to worry about, and no light pollution), and it is a surreal sight - with the sky absolutely littered with stars. However, it doesn't quite look like a Hubble image, though - there's very little colour in these fainter stars, and Hubble tends to focus on narrower fields of view, at higher zoom.

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Stars can be seen from space.

Astronauts, such as James Reilly Describes Seeing Stars in Space. In the video he notes that the stars in space don't twinkle, and that more stars are visible compared with Earth (he actually describes seeing "literally millions", but that is obviously hyperbole). He notes that it can be difficult to pick out the constellations as many of the fainter stars are visible.

In reality, several thousand stars are visible, but it depends on the acuity of your vision, and your skill at observing, so no exact number can be given.

If you are used to typical light polluted skies, then seeing the stars in a truly dark sky is pretty awe inspiring.

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You asked: For example, on the ISS during the night (the Earth that cover the Sun) looking at the deep sky, how many stars can you see?

The answer is 0 (at ISS-night) and 1 (at ISS-day) which is called the Sun. Well, that is what NASA, the visitors to the ISS, the Apollo astronauts, the videos of the ISS space walks and other pictures produced by NASA say. There are no pictures still or video taken from the ISS that show more than 1 star, that well known nearest one.

Interestingly there was one Russian astronaut (Not on ISS but early Russian space craft) who saw something different when he was up in space. He saw the sky full of stars and the light from them.(I'll try to find you a link later).

There is clearly one obvious conclusion. You know what it is and it has nothing to do with optical effects, the physics of light or the way your eyeball works.

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    $\begingroup$ Note the link in my answer. James Reilly says that he could see more stars from space than in Colorado. Armstrong noted that, on the moon, the stars weren't easy to see. That was due to his eyes being adapted to the bright lunar surface. Apollo astronauts reported that stars were quite visible when they stood in the shadow of the Lunar Module, and opened the outer sun shade of their helmets. $\endgroup$ – James K Apr 22 '17 at 19:23
  • $\begingroup$ Here you go - second hit on a google search. take your tinfoil hat off.. nasa.gov/content/starry-sky-from-the-space-station $\endgroup$ – Rob Jeffries Apr 23 '17 at 7:34

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