If you do a little research on the web you will find that the majority of the so-called experts insist that stars are visible by the naked eye in space. What bothers me is that none of those “experts” are astronauts. If you narrow down your search to what astronauts are saying then you find that only some of them are answering this question. Only a minority of astronauts clearly stated they could see the stars unaided by camera if they adjusted their eyes. The key phrase is “adjusting the eyes.”
The Hubble Space Telescope (HST) is in space and it sees the stars. The proof is the beautiful pictures of stars and galaxies taken by the HST. So, if HST can see the stars the human eye should be able to see them as well, right? The problem is that the aperture of the human eye is very small compared to the aperture of the Hubble Space Telescope. Besides, HST takes long exposure photographs.
In outer space, chances of receiving few photons coming from a distant star in the aperture of the human eye is very small. For a particular star, the total intensity of the light in the eye would be extremely small.
The pupil of the human eye is its aperture; the iris is the diaphragm that serves as the aperture stop. The aperture of the human eye is typically 4 mm in diameter, although it can be as big as 8 mm in the dark. The aperture of the Hubble Space Telescope is 2.4 meters in diameter (600 times bigger than the aperture of the human eye).
Seeing stars with a camera in space is different. First of all, the aperture of most cameras are much bigger than the pupil of a human eye. The camera aperture can be made bigger and many other controls and light amplification can be applied. Obviously, with a camera you can also take long exposure photographs. You cannot do this with the naked eye.
We see the stars on Earth with the naked eye because the atmosphere acts like a lens or a big camera aperture. The air molecules scatter the photons coming from the distant star. Our eyes receive the light of the star from different directions because of the scattering. There may be millions of molecules in the air scattering the star light towards our eye. Therefore the star light is amplified million times compared to the situation in space where there is no air therefore no scattering and no amplification.
In outer space, the only way to develop pictures of stars is by long exposure photography. I am amazed that some humans can actually see the stars by the naked eye in space. Few astronauts talked about seeing the closer objects such as Mars and the haze of the Milky Way by the naked eye. I am impressed by their super talent in “adjusting eyes”.