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I'm asking here about the diffuse glow seen by the human eye, when looking at the Milky Way.

Where do those visible photons come from exactly?

  • the surface of a star?
  • starlight bouncing off something (dust or a molecule)?
  • emitted by gas in the interstellar medium?
  • all of the above?
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I believe that the answer is mainly the first (directly from stars), although there will certainly be some of each of the others. The diffuseness comes from the fact that there are many more relatively dim stars than rods in the retina of the human eye. My argument for this is that magnified photographs of most of the Milky Way at least, break down into individiual stars, as seen in this image from astropixels.com

image of Milky Way from astropixels.xom

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    $\begingroup$ It's not just the limited number of receptors, the optics of the eye are unable to focus the galaxy well enough to separate the light from the different stars. The diffraction limit means even ideal optics would need to be far larger than the lens of the human eye, and the optics of human eyes are far from ideal. $\endgroup$ May 17 '20 at 1:39

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