Why Do Stars Look Like Points of Light Rather than a Uniform Haze of Light?
Question: Hi, I’ve a question I’ve wondered about for some time. I will try to be as succinct as possible. As I look at the night sky, I see many stars. These stars are all emitting photons at an astounding volume. A very small amount (respectively) of these photons reach earth. How is it that the human eye can sort all of the into individual points of light? This may be a silly question, but I’m amazed that these particles from so many points so far apart have the energy to travel so far, then are not diffused and mingled to a point that prohibits the human eye from seeing anything but a haze of light. Sorry for a lengthy question but I’ve always wondered and can never find material to address this specifically. — Daniel
Answer: One aspect of your question relates to what is generally called Olbers Paradox, which points out that if the universe is uniform and infinite that it should be completely filled with stars, thus filling the night sky with bright stars. The explanation to the paradox is that the universe is expanding, and this expansion shifts the light from stars to longer wavelengths, beyond the wavelength range to which our eyes are sensitive. The other aspect of your question relates to why we see stars as points of undiffused light even though the light they have emitted has travelled so far. The fact is that the light from some stars does get diffused by interstellar gas and dust which lies between us and the star. The light from stars which is not diffused comes from relatively nearby stars, lessening the chance of being blocked by interstellar matter. On average, the space between us and a nearby star is relatively empty, with about one hydrogen atom per cubic meter of volume, so there is on average very little interstellar matter to diffuse starlight.
Jeff Mangum