Excluding the Sun, stars are so far away that their angular diameter is effectively zero. However, when you take pictures of them, brighter stars appear as circles, not points. Why?
In theory, any star, regardless of brightness, should hit at most one small point of whatever medium is being used to take the photograph. Why do nearby points of the medium also respond? Does excessive light “bleed” into nearby points, and, if so, is the “bleeding” the same for digital and non-digital cameras?
Does it have something to do with the lens? Does the lens expand a single point of light into a small circle, depending on brightness?
I ran into this while trying to answer https://astronomy.stackexchange.com/questions/22474/how-to-find-the-viewing-size-of-a-star which effectively asks: what’s the function (if any) that relates star brightness to the size of a star’s disk on photographic film (or digital media)?
Note: I do realize that a star’s visual and photographic magnitudes can be different, and am assuming the answer will be based on photographic magnitude.
EDIT: Thanks for all the answers, I am still reviewing them. Here are some additional helpful links I found:
Photometry (astronomy) at Wikipedia
http://www.chiandh.eu/astphot/object.shtml, especially the discussion about “raw image units” and “full width at half maximum” (FWHM)
http://www.astro-imaging.com/Tutorial/MatchingCCD.html and its discussion of FWHM
[ Politics ] Open Question : Republicans keep claiming Biden has dementia, so why am I seeing him kicking Trump's butt by over 10 points in national polling?
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Am having problems getting my head around these questions.
I know what cluster points are, but can’t seem to get the question.