A very basic question about stars

I am posting here, though I have been in the hobby for some years.

Question: Why do brighter stars look "bigger" than less bright stars?

A pinpoint bright star should have the same "size" perception as a point just like a dim star, right? But why do all astrophotos show brighter as bigger? Is it because the camrea film or CCD detectors bleed the image outward with brightness?

Peter Glus said:

I am posting here, though I have been in the hobby for some years.

Question: Why do brighter stars look "bigger" than less bright stars?

A pinpoint bright star should have the same "size" perception as a point just like a dim star, right? But why do all astrophotos show brighter as bigger? Is it because the camrea film or CCD detectors bleed the image outward with brightness?

Peter: I have raised this question in the past, I have never gotten a totally satisfactory explanation... Brighter stars look bigger, not just somewhat bigger as one might expect were you just more of the disk and rings, rather many times bigger that suggests the eye-brain in involved.

I have done some experiments involving double stars and exit pupil size at constant magnification... At what brightness/magnication/exit pupil can you start to resolve the airy disk... That depends...

Jon

Peter Glus said:
A pinpoint bright star should have the same "size" perception as a point just like a dim star, right? But why do all astrophotos show brighter as bigger? Is it because the camrea film or CCD detectors bleed the image outward with brightness?

Although I have very limited experience with astrophotography, I've been into regular photography for over 30 years. I believe the assumption in your last sentence is correct. When a pinpoint of light strikes the emulsion side of film, which is made up of millions of tiny crystals, the light is bounced around and tends to bleed into otherwise unexposed emulsion. (If the exposure is long enough, it will fog the entire film frame.) I would guess that the same is true for the surface of CCD chips. There will also be a little diffused light around the pinpoint light rays, caused by the lenses and atmosphere.