B What process allows us to 'see' lightyears of distance with the natural eye?

[Runesmith]

Let me attempt a round on this merry-go-round. There were 2 comments on this thread by the OP that caught my attention. One was:

OK, then how can we still see the light from those two stars on the same scale we see the separation between them?

The other was something about the stars having to be super-massive (I am too lazy to look it up on the thread).

So I speculate that the OP is asking this:
The width of a star the size of the Sun is around 1.4 x 10^6 km. A light-year is 9.4 x 10^12 km. That means, the diameter of a star is 0.15 x 10^-6 light-years. So what he is asking is (if my speculation is correct) - on a scale where 5000 light-years = 180 degrees, how can we see a star, when it should have an angular diameter of 0.23 x 10^-9 degrees. Is that correct, OP?

The answer was already given - because it emits photons that reach your retina. The star is a point source in this scenario, because your eyes don't have the resolution required to resolve such a small angular diameter. Stars do appear as "blobs" when you look at them because of the "smearing" effect of the atmosphere, the imperfections in the lens of your eye, the way light is captured by the retina, and the way your brain interprets it.

 

[sophiecentaur]

The OP seems to be getting this the wrong way round. As usual in Physics, it's the numbers that count. The relevant thing is how many millions of TeraWatts of Power are being radiated by the source over the whole of a sphere of radius many light years and what fraction of that is intercepted by the tiny aperture of the eye (or the slightly bigger aperture of the telescope). If there is enough for the retina or detector to register it then the star is 'visible'.

 

[KameshwariDev]

This is a question of visual field.

The human visual field is about 114 degrees. This enables us to see anything that falls within the space. It should also be understood that as the distance between the observation frame and observer increases, the visual angle of the observation frame decreases.

An analogy would be that of a person standing one foot away from the lengthwise centre of the side of a square building of side 10 metres and looking towards the building. He would be staring at a wall but if he steps back 20 metres he would be able to see the entire side of the building. If he steps back another 20 metres, he would be able to see the building and even its immediate neighbours. This size of observation frame would increase as the distance between observation frame and observer increases.

It should also be noted that in the whole process the detail is lost. when the person stands one foot away he would be able to see the graffiti of a mickey mouse that his child might have drawn to make the blank face of the wall appealing. From a distance of 10 metres, though he is able to see the entire face of the wall, he would not be able to discern the features of mickey mouse and from a distance of twenty metres, when he is able to see the building and his neighbours’ houses, mickey mouse is indiscernible from the wall.

This is what happens with the stars. The distance between Earth and stars is so huge that the observation frame is infinitesimally large making the actual distance between the stars infinitesimally small enabling them to fall within the field of human vision.

<<Post edited by a Mentor to remove a dangerous suggestion>>

 

[sophiecentaur]

This is a question of visual field.
The human visual field is about 114 degrees. This enables us to see anything that falls within the space. It should also be understood that as the distance between the observation frame and observer increases, the visual angle of the observation frame decreases.
An analogy would be that of a person standing one foot away from the lengthwise centre of the side of a square building of side 10 metres and looking towards the building. He would be staring at a wall but if he steps back 20 metres he would be able to see the entire side of the building. If he steps back another 20 metres, he would be able to see the building and even its immediate neighbours. This size of observation frame would increase as the distance between observation frame and observer increases. It should also be noted that in the whole process the detail is lost. when the person stands one foot away he would be able to see the graffiti of a mickey mouse that his child might have drawn to make the blank face of the wall appealing. From a distance of 10 metres, though he is able to see the entire face of the wall, he would not be able to discern the features of mickey mouse and from a distance of twenty metres, when he is able to see the building and his neighbours’ houses, mickey mouse is indiscernible from the wall.
This is what happens with the stars. The distance between Earth and stars is so huge that the observation frame is infinitesimally large making the actual distance between the stars infinitesimally small enabling them to fall within the field of human vision.

<<Post edited by a Mentor to remove a dangerous suggestion>>

This is not as relevant as you think. The bottom line is how much Energy is gathered by the objective lens from the star. The focal length of a telescope lens is much less relevant than the aperture. For a 'good' lens, all stars will have an Airy Disc image (resolution) which is diffraction limited by the aperture and it corresponds to a [Edit: perceived] solid angle subtended by the star in the sky (not a point). All the energy in this disc will fall on a number of sensor elements and it is the sum of the contributions of all the elements that will determine whether or not a star is detected. Wide or narrow angle, the answer is largely the same. Astrophotographs of faint stars require the same exposure times for a given aperture, whatever the focal length. (Distributed objects are a different matter and 'f value' becomes relevant)

<<Post edited by a Mentor to remove a dangerous suggestion in the quoted text>>

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