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

[Surayabay]

Would it be correct to say, the distance we see between stars 'represents' or 'implies' ly's? And if that's true, how can we still see the light from those stars? Or is that a whole different question, (beacuse the light is constantly traveling towards us!)

newjerseyrunner, a little HELP, my friend!

 

[Janus]

Would it be correct to say, the distance we see between stars 'represents' or 'implies' ly's? And if that's true, how can we still see the light from those stars? Or is that a whole different question, (beacuse the light is constantly traveling towards us!)

We see the light from stars because they send light in all directions, and some of that light comes in our direction. If we are looking in the star's direction, some of that light enter through the pupil, and if it is enough, triggers the receptors on the retina. The fact that enough light can reach us from some far distance star is a testament of just how much light stars put out. Our own sun produces enough light to be seen for 10's of light years by the naked-eye. Most of the stars you see in the night sky are much, much brighter, which is why we can see them from much further distances away. (there are a lot more stars out there than we see with the naked eye. Tons of them are closer than the stars we can see. They are just too dim to see with the naked eye because they aren't putting out as much light to begin with. of the ~75 closest stars to us, only 9 of them are naked-eye visible)

As as far as the distance between stars goes. Angular separation is only a part of the problem.
Below is a diagram showing the sight-lines two stars.

The stars send light off in all directions, as shown by the small arrows, but only some of it ( the longer lines shown) comes in our direction.
The angle between the lines heading to our eyes is the angular separation. However, this doesn't tell us everything. One of those stars is much further away. So even though they look like they are close together to our eyes, they are really far apart. (This occurs within the groups of stars we call the constellations also. They look like a closely grouped bunch, but the actual distances between stars can be quite great. Stars in completely different constellations can be physically closer to each other than stars in the same constellation are.)

To figure out just how far apart the stars in the diagram really are, we also have to know how far away they are from us. There are a number of methods of doing this, depending on how far away the stars are.

I believe someone has already mentioned parallax. This is the apparent shift of foreground objects compared to background objects when viewed from different points of view. A simple example of this is to hold up 1 finger at arm's length and look at it with just one eye and then the other. It will "jump" back and forth relative to the background.

If we look at one of the nearer stars, we can measure how much it appears to shift compared to further away background stars when we look at it from different sides of the Earth's orbit. (this means you have to wait 6 months between measurements) This allows us to work out how far away they are. Closer stars will shift more than further ones.
With our two stars above, it might look like this.
The vertical lines are the background reference and the upper and lower pairs of stars are how they look when seen from different points of our orbit around the Sun.

Beyond a certain distance, this becomes too small to measure, and we have to rely on other methods. The star on the left is the closer star and shifts more relative to the background, than the right star, which is further away.

This shift gets smaller an smaller the further stars are from us, and beyond a certain distance becomes too small to accurately measure. Other methods for determining star distances are then used.

 

[jbriggs444]

Would it be correct to say, the distance we see between stars 'represents' or 'implies' ly's? And if that's true, how can we still see the light from those stars? Or is that a whole different question, (beacuse the light is constantly traveling towards us!)

It is still far from clear what problem you imagine is present.

We see the light from the stars that were, at a particular time in the past, positioned in the direction we are looking now. Those stars were, at that time, shining. Some of their light happened to be shining directly at the place where your pupil was positioned when you looked that way.

The light from a particular direction in the sky is focused (by the lens in your eye) on a particular place on the retina. That mapping of direction in sky to position on retina is the primary way in which we sense where (i.e. in what direction) a particular object is located.

We determine angular separation because the rods or cones that are activated by one star are at some distance (along the curve of the retina) from the rods or cones that were activated by another star. Or by how far we have to turn our head or swivel our eyes so that the same rods and cones are activated by the other star.

We determine range with a variety of other methods.

With telescopes, we can measure angular separation more precisely, for instance, by measuring the separation of black dots on a negative image with a known scale. But the principle is the same.

 

[sophiecentaur]

It is still far from clear what problem you imagine is present.

The OP really needs to do some observing and get immersed with the practicalities, rather than worrying about why what and how, I think. Most of his/her questions could just as well apply to looking at a cinema film in which the whole image sits on a plane screen. The brain decodes that image and we 'see' the most likely solution to the puzzle of what's actually there.

 

[Surayabay]

The OP really needs to do some observing and get immersed with the practicalities, rather than worrying about why what and how, I think. Most of his/her questions could just as well apply to looking at a cinema film in which the whole image sits on a plane screen. The brain decodes that image and we 'see' the most likely solution to the puzzle of what's actually there.

Hey, what am I, chopped liver?

sophiecentaur, and russ_watters, Please don't get upset with me! What I'm not understanding is, we see the angular separation with our eyes. it could be a few, to many K-ly's. (even though it appears and looks like a short distance between those two stars, to us.) But it's really ly's, (assume for this model, they are a known distance between one another and have a realative Mag brightness.) So, that separation, (the 'separation angle' / distance from one to the other we see would imply or represent a few or many K ly's, right? Please answer, (Yes or No?) If yes, then in a sense, we can see ly's from one point of light, (star) to another! Right? (yes or no? - then explain, please). OK, then how can we still see the light from those two stars on the same scale we see the separation between them? When I compare the scale between the stars were looking at in this model and the ly distance of separation we can see, it doesn't seem to fit!? (I can see two stars, points of light, 'AND' the ly distance between them, with my eye, from my observation point on earth?) That's what I can't figure out! (yes, I know, the stars in this example may be dead, and their light is still traveling towards us.) Thanks again!

 

[Surayabay]

What you are describing is the angle between them. If two objects are each 1m away from you and to your eyes are 45 degrees apart, they are 1m away from each other. Or scale that up to a million light years. It gets more complicated if the stars are different distances from us, but either way, it's just triangles.

Think about a photo of Jupiter vs the real Jupiter. The real Jupiter is so small looking because it is far away. In the photo it looks big because it is closer and subtends a larger angle, even though the photo is smaller than the real thing.

Russ,
russ_watters, Please don't get upset with me! What I'm not understanding is, we see the angular separation with our eyes. it could be a few, to many K-ly's. (even though it appears and looks like a short distance between those two stars, to us.) But it's really ly's, (assume for this model, they are a known distance between one another and have a realative Mag brightness.) So, that separation, (the 'separation angle' / distance from one to the other we see would imply or represent a few or many K ly's, right? Please answer, (Yes or No?) If yes, then in a sense, we can see ly's from one point of light, (star) to another! Right? (yes or no? - then explain, please). OK, then how can we still see the light from those two stars on the same scale we see the separation between them? When I compare the scale between the stars were looking at in this model and the ly distance of separation we can see, it doesn't seem to fit!? (I can see two stars, points of light, 'AND' the ly distance between them, with my eye, from my observation point on earth?) That's what I can't figure out! (yes, I know, the stars in this example may be dead, and their light is still traveling towards us.) Thanks again!

 

[Surayabay]

Are you simply asking how can we figure out how far apart two stars are? None of that is done with the naked eye, it requires extremely precise measurements.

The closest stars we determine their position through parallax. If you take two objects that are different distances from your eye and move your head, these objects will change position relative to your eyes. That change is related to how far apart they are and how much you are moving your head. The Earth moves back and forth about 300 million kilometers every year, this cause the positions of the closest stars to change ever so slightly, but by a large enough amount that we can measure them.

newjerseyrunner,
Please don't get upset with me! What I'm not understanding is, we see the angular separation with our eyes. it could be a few, to many K-ly's. (even though it appears and looks like a short distance between those two stars, to us.) But it's really ly's, (assume for this model, they are a known distance between one another and have a realative Mag brightness.) So, that separation, (the 'separation angle' / distance from one to the other we see would imply or represent a few or many K ly's, right? Please answer, (Yes or No?) If yes, then in a sense, we can see ly's from one point of light, (star) to another! Right? (yes or no? - then explain, please). OK, then how can we still see the light from those two stars on the same scale we see the separation between them? When I compare the scale between the stars were looking at in this model and the ly distance of separation we can see, it doesn't seem to fit!? (I can see two stars, points of light, 'AND' the ly distance between them, with my eye, from my observation point on earth?) That's what I can't figure out! (yes, I know, the stars in this example may be dead, and their light is still traveling towards us.) Thanks again!

 

[sophiecentaur]

But it's really ly's,

It's just as "really" metres or AU, remember. Parsecs are also a very useful measure of distances and the parsec also reminds us that it's apparent angular displacement (parallax) that give us the clue about the distance away.

I can see two stars, points of light, 'AND' the ly distance between them, with my eye, from my observation point on earth?

How do you achieve that?
I don't know how much you have read about all this but I think you would benefit from some self study, rather than trying to do it by Q and A. You seem to be chasing your tail a bit with this and repeating your questions despite having been given answers. Try this link and Google "Measuring distances in Space", for example. The history of how people first measured the distances to objects is very interesting and could help resolve some of your difficulties.

 

[Surayabay]

It's just as "really" metres or AU, remember. Parsecs are also a very useful measure of distances and the parsec also reminds us that it's apparent angular displacement (parallax) that give us the clue about the distance away.

How do you achieve that?
I don't know how much you have read about all this but I think you would benefit from some self study, rather than trying to do it by Q and A. You seem to be chasing your tail a bit with this and repeating your questions despite having been given answers. Try this link and Google "Measuring distances in Space", for example. The history of how people first measured the distances to objects is very interesting and could help resolve some of your difficulties.

That's an excellent idea, I will read it. I thought this site was for any level of cosmic/astronomy questions. Thank you!

 

[jbriggs444]

I thought this site was for any level of cosmic/astronomy questions.

Trying to be gentle...

Better answers can be obtained if more focused questions are asked. It is not yet clear what question you are asking.

 

[sophiecentaur]

That's an excellent idea, I will read it. I thought this site was for any level of cosmic/astronomy questions. Thank you!

True - any level but PF is not a Physics Course or an Astronomy Course. It's hopeless as a sole source of information because, for a start, there is no 'organisation' between contributors or information structure. Once you have read a few other sources, you will find that your PF questions will be better formed and then they can be answered better too.
PS Q and A was first used, so I believe, by posh Ancient Greek boys with their tutors. Books and didactic teaching took over, with bursts of 'tutorial' time because it is more efficient.

 

[JLowe]

Trying to be gentle...

Better answers can be obtained if more focused questions are asked. It is not yet clear what question you are asking.

I think it has been answered but OP has yet to understand the answer.

But maybe I’m wrong.

 

[jbriggs444]

So, that separation, (the 'separation angle' / distance from one to the other we see would imply or represent a few or many K ly's, right? Please answer, (Yes or No?)

No. The separation angle does not represent a spatial separation.

Together with range measurements from viewer to each star, one could perform some simple trigonometry and compute the spatial separation based on the angular separation. But there is no sense in which one is seeing the spatial separation.

 

[sophiecentaur]

This thread is beginning to remind me of the Father Ted scene in the caravan.

 

[davenn]

That's an excellent idea, I will read it. I thought this site was for any level of cosmic/astronomy questions. Thank you!

It is, but it also helps if you have a very basic understanding
You are asking questions but lack the basic knowledge to understand the answers you have been given

Once you have read a few other sources, you will find that your PF questions will be better formed and then they can be answered better too.

... and that the answers so far given would be understood or more likely to be understood by the questioner

I think it has been answered but OP has yet to understand the answer.

Exactly ... because of my previous two comments

 

[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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