Surayabay said:
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.
