Seeing star light -- Why does it take so long?

[Don H]

O.K. I'm new here so if this has been answered I don't know how or where to find the answer.

I'm not a scientist just a mechanic so if you could explain this simply maybe i can understand it.
Maybe ELI5.

Light I see in the sky has been traveling for a long time and "is here now".
As far as I figure, the light is far past the Earth also, so I'm in their light beam already.
Why don't I see all the stars in the sky "now", because their light is already here, right?

 

[Bystander]

Have you tried "Olber's Paradox?"

 

[mathman]

You can see all the stars that are bright enough. With a telescope you can see more. (I assume you are talking about on a clear night.)

 

[Don H]

Have you tried "Olber's Paradox?"

I don't think Olber accounted for the exponential dissipation of light.

I also don't think this addresses my question.
Thank you anyway.

 

[Don H]

You can see all the stars that are bright enough. With a telescope you can see more. (I assume you are talking about on a clear night.)

I think you are hitting more closely on what I'm asking.

The longer I stare into the sky the more stars I see.

As in your telescope statement, I'm increasing my depth of vision and so seeing more stars more quickly by magnification, as in the light is not hear yet.

Thus my question; if the light is already here, why can't I see it "Now"?
And yes obviously on a clear night.

 

[Nugatory]

Why don't I see all the stars in the sky "now", because their light is already here, right?

The light that is here now left the star years ago. If your telescope or your eyes didn't catch it, it would keep on moving past you - but there's more light behind it, so there is always light where you are.

Whether you can see it or not depends on how bright the light is; if it's too dim for your eyes to register it, you won't see it even though it's there. The telescope concentrates the light, increasing its brightness so that you can see it.

 

[davenn]

The longer I stare into the sky the more stars I see.

Tho you probably don't realize it, you are referring to dark adaption of your eyes.
Only up to a certain point ... your eye dark adaption only takes you so far ... for the avg eyesight person in a dark location around
6th magnitude and up to around 7th mag for a person with very good eyesight

As in your telescope statement, I'm increasing my depth of vision and so seeing more stars more quickly by magnification, as in the light is not hear yet.

Here, not hear

that is incorrect...
the telescope just let's you see fainter stars ... the light is already here, else you wouldn't see it, telescope or not

Ohh and the only stars you see ( naked eye) are ones from within our Milky Way galaxy ... and there is not an infinite number
of them. The galaxy has a finite number of stars and not all of them are visible from our place (perspective) in the galaxy

Thus my question; if the light is already here, why can't I see it "Now"?
And yes obviously on a clear night.

As I and others have stated ... you can ... but ONLY those bright enough to be seenDave

 

[phinds]

Check out the Hubble Deep Field.

 

[Don H]

The light that is here now left the star years ago. If your telescope or your eyes didn't catch it, it would keep on moving past you - but there's more light behind it, so there is always light where you are.

Whether you can see it or not depends on how bright the light is; if it's too dim for your eyes to register it, you won't see it even though it's there. The telescope concentrates the light, increasing its brightness so that you can see it.

That is part of what I was looking for. The other part is why does it take so long for my eyes to see some/most light if it's already here? The longer I stare the more I see. Is this because of the dim factor? There isn't much left in the way of photons after traveling so far with the exponential dissipation factor that it is my eyes that are not picking up enough light to register it, not that the light is not here?
Thanks in advance.

 

[Don H]

Tho you probably don't realize it, you are referring to dark adaption of your eyes.
Only up to a certain point ... your eye dark adaption only takes you so far ... for the avg eyesight person in a dark location around
6th magnitude and up to around 7th mag for a person with very good eyesight

Here, not hear

that is incorrect...
the telescope just let's you see fainter stars ... the light is already here, else you wouldn't see it, telescope or not

Ohh and the only stars you see ( naked eye) are ones from within our Milky Way galaxy ... and there is not an infinite number
of them. The galaxy has a finite number of stars and not all of them are visible from our place (perspective) in the galaxy

As I and others have stated ... you can ... but ONLY those bright enough to be seenDave

Thank you.

 

[Don H]

Thank you.

Check out the Hubble Deep Field.

My favorite.
I think I understand it as my eyes aren't registering the dim light.
Thank you.

 

[Nugatory]

The other part is why does it take so long for my eyes to see some/most light if it's already here? The longer I stare the more I see.

Several things going on here, all part of how the eye and brain work. It takes a few minutes for your optical system to adjust to looking for very dim objects against a dark background.

 

[Don H]

Several things going on here, all part of how the eye and brain work. It takes a few minutes for your optical system to adjust to looking for very dim objects against a dark background.

O.K., I think this is my answer. I just didn't think it took my eyes that long to gather light.
Thank you.

 

[pinball1970]

O.K., I think this is my answer. I just didn't think it took my eyes that long to gather light.
Thank you.

Try and pick a spot with as little ambient light as possible, city lights obscure star light. The extreme of this scenario is the sun, we don’t see any stars in day because the sun is obscuring and overpowering all the star light.

It depends where you live but get yourself out to country if you can far from the city.

I am not sure if this will work but there are more rods in your peripheral vision, rods are more sensitive to light than cones. Perhaps you can see the dimmer ones better? http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html

 

[Drakkith]

I just didn't think it took my eyes that long to gather light.

Indeed, you eyes don't take long to gather light at all. In my astrophotography camera, I can set my 'integration time', the time between opening the shutter and closing it again, to be anything from a fraction of a second up to an entire hour. The longer the shutter is open the more light falls onto the sensor. In a perfect sensor, each photon would move a single electron into the region of the sensor where it is subsequently 'read out', corresponding to a detection of that photon. Many detection events per pixel, corresponding to a lot of light falling on that pixel, makes that pixel brighter in your image. The longer my shutter is open the more light falls onto the sensor, the more electrons end up being read out, and thus the image is brighter overall and can see fainter objects.

Your eye is not like this.

Instead, the rods and cones in your eye require a sort of 'minimum signal' to activate, at which point it's an all-or-nothing affair. The cells are either active or they are not. Each cell is directly connected to a network of other cells that perform basic processing before the signal is sent to the optic nerve and then the visual centers of the brain. But here's the thing. The first part of this processing network has multiple cones/rods connected together. To prevent visual overload, many different cone cells are required to fire within a short span of time for their part of the network to send its signal. But for rods, it's often sufficient for just a single cell to fire to trigger the network and send the signal to the brain that something has been seen. After firing, both rods and cones require a short cooldown while their photosensitive chemicals are refreshed and the cell 'resets'.

My overall point, other than to quickly describe the basics of how the visual system works, is to get across the point that your eyes are not cameras. They don't really have an 'integration time'. This is one of the main reasons your night vision is so limited. You can't stare at a spot for several seconds while your eye gathers photons before sending the signal to your brain. Instead we have to make do with a continual process that generates a very dim visual field with only very limited options to improve it.

 

[Janus]

I am not sure if this will work but there are more rods in your peripheral vision, rods are more sensitive to light than cones. Perhaps you can see the dimmer ones better?

This is actually a common tactic for viewing dim sky objects better. Rather than look directly at it, you look to one side of it.

 

[davenn]

O.K., I think this is my answer. I just didn't think it took my eyes that long to gather light.
Thank you.

For the average person, it takes a good 20 minutes for your eyes to become dark adapted

I am not sure if this will work but there are more rods in your peripheral vision, rods are more sensitive to light than cones. Perhaps you can see the dimmer ones better?

This is actually a common tactic for viewing dim sky objects better. Rather than look directly at it, you look to one side of it.

yes to both comments ... commonly called averted vision

 

[DaveC426913]

O.K., I think this is my answer. I just didn't think it took my eyes that long to gather light.
Thank you.

Specifically, it takes that long for your pupils to expand. Full dark adaptation takes as much as 30 minutes.
When your pupils are small, they let in a proportionately small number of photons. Your rods and cones have a lower limit of sensitivity.

Ah. Drakkith might have already addressed this.

 

[davenn]

Ah. Drakkith might have already addressed this.

yeah, several of us had

I initially did a way back in post #7

 

[stefan r]

O.K., I think this is my answer. I just didn't think it took my eyes that long to gather light.
Thank you.

You can go the other way. go to a dark room with a mirror. look at your eyes. Turn on a bright flashlight and burn your retinas. You should see your irises clamp down on the pupil. Pupils will not reopen all the way for a long time.