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If light keeps travelling in a straight line, why can't we see distant galaxies with the naked eye? Surely if you stared long enough, the light from them would eventually hit your eye? I apologize if this is a dumb question:)

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    $\begingroup$ The TL;DR answer: Your eyes don't work anything like a telescope with an arbitrarily long exposure time. Our eyes evolved to best suit human's diurnal nature and living in a dangerous world. A number of those adaptations that suited our survival militate against seeing very dim stationary objects at night. $\endgroup$ – David Hammen Jan 15 '15 at 14:12
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    $\begingroup$ Because you're not a frog $\endgroup$ – Brian S Jan 15 '15 at 20:11
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    $\begingroup$ Stare at the sun (don't). All the light is hitting your eyes, so why can't you make out the details? There's more to vision than just light reaching you. $\endgroup$ – Matthew Read Jan 16 '15 at 5:31
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Surely if you stared long enough, the light from them would eventually hit your eye?

Collecting light over a long span of time is how telescopes can see very dim objects. The human visual system doesn't work that way.

For one thing, even when you think you are staring at something, your eyes still dance around a bit. It's a built-in response called ocular microtremors. These microtremors appear to be an essential part of making the vision system functional.

For another, your eye does not and cannot collect light for arbitrarily long periods of time (the way a photographic telescope can). There's an immense amount of signal processing that happens in the eye and along the way to the brain. This signal processing depends on light being collected for short spans of time.

Our vision system evolved to see food, friends, and dangers under well-lit conditions. We are very good at seeing motion in broad daylight. We're not so good at seeing stationary objects, and we're not good at all at seeing barely visible sources under very dark skies.

Naked eye astronomy is limited by the nature of the human vision system. The most remote object we can see is the Triangulum Galaxy, and that's only under conditions of extremely dark and extremely clear skies.

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    $\begingroup$ Note that in that particular case, we can see a distant galaxy. $\endgroup$ – Mooing Duck Jan 15 '15 at 20:35
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    $\begingroup$ @MooingDuck - To me, a "distant galaxy" is one whose light we see just now was in fact emitted several billion year ago. The Triangulum Galaxy is just next door by that standard. It is not a "distant galaxy". Another way to look at it: There are (cue Carl Sagan) billions and billions of galaxies in the observable universe. Of that huge number, we can see but four with our naked eye. $\endgroup$ – David Hammen Jan 15 '15 at 20:55
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    $\begingroup$ It may not be a distant galaxy, but it is a galaxy and it is distant. :P $\endgroup$ – Lightness Races with Monica Jan 16 '15 at 16:22
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Not at all a dumb question, but actually you can see distant galaxies with the naked eye. From the northern hemisphere, the Andromeda Galaxy, our biggest neighboring galaxy, is visible if you know where to look, and is at a reasonably dark place. From the southern hemisphere, the two smaller, but nearer, irregular galaxies called the Small and Large Magellanic Clouds are visible.

The reason that more distant galaxies are not visible, is due to the inverse-square law: As the light particles (photons) recede from the galaxy (or any other light source), they are distributed over an ever-increasing surface. That means that a detector (e.g. your eye) of a given area will catch less photons, the farther it is placed from the galaxy. The law says that if in a time interval Δt on average it detects, say, 8 photons at a distance D, then in the same time interval, at a distance 2D it will detect 8/22 = 2 photons. At a distance of 4D, it will detect 8/42 = 0.5 photons. Or, equivalently, it will need twice the time to detect a single photon.

The bottom line is that in principle you can see the very distant galaxies, but the photons are so few and arrive so rarely, that your eye is not a good enough detector. The benefit of a telescope is that 1) it has a larger area than your eye, and 2) you can put a camera at its focal point instead of your eye and take a picture with a large exposure time, i.e. increasing the Δt.

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    $\begingroup$ In principal, you can't see very distant galaxies with your naked eye. The Andromeda Galaxy, the Magellanic Clouds, and the Triangulum Galaxy are nearby galaxies, not very distant galaxies, and those are the limits of naked eye astronomy. There are a number of characteristics of the human vision system that militate against seeing anything more remote than those four. $\endgroup$ – David Hammen Jan 15 '15 at 14:08
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    $\begingroup$ Okay, I guess I interpreted the word "distant" in relation to the stars in the Milky Way, but I suppose @DavidHammen is right that the intention was more like "more distant than the galaxies that we can see". As I tried to describe in the end of my answer but David explains better in his, the eye is not a good enough detector. That's what I meant by "In principle... but..." $\endgroup$ – pela Jan 15 '15 at 15:12
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    $\begingroup$ @DavidHammen It might be worth noting that you can't even see very much of Andromeda with the naked eye (just the core) - the whole Andromeda galaxy is about 6 times wider than the full moon! $\endgroup$ – Michael Jan 15 '15 at 18:56
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Your reasoning would be valid not only for galaxies, but also for stars and anything it shines in the Universe, but there is an important effect which invalidates it: absorption of light.

Intergalactic and interstellar medium is filled with dust and gas, which contributes to absorb and scatter the light from distant objects. Especially on the plane of our Galaxy, we still have plenty of gas and dust (Milky Way is a relatively young galaxy): indeed, to look at distant object we try to orient our telescopes towards the Lockman Hole, whenever it is possible.

This is especially valid for low frequencies light: at higher energies, scattering and absorption of X-ray and gamma-rays from standard amount of absorbing material is negligible (even if, the more distant you look at, the younger the objects, the more is the dust and gas available which is still not locked in stars).

Also, consider the Olbers' paradox, which indicates for an expanding Universe to account for the "dark sky".

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    $\begingroup$ True, in my answer I didn't consider absorption. At visible wavelengths, however, which is by definition what the eye can detect, that's not the major reason that we cannot see distant galaxies, except if, as @Py-ser mentions, you look through the plane of the Milky Way. In most directions, the extinction (called A_V) at visible wavelengths is of the order 0.1-0.5 or so, meaning that only 10% to 30% of the light is absorbed. $\endgroup$ – pela Jan 15 '15 at 13:29
  • $\begingroup$ Agreed. Optical absorption is only important in certain directions. It does not prevent us seeing distant galaxies unless the absorption is intrinsic to them. $\endgroup$ – Rob Jeffries Jan 16 '15 at 15:51
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  1. Few photons -- You have tiny pupils. Only photons that manage to travel that far over that much distance along a path that manages to intersect with your tiny pupils will have a chance of being seen. And only some photons that reach your retina actually interact with molecules that register their arrival.

  2. Interference -- The molecules of the atmosphere, dust in the atmosphere, reflection/refraction of and in your eye, dust in the solar system, the Oort cloud, interstellar dust in our galaxy, dust in intergalactic space, any molecule at all along the path, all may absorb any of the few photons and re-emit them in a different direction.

  3. Stability -- Telescopes, especially such as Hubble, can be really, really still compared to your eyes. Not only do your eyes constantly make tiny shifts, but you breathe and your heart beats and other things keep very dim images from being able to form.

  4. Exposure -- The very first Hubble Deep Field image was collected over around 100 hours of exposure. You might find that difficult with your eyes.

  5. Retention -- Exposure time affects how much 'data' is retained about where photons have hit the recording surface. Your eyes will not remember that a photon registered at a receptor even a minute earlier. Your eyes are not good at all for 'still photography'.

  6. Light pollution/universal expansion -- The universe has been expanding for billions of years. As it expands, light propagating through space 'stretches' more to the red end of the visible spectrum. For distant galaxies this effectively means that the visible light from them has shifted far enough to be infrared and invisible by the time it gets here. Now, ultraviolet light would also shift, some of it becoming 'visible'. But it then starts getting mixed with any scattered 'light pollution' effects once it gets to our atmosphere. Your eyes aren't good at all in keeping track of which photons come from what sources.

There are probably other factors, but maybe those are more than enough to indicate how big the problem is. Note that the early 100-hour Hubble image was a big surprise to astronomers. Even with the large light-gathering telescopes previously available, they were unable to get enough light for useful data. That earlier equipment had much larger pupils than yours, more sensitive imaging surfaces and could 'sit still' much longer than you; and it still had difficulty with distant galaxies.

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  • $\begingroup$ You are the only one that has mentioned redshift - surely important for "distant galaxies". I'll give you +1 if you can work in the Lyman break. It's this that prevents us seeing redshifted UV. Not sure what you mean by mixed with scattered light pollution? $\endgroup$ – Rob Jeffries Jan 16 '15 at 15:48
  • $\begingroup$ @RobJeffries Light pollution affects our vision of any dim celestial objects by coming from all directions as the (dusty/smoky/etc.) atmosphere scatters light from any nearby emitting sources, overwhelming dim light. The Lyman-break effect would probably fit under '2. Interference' since it's a good example of "stuff" in space absorbing photons. But it shouldn't be as significant for truly 'distant' galaxies since the red-shifted light ought to be less or even not absorbed by more local interstellar gasses. (I'm no expert on that by any means, but it seems reasonable.) $\endgroup$ – user2338816 Jan 16 '15 at 19:35
  • $\begingroup$ "Lyman break" galaxies are only at large distances. Their rest frame absorption by neutral H takes out intrinsic UV light, and the galaxies become invisible to us at their redshifted wavelengths. $\endgroup$ – Rob Jeffries Jan 16 '15 at 20:19
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Just because you can keep your eyelids open for $x$ seconds doesn't mean you are collecting light for $x$ seconds and using it to form a single image in your brain. How would you "save" the photo? How would you decide when to end light collection? You know as well as I do that you can't simply lift your finger off your brain's shutter release!

And that's on top of all the other factors as explained in other answers (but I wanted to belabour this particular point a little further than do the other answers.).

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I think your question is a reframing of what is known as "Obler's paradox" - namely if the universe is infinite why is the night sky not white, as sooner or later our line of sight hits a star, and even if very far away there would be infinite stars out there.

The answer to this is either (a) the Universe is not infinite or (b) the Universe has not been here for ever, so even if it is infinite, light from very far away is yet to reach us.

Case (b) is generally accepted - ie the Universe began a finite time ago in the "big bang" - though (a) is disputed - ie it may be that the universe is not infinite in any case.

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Can a Human See a Single Photon?

The human eye is very sensitive but can we see a single photon? The answer is that the sensors in the retinacanrespond to a single photon. However, neural filters only allow a signal to pass to the brain to trigger a conscious response when at least about five to nine arrive within less than 100 ms. If we could consciously see single photons we would experience too much visual "noise" in very low light, so this filter is a necessary adaptation, not a weakness.

According to this paper http://math.ucr.edu/home/baez/physics/Quantum/see_a_photon.html

As this is not always possible for distant galaxies we cannot see distant galaxies.

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The core of the question has already been answered, but it's still interesting to illustrate just how difficult it is to make a naked eye observations of a the extremely bright nearby galaxy M81. The astronomer Brian Skiff gives an account of his successful naked eye observation of this galaxy here.

Now, galaxies of a given brightness are more difficult to spot than stars of the same brightness, because of their extended nature. If the sky is sufficiently dark, then you can see stars as faint as magnitude 8, but you'll still struggle to spot M81 that has a brighness of magnitude 7. The magnitude 7 is an artificial figure obtained by adding up the light that comes from slightly different directions.

Also, you only need very slight amount of light polution to make the sky backgound just a tiny bit grey to make the galaxy vanish from view, while the visibility of faint stars remains essentially unaffected. This is because the brighness as a function of the position in the sky in the case of a star has a very strong and narrow peak while in case fo a galaxy, due to its extended nature, doesn't show a big peak. The integrated brightness may be the same for both cases, but the amount of background light you need to make the galaxy invisible is obviously much less than what you need for the star.

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