<p>As Rob Jeffries says, the universe is mostly empty space. A photon can easily travel thousands of light years without interacting with anything. Most of the interaction would occur when photons entered the earth's atmosphere. The Hubble avoids this. These photos were most likely from combining several viewing sessions giving basically an extended time period for observing the galaxy.</p>

CC BY-SA 3.0
2017-09-10T19:42:45.430Z
Galaxies were observed more than 200 years before Hubble, which just shows that light can travel a long way through even a relatively dense medium (our atmosphere) without being absorbed to a great extent.
Dr Chuck
CC BY-SA 3.0
2017-09-11T20:22:25.523Z
@DrChuck The Andromeda galaxy has been observed for much longer than that since it's very well visible with the naked eye. If there's one thing I'm jealous of the good old days, it's the lack of light pollution.
Eric Duminil
CC BY-SA 3.0
2017-09-11T22:26:16.800Z
Or as Douglas Adams said, "Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space."
T.E.D.
CC BY-SA 3.0
2017-09-10T19:31:09.377Z
2017-09-10T21:17:33.587Z
2017-09-10T21:17:33.587Z
Natsfan
<p>There's a misconception in your question I don't think the other answers have addressed. </p>

<blockquote>
  <p>If light emitted from the galaxy travels in all directions, then how
  is it that we can still map out the entire galaxy</p>
</blockquote>

<p>Light <strong>is</strong> emitted from the galaxy in all directions. Only a tiny, tiny fraction of it is directed to Earth, and of that, an even tinier fraction is collected by any given telescope. But we can still see it, because galaxies are very, very bright. Andromeda contains about a trillion stars. </p>

CC BY-SA 3.0
2017-09-11T12:57:42.363Z
This is a useful point to make. If you spend the effort to calculate the total number of photons Andromeda emits per second, you'll find an astronomically high number (think $10^{60}\:\mathrm{photons/s}$ at a minimum). So its no wonder we can collect $\sim 10^3\:\mathrm{photons/pixel}$ in a single, longer-than-a-second observation with a telescope.
zephyr
CC BY-SA 3.0
2017-09-11T03:02:18.510Z
2017-09-11T03:02:18.510Z
Rupert Morrish
<p>Sorry if this logic seems a bit circular, but we can get unobscured pictures of galaxies because they are unobscured.</p>

<p>As has been mentioned - space is really, really big and really, really empty. This is hard for us to contemplate, because there's so much stuff right next to us - but this is actually a really unusual condition. The next star to the Sun is over 4 light-years away, but we get almost all (99.9999999999...%) the light from it that heads in our direction - the same with light from further away - we get a huge number of photons sent to us from objects very far away.</p>

<p>Hubble also uses the simple camera techniques of lensing and long exposures to take images of distant objects - so more light is received to construct the image.</p>

<p>But, the other part of this, it is almost impossible to take a picture of a galaxy (or star) that is behind another galaxy or dust cloud. For example, we can't easily see past the centre of our own galaxy, because there's a lot of dust and gas and stars in the way. The picture in your question, on the other, seems to be Andromeda, which is above the plane of the galaxy. Our galaxy is quite thin compared to its diameter, and we're a decent way out of the galactic centre, meaning there's a lot less stuff in the way.</p>

<p>And there are <a href="https://en.wikipedia.org/wiki/NGC_3628" rel="nofollow noreferrer">some galaxies</a> we've taken images of which are obscured by dust:</p>

<p><a href="https://i.sstatic.net/c5Ukh.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/c5Ukh.jpg" alt="enter image description here"></a></p>

CC BY-SA 3.0
2017-09-12T20:45:27.123Z
"a lot less crap in the way" -- can we try to answer without so much technical jargon?
Barmar
CC BY-SA 3.0
2017-09-11T02:27:27.400Z
2017-09-12T22:10:44.853Z
2017-09-12T22:10:44.853Z
HorusKol
<p>There's been some good answers already, but I'd like to add my two-pennyworth:</p>

<blockquote>
  <p>How do we have photos of galaxies so far away?</p>
</blockquote>

<p>Because there's nothing much between them and us that interferes with the light that reaches our cameras. </p>

<blockquote>
  <p>A possible answer for this is that, light emitted from the galaxies travelled a billion miles all the way to earth, where the Hubble space telescope picked up this light through its sensors, and was able to construct an image of the galaxy</p>
</blockquote>

<p>It's a billion miles to <em>Saturn</em>. Well actually the distance varies with the orbits, but see <a href="https://www.space.com/18477-how-far-away-is-saturn.html" rel="nofollow noreferrer">this Space.com article</a>: <em>"At their most distant, when they lie on opposite sides of the sun from one another, they are just over a billion miles (1.7 billion km) apart"</em>. The <a href="https://www.google.co.uk/search?q=andromeda+galaxy+how+far+miles&amp;oq=andromeda+galaxy+how+far+miles&amp;gs_l=psy-ab.3..0i22i30k1l2.82831.84293.0.84392.6.6.0.0.0.0.134.542.2j3.5.0....0...1.1.64.psy-ab..1.5.538...0.UhwIvdw4S-A" rel="nofollow noreferrer">Andromeda galaxy</a> is circa fifteen billion <em>billion</em> miles away. Or circa fifteen quintillion miles. </p>

<blockquote>
  <p>but if this is true, and galaxies are billions of miles away, shouldn't the light particles emitted from the galaxies be scattered all over the place?</p>
</blockquote>

<p>Don't forget that photons have an E=hf wave nature. And that even though they are scattered in the air, you can still see the Moon. Yes, there's a bit of light going astray in space. But not so much that the night-time sky is some blank foggy fug. You can see Saturn too. And the stars. And the galaxies, but they are <a href="https://www.google.co.uk/search?q=andromeda+galaxies+very+dim&amp;oq=andromeda+galaxies+very+dim&amp;gs_l=psy-ab.3...7848.14077.0.14358.16.16.0.0.0.0.127.1150.13j3.16.0....0...1.1.64.psy-ab..1.0.0.3AYbIEWlZKk" rel="nofollow noreferrer">rather dim</a>. </p>

<blockquote>
  <p>after all they have been travelling from millions of years, and have probably collided with asteroids and other foreign objects. What were the chances that about 95% of the photons actually reached earth, giving us a very detailed image. </p>
</blockquote>

<p>The chances are high. We have pictures of planets and things <em>because</em> the chances are high. </p>

<blockquote>
  <p>Consider the Andromeda galaxy which has a distance of 1.492 × 10^19 mi from earth. If light emitted from the galaxy travels in all directions, then how is it that we can still map out the entire galaxy, evident from the photo below?</p>
</blockquote>

<p>If I was covered in lights, I would emit light in all directions, and you would see me because some of that light goes into your eye. The Andomeda galaxy is similar. </p>

<blockquote>
  <p>Shouldn't like half of the galaxy be missing since photons could have hit other objects, and "never have reached earth"? </p>
</blockquote>

<p>No. And if half the photons didn't reach Earth, you'd just see a dimmer galaxy, that's all. </p>

CC BY-SA 3.0
2017-09-12T16:18:45.817Z
2017-09-12T16:18:45.817Z
John Duffield
<p>There are two reasons that often — but not always — light from galaxies millions and even billions of lightyears away make it through the Universe and down to us:</p>

<strong>Particle <em>number</em> and particle <em>size</em></strong>

<ol>
<li><p>First, the intergalactic medium (IGM) is <em>extremely</em> dilute. The number density of particles out there is of the order $n\sim10^{-7}\,\mathrm{cm}^{-3}$, or roughly 26 orders of magnitude lower that the air at sea level! That means that if you consider a tube from Andromeda to the Milky Way with cross-sectional area of $1\,\mathrm{cm}^{2}$, it will contain roughly one microgram of matter (thanks to Rob Jeffries for catching a factor $10^6$ error).</p></li>
<li><p>Second, even if a photon comes close to an atom, it will only be absorbed if its energy matches closely some transition in the atom. Since most of the atoms are ionized (and thus should be called plasma instead, but in astronomy the distinction if often not made), there are no electrons to absorb the photon. The photons are more likely to interact with the free electrons via Thomson scattering, but the Thomson cross section is immensely small $(\sim10^{-24}\,\mathrm{cm}^{2})$, so even if you consider the <a href="https://en.wikipedia.org/wiki/Cosmic_microwave_background" rel="noreferrer">CMB photons</a> — which have traveled through the Universe almost since the Big Bang — only around 5% of them have interacted with electrons on their way.</p></li>
</ol>

<p>In other words: The amount of transmitted light depends on two factors: 1) The amount of matter along the line of sight, and 2) that matter's ability to absorb the light. In the IGM, both are tremendously small. When the light enters the inter<em>stellar</em> medium (ISM) inside our galaxy, it may encounter denser clouds with atoms that are able to absorb the light. But usually (although not always) "dense" is still very dilute compared to Earth's atmosphere.</p>

<strong>Mathematical expression</strong>

<p>In general, if a beam of light traverses a region of particles, each with a cross section $\sigma$ (measured e.g. in cm$^2$), passing $N$ particles per area of the beam (measured e.g. in cm$^{-2}$), then the opacity of the medium is given by the <em>optical depth</em> $\tau$, defined by
$$
\tau \equiv N \, \sigma.
$$
The transmitted fraction $f$ of photons is then
$$
f = e^{-\tau}.
$$
In general $\sigma$ depends on the wavelength, and thus part of the spectrum may pass unhindered, while another part may be completely absorbed.</p>

<p>The figure below (from <a href="http://spiff.rit.edu/classes/phys443/lectures/qso/qso.html" rel="noreferrer">here</a>) shows the spectrum of a quasar lying at a distance of 22 billion lightyears, i.e. $10\,000$ times farther away than Andromeda. You see that there are several thin absorption lines (caused by intervening hydrogen clouds whose densities are a factor of 10-100 higher than the IGM), but still most of the light makes it down to us.</p>

<p><a href="https://i.sstatic.net/WSHff.png" rel="noreferrer"><img src="https://i.sstatic.net/WSHff.png" alt="qso"></a></p>

<p>Because the light we see from this quasar was emitted so long ago, the Universe was considerably smaller at that time, and thus the density was larger. Nonetheless, only a small fraction is absorbed. The farther away the light is emitted, the longer ago it was, which means smaller Universe, and higher density, and thus the more light is absorbed. If you consider this quasar (from <a href="http://classic.sdss.org/news/releases/20000413.qso.img2.html" rel="noreferrer">here</a>) which lies 27 billion lightyears away, you see that much more light is absorbed in part of the spectrum. Still, however, much light make it through to us.</p>

<p><a href="https://i.sstatic.net/fP9Zm.gif" rel="noreferrer"><img src="https://i.sstatic.net/fP9Zm.gif" alt="qso2"></a></p>

<p>The reason that it is only the short wavelengths that are absorbed is quite interesting — but that's another story.</p>

CC BY-SA 3.0
2017-09-11T12:00:28.000Z
The distance to Andromeda is $2\times 10^{24}$ cm. A 1cm$^2$ cylinder contains $2\times 10^{18}$ H atoms/ions, with mass $4\times 10^{-6}$ g ? If you had this surface density of tinfoil it would be a micron thick and I suspect not opaque to light, however the tinfoil argument is a red herring since the reflectivity of tin arises directly from its density (and electron degeneracy), not the total number of atoms present along the line of sight. @user18458
ProfRob
CC BY-SA 3.0
2017-09-11T12:34:00.560Z
Oops, thanks @RobJeffries. I don't know how I missed a factor of a million. Guess I should stop doing calculations in my head. I'll edit.
pela
CC BY-SA 3.0
2017-09-11T15:38:53.890Z
Is it correct to say the quasar is >20 billion ly away when the universe is <14 billion years old? It may now be that far away, but we are talking about the light we are measuring from it, which was not emitted from that distance. Just a bit misleading I think.
mao47
CC BY-SA 3.0
2017-09-11T18:56:44.197Z
@mao47: It is quite customary, when talking about distances to a given cosmological object, to refer to the distance to that object _now_. The distance it had when it emitted the light we see today is less commonly of interest, but is easily found: For instance, the last quasar I mention lies at redshift z = 5.82. At a given redshift z, everything was a factor (1+z) closer to each other than today, so the distance to that quasar was 27 Gly / (1+5.82) = 4 Gly (despite the Universe only being 1 Gyr old at the time).
pela
CC BY-SA 3.0
2017-09-12T11:24:08.407Z
Do you have a link to the explanation of why only short wavelengths are absorbed?
Beta Decay
CC BY-SA 3.0
2017-09-12T12:56:11.830Z
@BetaDecay: It has to do with the [Lyman α](https://en.wikipedia.org/wiki/Lyman-alpha_line) transition of neutral hydrogen at 1216 Å. As light from the quasar travels through the Universe, it is redshifted. Thus, light that is initially blueward of Lyα, will at some point become Lyα. If at this point there happens to be a hydrogen cloud, it will produce an absorption line. [cont'd below]
pela
CC BY-SA 3.0
2017-09-12T12:56:21.113Z
Depending on the epoch at which the quasar is observed, there may be many clouds (wiping out all light blueward of 1216 Å and producing a [Gunn–Peterson trough](https://en.wikipedia.org/wiki/Gunn–Peterson_trough) seen in the last spectrum above), some clouds (producing the [Lyman α forest](https://en.wikipedia.org/wiki/Lyman-alpha_forest) seen in the first spectrum), or few clouds (transmitting most of the spectrum).
pela
CC BY-SA 3.0
2017-09-12T13:00:25.500Z
Similar absorption may be seen for other transitions (e.g. Lyman β, or singly ionized metals such as Mg II or Fe II), but they will be quite a lot weaker.
pela
CC BY-SA 3.0
2017-09-10T19:36:24.047Z
2017-09-12T06:25:05.640Z
2017-09-12T06:25:05.640Z
pela
<p>A possible answer for this is that, <strong>light emitted from the galaxies travelled a billion miles all the way to earth, where the hubble space telescope picked up this light through its sensors, and was able to construct an image of the galaxy</strong></p>

<p>but if this is true, and galaxies are billions of miles away, shouldn't the light particles emitted from the galaxies be <strong>scattered</strong> all over the place? after all they have been travelling from millions of years, and have probably collided with asteroids and other foreign objects. What were the chances that about 95% of the photons actually reached earth, giving us a very detailed image. </p>

<p>Consider the andromeda galaxy which has a distance of 1.492 × 10^19 mi from earth. If light emitted from the galaxy travels in all directions, then how is it that we can still map out the entire galaxy, evident from the photo below?</p>

<p>Shouldn't like half of the galaxy be missing since photons could have hit other objects, and "never have reached earth"? </p>

<p><a href="https://i.sstatic.net/DVW2m.png" rel="noreferrer"><img src="https://i.sstatic.net/DVW2m.png" alt="enter image description here"></a></p>

CC BY-SA 3.0
2017-09-10T19:04:16.700Z
Because space is largely just that. The entire premise of your question - that light is likely to interact with something - is incorrect.
ProfRob
CC BY-SA 3.0
2017-09-10T19:24:33.657Z
But would you not think that among vast distances, that there must be some chance that light interacts with other objects
K Split X
CC BY-SA 3.0
2017-09-10T20:12:12.420Z
@KSplitX You're going about it the wrong way. We can see the galaxy from here because there's nothing in between. (That is, the fact that we can see it from here is evidence that nothing is.) If there are galaxies that are obscured by something in between, then we couldn't see those, no.
Mr Lister
CC BY-SA 3.0
2017-09-10T22:15:16.477Z
@KSplitX No. That's just how empty space really is.
chepner
CC BY-SA 3.0
2017-09-11T04:43:35.383Z
Light from galaxies travelled a billion miles?  Sorry, but a billion miles barely gets you past the orbit of Saturn :-)  As for why we can see galaxies a billion or more light years away, 1) They emit a lot of photons; 2) We use big mirrors to catch as many photons as possible; and 3) We stare at the same patch of sky for hundreds of hours (for the Hubble Deep fField images) to collect photons.  Indeed, in real time there is pretty much nothing to be seen in the patches of sky they look at - that's part of the reason why they were chosen.
jamesqf
CC BY-SA 3.0
2017-09-11T07:14:21.303Z
On top of the many excellent answers here, there's also the fact the galaxies are pretty damn big in the first place. The points of light you see in that photo are stars in our galaxy that got in the way of the camera! The misty oval thing is the real galaxy: the light of a billion stars that can't be resolved to individual points in the photo!
Ben Hillier
CC BY-SA 3.0
2017-09-11T07:37:44.707Z
The premise of this question is a rather good example of an *Argument from Personal Incredulity* (I can't understand how X can be true, therefore, I doubt X to be true).
Oscar Bravo
CC BY-SA 3.0
2017-09-11T07:45:33.950Z
@OscarBravo: I don't think the OP doubts that we can see the galaxies, I just think s/he want to understand _why_.
pela
CC BY-SA 3.0
2017-09-11T07:49:23.077Z
@BenHillier: Actually, that's not true. We _are_ able to see individual stars in Andromeda. Have a look [here](https://www.nasa.gov/content/hubble-s-high-definition-panoramic-view-of-the-andromeda-galaxy).
pela
CC BY-SA 3.0
2017-09-11T08:03:12.227Z
Don't forget the flip side, that if there *is* sufficient matter in the way the light *is* absorbed and we don't see it.  Exhibit A: the center of our own galaxy, the Milky Way.  Were it not for the dust and gas blocking our view we'd have a _spectacular_ view.
Jim Garrison
CC BY-SA 3.0
2017-09-11T09:31:18.083Z
"Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is."
PlasmaHH
CC BY-SA 3.0
2017-09-11T11:53:52.307Z
By curious coincidence, one billion miles is almost exactly 10 AU ([here is a handy converter](http://www.unit-conversion.info/astronomical.html) that I found, if you want to play with the units). 10 AU, in turn, is almost exactly the orbital radius of Saturn around the Sun (I cheated and looked at [Wikipedia](https://en.wikipedia.org/wiki/Solar_System#Distances_and_scales) for the numbers). The orbital radius of Earth around the Sun is almost exactly 1 AU, so one billion miles is approximately the distance between Earth and Saturn on closest approach.
user
CC BY-SA 3.0
2017-09-11T18:16:57.837Z
I like questions like this.  It's a good question, with a easy to defend answer, and it demonstrates how difficult it is to comprehend astronomical distances while living a life that cares about meters and kilometers, or feet and miles.  The scale is so extravagant, it's hard to put both viewpoints into a single brain.
Cort Ammon
CC BY-SA 3.0
2017-09-12T00:27:17.267Z
Not at all the same thing, but [Olbers's paradox](https://en.wikipedia.org/wiki/Olbers%27_paradox) seems to have some similarities in its reasoning.
user541686
CC BY-SA 3.0
2017-09-12T07:17:20.650Z
@OscarBravo This **isn't** a good example of an *'Argument from Personal Incredulity'*. The OP is saying s asking how something happens despite thinking that is should not be possible, as opposed to proclaiming that the thing must be falsified because they don't understand it.
Steve Ives
CC BY-SA 3.0
2017-09-12T17:03:41.717Z
@Ben Hillier: I think not all of those points of light are actually stars.  The oval blob to the lower left of center looks like an even more distant galaxy seen through the outer fringes of the main one.
jamesqf
CC BY-SA 3.0
2017-09-13T04:45:36.950Z
Yes. Things do get in the way. Hubble and other astronomy satellites are launched into space to escape the atmosphere that filters out much of the light emitted by distant galaxies and other objects. There is also gravitational lensing of light from distant galaxies, by the cumulative weight of "nearer" galaxies and, possibly, dark matter, along the same line of sight. But, there was also the "Deep Field" project where Hubble stared at dark, seemingly empty points in space; only to find more galaxies much further afield. It is hard to appreciate the scale: billions of galaxies, each with billi
Manny Martines
CC BY-SA 3.0
2017-09-13T04:56:51.287Z
Consider this: Our galaxy, the "Milky Way", is on a collision course with the nearest galaxy: "Andromeda". From here, now, Andromeda appears as a dense package. But, there is enough distance between all the stars in both galaxies, that they are expected to just pass through each other with close to zero actual collisions.
Manny Martines
CC BY-SA 3.0
2017-09-13T07:17:52.723Z
@jamesqf I'm pretty certain that's one of the several large satellite galaxies orbiting Andromeda.
Ben Hillier
CC BY-SA 3.0
2017-09-13T09:05:33.803Z
To illustrate how really, really, really big and how really, really, really empty space is you might want to look at http://1pixelmoon.com.
RobIII
CC BY-SA 3.0
2018-04-10T20:02:16.707Z
@RobJeffries The asker's mistaken assumption could have originated with creationist pseudoscience. I've heard creationists claim that redshift is caused by interstellar dust. (I'm guessing that spectral lines are shifted by witchcraft.)
StackOverthrow
<galaxy><light><photography>
How do we have photos of galaxies so far away?
K Split X
# How do we have photos of galaxies so far away?

(Asked by: username_0 on 2017-09-10)

A possible answer for this is that, **light emitted from the galaxies travelled a billion miles all the way to earth, where the hubble space telescope picked up this light through its sensors, and was able to construct an image of the galaxy**

but if this is true, and galaxies are billions of miles away, shouldn't the light particles emitted from the galaxies be **scattered** all over the place? after all they have been travelling from millions of years, and have probably collided with asteroids and other foreign objects. What were the chances that about 95% of the photons actually reached earth, giving us a very detailed image.

Consider the andromeda galaxy which has a distance of 1.492 × 10^19 mi from earth. If light emitted from the galaxy travels in all directions, then how is it that we can still map out the entire galaxy, evident from the photo below?

Shouldn't like half of the galaxy be missing since photons could have hit other objects, and "never have reached earth"?

[![enter image description here](https://i.sstatic.net/DVW2m.png)](https://i.sstatic.net/DVW2m.png)

**Tags:** galaxy, light, photography
> **Comments:**
> **username_1**: Because space is largely just that. The entire premise of your question - that light is likely to interact with something - is incorrect.  
> **username_0**: But would you not think that among vast distances, that there must be some chance that light interacts with other objects  
> **username_2**: @KSplitX You're going about it the wrong way. We can see the galaxy from here because there's nothing in between. (That is, the fact that we can see it from here is evidence that nothing is.) If there are galaxies that are obscured by something in between, then we couldn't see those, no.  
> **username_3**: @KSplitX No. That's just how empty space really is.  
> **username_4**: Light from galaxies travelled a billion miles?  Sorry, but a billion miles barely gets you past the orbit of Saturn :-)  As for why we can see galaxies a billion or more light years away, 1) They emit a lot of photons; 2) We use big mirrors to catch as many photons as possible; and 3) We stare at the same patch of sky for hundreds of hours (for the Hubble Deep fField images) to collect photons.  Indeed, in real time there is pretty much nothing to be seen in the patches of sky they look at - that's part of the reason why they were chosen.  
> **username_5**: On top of the many excellent answers here, there's also the fact the galaxies are pretty damn big in the first place. The points of light you see in that photo are stars in our galaxy that got in the way of the camera! The misty oval thing is the real galaxy: the light of a billion stars that can't be resolved to individual points in the photo!  
> **username_6**: The premise of this question is a rather good example of an *Argument from Personal Incredulity* (I can't understand how X can be true, therefore, I doubt X to be true).  
> **username_7**: @OscarBravo: I don't think the OP doubts that we can see the galaxies, I just think s/he want to understand _why_.  
> **username_7**: @BenHillier: Actually, that's not true. We _are_ able to see individual stars in Andromeda. Have a look [here](https://www.nasa.gov/content/hubble-s-high-definition-panoramic-view-of-the-andromeda-galaxy).  
> **username_8**: Don't forget the flip side, that if there *is* sufficient matter in the way the light *is* absorbed and we don't see it.  Exhibit A: the center of our own galaxy, the Milky Way.  Were it not for the dust and gas blocking our view we'd have a _spectacular_ view.  
> **username_9**: "Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is."  
> **username_10**: By curious coincidence, one billion miles is almost exactly 10 AU ([here is a handy converter](http://www.unit-conversion.info/astronomical.html) that I found, if you want to play with the units). 10 AU, in turn, is almost exactly the orbital radius of Saturn around the Sun (I cheated and looked at [Wikipedia](https://en.wikipedia.org/wiki/Solar_System#Distances_and_scales) for the numbers). The orbital radius of Earth around the Sun is almost exactly 1 AU, so one billion miles is approximately the distance between Earth and Saturn on closest approach.  
> **username_11**: I like questions like this.  It's a good question, with a easy to defend answer, and it demonstrates how difficult it is to comprehend astronomical distances while living a life that cares about meters and kilometers, or feet and miles.  The scale is so extravagant, it's hard to put both viewpoints into a single brain.  
> **username_12**: Not at all the same thing, but [Olbers's paradox](https://en.wikipedia.org/wiki/Olbers%27_paradox) seems to have some similarities in its reasoning.  
> **username_13**: @OscarBravo This **isn't** a good example of an *'Argument from Personal Incredulity'*. The OP is saying s asking how something happens despite thinking that is should not be possible, as opposed to proclaiming that the thing must be falsified because they don't understand it.  
> **username_4**: @Ben Hillier: I think not all of those points of light are actually stars.  The oval blob to the lower left of center looks like an even more distant galaxy seen through the outer fringes of the main one.  
> **username_14**: Yes. Things do get in the way. Hubble and other astronomy satellites are launched into space to escape the atmosphere that filters out much of the light emitted by distant galaxies and other objects. There is also gravitational lensing of light from distant galaxies, by the cumulative weight of "nearer" galaxies and, possibly, dark matter, along the same line of sight. But, there was also the "Deep Field" project where Hubble stared at dark, seemingly empty points in space; only to find more galaxies much further afield. It is hard to appreciate the scale: billions of galaxies, each with billi  
> **username_14**: Consider this: Our galaxy, the "Milky Way", is on a collision course with the nearest galaxy: "Andromeda". From here, now, Andromeda appears as a dense package. But, there is enough distance between all the stars in both galaxies, that they are expected to just pass through each other with close to zero actual collisions.  
> **username_5**: @jamesqf I'm pretty certain that's one of the several large satellite galaxies orbiting Andromeda.  
> **username_15**: To illustrate how really, really, really big and how really, really, really empty space is you might want to look at http://1pixelmoon.com.  
> **username_16**: @RobJeffries The asker's mistaken assumption could have originated with creationist pseudoscience. I've heard creationists claim that redshift is caused by interstellar dust. (I'm guessing that spectral lines are shifted by witchcraft.)  

---

## 5 answers

### Answer by username_7 on 2017-09-10. Score: 54 | Accepted answer

There are two reasons that often — but not always — light from galaxies millions and even billions of lightyears away make it through the Universe and down to us:

**Particle *number* and particle *size***

1. First, the intergalactic medium (IGM) is *extremely* dilute. The number density of particles out there is of the order $n\sim10^{-7}\,\mathrm{cm}^{-3}$, or roughly 26 orders of magnitude lower that the air at sea level! That means that if you consider a tube from Andromeda to the Milky Way with cross-sectional area of $1\,\mathrm{cm}^{2}$, it will contain roughly one microgram of matter (thanks to Rob Jeffries for catching a factor $10^6$ error).
2. Second, even if a photon comes close to an atom, it will only be absorbed if its energy matches closely some transition in the atom. Since most of the atoms are ionized (and thus should be called plasma instead, but in astronomy the distinction if often not made), there are no electrons to absorb the photon. The photons are more likely to interact with the free electrons via Thomson scattering, but the Thomson cross section is immensely small $(\sim10^{-24}\,\mathrm{cm}^{2})$, so even if you consider the [CMB photons](https://en.wikipedia.org/wiki/Cosmic_microwave_background) — which have traveled through the Universe almost since the Big Bang — only around 5% of them have interacted with electrons on their way.

In other words: The amount of transmitted light depends on two factors: 1) The amount of matter along the line of sight, and 2) that matter's ability to absorb the light. In the IGM, both are tremendously small. When the light enters the inter*stellar* medium (ISM) inside our galaxy, it may encounter denser clouds with atoms that are able to absorb the light. But usually (although not always) "dense" is still very dilute compared to Earth's atmosphere.

**Mathematical expression**

In general, if a beam of light traverses a region of particles, each with a cross section $\sigma$ (measured e.g. in cm$^2$), passing $N$ particles per area of the beam (measured e.g. in cm$^{-2}$), then the opacity of the medium is given by the *optical depth* $\tau$, defined by
$$
\tau \equiv N \, \sigma.
$$
The transmitted fraction $f$ of photons is then
$$
f = e^{-\tau}.
$$
In general $\sigma$ depends on the wavelength, and thus part of the spectrum may pass unhindered, while another part may be completely absorbed.

The figure below (from [here](http://spiff.rit.edu/classes/phys443/lectures/qso/qso.html)) shows the spectrum of a quasar lying at a distance of 22 billion lightyears, i.e. $10\,000$ times farther away than Andromeda. You see that there are several thin absorption lines (caused by intervening hydrogen clouds whose densities are a factor of 10-100 higher than the IGM), but still most of the light makes it down to us.

[![qso](https://i.sstatic.net/WSHff.png)](https://i.sstatic.net/WSHff.png)

Because the light we see from this quasar was emitted so long ago, the Universe was considerably smaller at that time, and thus the density was larger. Nonetheless, only a small fraction is absorbed. The farther away the light is emitted, the longer ago it was, which means smaller Universe, and higher density, and thus the more light is absorbed. If you consider this quasar (from [here](http://classic.sdss.org/news/releases/20000413.qso.img2.html)) which lies 27 billion lightyears away, you see that much more light is absorbed in part of the spectrum. Still, however, much light make it through to us.

[![qso2](https://i.sstatic.net/fP9Zm.gif)](https://i.sstatic.net/fP9Zm.gif)

The reason that it is only the short wavelengths that are absorbed is quite interesting — but that's another story.

> **Comments:**
> **username_1**: The distance to Andromeda is $2\times 10^{24}$ cm. A 1cm$^2$ cylinder contains $2\times 10^{18}$ H atoms/ions, with mass $4\times 10^{-6}$ g ? If you had this surface density of tinfoil it would be a micron thick and I suspect not opaque to light, however the tinfoil argument is a red herring since the reflectivity of tin arises directly from its density (and electron degeneracy), not the total number of atoms present along the line of sight. @user18458  
> **username_7**: Oops, thanks @RobJeffries. I don't know how I missed a factor of a million. Guess I should stop doing calculations in my head. I'll edit.  
> **username_26**: Is it correct to say the quasar is >20 billion ly away when the universe is <14 billion years old? It may now be that far away, but we are talking about the light we are measuring from it, which was not emitted from that distance. Just a bit misleading I think.  
> **username_7**: @mao47: It is quite customary, when talking about distances to a given cosmological object, to refer to the distance to that object _now_. The distance it had when it emitted the light we see today is less commonly of interest, but is easily found: For instance, the last quasar I mention lies at redshift z = 5.82. At a given redshift z, everything was a factor (1+z) closer to each other than today, so the distance to that quasar was 27 Gly / (1+5.82) = 4 Gly (despite the Universe only being 1 Gyr old at the time).  
> **username_27**: Do you have a link to the explanation of why only short wavelengths are absorbed?  
> **username_7**: @BetaDecay: It has to do with the [Lyman α](https://en.wikipedia.org/wiki/Lyman-alpha_line) transition of neutral hydrogen at 1216 Å. As light from the quasar travels through the Universe, it is redshifted. Thus, light that is initially blueward of Lyα, will at some point become Lyα. If at this point there happens to be a hydrogen cloud, it will produce an absorption line. [cont'd below]  
> **username_7**: Depending on the epoch at which the quasar is observed, there may be many clouds (wiping out all light blueward of 1216 Å and producing a [Gunn–Peterson trough](https://en.wikipedia.org/wiki/Gunn–Peterson_trough) seen in the last spectrum above), some clouds (producing the [Lyman α forest](https://en.wikipedia.org/wiki/Lyman-alpha_forest) seen in the first spectrum), or few clouds (transmitting most of the spectrum).  
> **username_7**: Similar absorption may be seen for other transitions (e.g. Lyman β, or singly ionized metals such as Mg II or Fe II), but they will be quite a lot weaker.  

---

### Answer by username_17 on 2017-09-10. Score: 26

As Rob Jeffries says, the universe is mostly empty space. A photon can easily travel thousands of light years without interacting with anything. Most of the interaction would occur when photons entered the earth's atmosphere. The Hubble avoids this. These photos were most likely from combining several viewing sessions giving basically an extended time period for observing the galaxy.

> **Comments:**
> **username_18**: Galaxies were observed more than 200 years before Hubble, which just shows that light can travel a long way through even a relatively dense medium (our atmosphere) without being absorbed to a great extent.  
> **username_19**: @DrChuck The Andromeda galaxy has been observed for much longer than that since it's very well visible with the naked eye. If there's one thing I'm jealous of the good old days, it's the lack of light pollution.  
> **username_20**: Or as Douglas Adams said, "Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space."  

---

### Answer by username_21 on 2017-09-11. Score: 23

There's a misconception in your question I don't think the other answers have addressed.

> If light emitted from the galaxy travels in all directions, then how
> is it that we can still map out the entire galaxy

Light **is** emitted from the galaxy in all directions. Only a tiny, tiny fraction of it is directed to Earth, and of that, an even tinier fraction is collected by any given telescope. But we can still see it, because galaxies are very, very bright. Andromeda contains about a trillion stars.

> **Comments:**
> **username_22**: This is a useful point to make. If you spend the effort to calculate the total number of photons Andromeda emits per second, you'll find an astronomically high number (think $10^{60}\:\mathrm{photons/s}$ at a minimum). So its no wonder we can collect $\sim 10^3\:\mathrm{photons/pixel}$ in a single, longer-than-a-second observation with a telescope.  

---

### Answer by username_23 on 2017-09-11. Score: 14

Sorry if this logic seems a bit circular, but we can get unobscured pictures of galaxies because they are unobscured.

As has been mentioned - space is really, really big and really, really empty. This is hard for us to contemplate, because there's so much stuff right next to us - but this is actually a really unusual condition. The next star to the Sun is over 4 light-years away, but we get almost all (99.9999999999...%) the light from it that heads in our direction - the same with light from further away - we get a huge number of photons sent to us from objects very far away.

Hubble also uses the simple camera techniques of lensing and long exposures to take images of distant objects - so more light is received to construct the image.

But, the other part of this, it is almost impossible to take a picture of a galaxy (or star) that is behind another galaxy or dust cloud. For example, we can't easily see past the centre of our own galaxy, because there's a lot of dust and gas and stars in the way. The picture in your question, on the other, seems to be Andromeda, which is above the plane of the galaxy. Our galaxy is quite thin compared to its diameter, and we're a decent way out of the galactic centre, meaning there's a lot less stuff in the way.

And there are [some galaxies](https://en.wikipedia.org/wiki/NGC_3628) we've taken images of which are obscured by dust:

[![enter image description here](https://i.sstatic.net/c5Ukh.jpg)](https://i.sstatic.net/c5Ukh.jpg)

> **Comments:**
> **username_24**: "a lot less crap in the way" -- can we try to answer without so much technical jargon?  

---

### Answer by username_25 on 2017-09-12. Score: 1

There's been some good answers already, but I'd like to add my two-pennyworth:

> How do we have photos of galaxies so far away?

Because there's nothing much between them and us that interferes with the light that reaches our cameras.

> A possible answer for this is that, light emitted from the galaxies travelled a billion miles all the way to earth, where the Hubble space telescope picked up this light through its sensors, and was able to construct an image of the galaxy

It's a billion miles to *Saturn*. Well actually the distance varies with the orbits, but see [this Space.com article](https://www.space.com/18477-how-far-away-is-saturn.html): *"At their most distant, when they lie on opposite sides of the sun from one another, they are just over a billion miles (1.7 billion km) apart"*. The [Andromeda galaxy](https://www.google.co.uk/search?q=andromeda+galaxy+how+far+miles&oq=andromeda+galaxy+how+far+miles&gs_l=psy-ab.3..0i22i30k1l2.82831.84293.0.84392.6.6.0.0.0.0.134.542.2j3.5.0....0...1.1.64.psy-ab..1.5.538...0.UhwIvdw4S-A) is circa fifteen billion *billion* miles away. Or circa fifteen quintillion miles.

> but if this is true, and galaxies are billions of miles away, shouldn't the light particles emitted from the galaxies be scattered all over the place?

Don't forget that photons have an E=hf wave nature. And that even though they are scattered in the air, you can still see the Moon. Yes, there's a bit of light going astray in space. But not so much that the night-time sky is some blank foggy fug. You can see Saturn too. And the stars. And the galaxies, but they are [rather dim](https://www.google.co.uk/search?q=andromeda+galaxies+very+dim&oq=andromeda+galaxies+very+dim&gs_l=psy-ab.3...7848.14077.0.14358.16.16.0.0.0.0.127.1150.13j3.16.0....0...1.1.64.psy-ab..1.0.0.3AYbIEWlZKk).

> after all they have been travelling from millions of years, and have probably collided with asteroids and other foreign objects. What were the chances that about 95% of the photons actually reached earth, giving us a very detailed image.

The chances are high. We have pictures of planets and things *because* the chances are high.

> Consider the Andromeda galaxy which has a distance of 1.492 × 10^19 mi from earth. If light emitted from the galaxy travels in all directions, then how is it that we can still map out the entire galaxy, evident from the photo below?

If I was covered in lights, I would emit light in all directions, and you would see me because some of that light goes into your eye. The Andomeda galaxy is similar.

> Shouldn't like half of the galaxy be missing since photons could have hit other objects, and "never have reached earth"?

No. And if half the photons didn't reach Earth, you'd just see a dimmer galaxy, that's all.


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