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

• 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"?

Because space is largely just that. The entire premise of your question - that light is likely to interact with something - is incorrect.

But would you not think that among vast distances, that there must be some chance that light interacts with other objects

@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.

@KSplitX No. That's just how empty space really is.

Because there are a lot of light particles.

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.

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!

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).

@OscarBravo: I don't think the OP doubts that we can see the galaxies, I just think s/he want to understand _why_.

@BenHillier: Actually, that's not true. We _are_ able to see individual stars in Andromeda. Have a look here.

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.

"Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is."

By curious coincidence, one billion miles is almost exactly 10 AU (here is a handy converter 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 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.

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.

Not at all the same thing, but Olbers's paradox seems to have some similarities in its reasoning.

@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.

@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.

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

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.

@jamesqf I'm pretty certain that's one of the several large satellite galaxies orbiting Andromeda.

To illustrate how really, really, really big and how really, really, really empty space is you might want to look at http://1pixelmoon.com.

@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.)

4 years ago

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 — 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 interstellar 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) 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.

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) 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.

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

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

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.

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: 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).

Do you have a link to the explanation of why only short wavelengths are absorbed?

@BetaDecay: It has to do with the Lyman α 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]

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 seen in the last spectrum above), some clouds (producing the Lyman α forest seen in the first spectrum), or few clouds (transmitting most of the spectrum).

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.