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I'm impressed that we have any telescope imagery at all. Take the images we have from the "Pillars of Creation".

The Pillars of Creation is in the Eagle Nebula, some 7,000 light-years away from Earth. It's estimated its size is about 4 light years.

Does that mean that between those 7,000 light-years, there's absolute nothing in the way that obscures the pillars? Given its arcseconds, I'd expect that any kind of disturbance would make it simply impossible to visualize it.

Also, does that also means that the pillars occupy that portion (albeit minuscule) of the sky permanently? What I wonder is, what is behind the ever growing portion of the sky being hidden by those 4 light years of size?

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    $\begingroup$ Mostly yes. Space is large and empty $\endgroup$ Oct 10, 2023 at 15:50
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    $\begingroup$ Cue Hitchhikers Guide to the Galaxy… $\endgroup$
    – Jon Custer
    Oct 10, 2023 at 15:54
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    $\begingroup$ The fact that space objects block the things behind them is one of the difficulties of viewing our own galaxy -- you can't see the center of the Milky Way very well because it's so dense with stars and dust that it blocks the view of whatever is behind it. $\endgroup$ Oct 10, 2023 at 19:04
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    $\begingroup$ The pillars are the thing obstructing the view the objects behind them. $\endgroup$ Oct 11, 2023 at 18:33
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    $\begingroup$ That's a testament to how empty and big space is. That's why it's called space. ;) $\endgroup$
    – DKNguyen
    Oct 11, 2023 at 20:56

3 Answers 3

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Yes, space is very empty. There is not nothing between us an the Eagle nebula, but little enough that we can still get a reasonable view of it.

The pillars are ephemeral, they are evolving on timescales that, whilst slow on human timescales, are quite rapid on the timescales of galactic evolution. There probably won't be much evidence of them in a few million years.

The pillars do "hide" a portion of space behind them (and inside them). This is why you often see observations taken at infrared wavelengths because light at this wavelength can penetrate the gas and (mainly) dust making up the pillars more easily than visible wavelengths and we can see into them and behind them. See here for example.

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    $\begingroup$ On the last paragraph, esa.int/ESA_Multimedia/Images/2022/10/… is useful -- It shows the difference between Hubble's (visible) and Webb's (infrared) view of the pillars. $\endgroup$
    – R.M.
    Oct 11, 2023 at 13:17
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Does that mean that between those 7,000 light-years, there's absolute nothing in the way that obscures the pillars? Given it's arcseconds, I'd expect that any kind of disturbance would make it simply impossible to visualize it.

Almost, with a few details.

It means there's nothing opaque in the way. At risk of being trite, the telescope lens and the atmosphere (well, not for Hubble) is often "in the way", but they're transparent. Many substances are opaque only in some wavelengths and transparent in others. Astronomical photographs are sometimes not literally what you would see with your eye, they are false-color.

While it may seem amazing that no object happens to be in a 7k lyr cone, keep in mind that's not the "experiment". It is actually an infinitely long cone and we see only the first object. For example, there could be an object directly behind Proxima Centauri, and we wouldn't see it. So the true "property" is the distribution of distances to the foremost object in each cone coming out of the telescope. This distribution is generally as you would expect given the number, size and type of objects you find in the universe.

Of course if there was actually an object behind Proxima Centauri, we could still detect it as it moves around (and we do) - it would eventually "peek out". Incidentally, other planets, the Moon, the Earth, the Sun often obscure celestial bodies - but not all the time. But if by unlikely coincidence the obstacle's orbit was in perfect sync with the telescope and the obstacle, we might still be able to detect it due to gravity bending light around the obstacle.

So in sum:

  • Not every obstacle blocks every wavelength
  • Most objects (including us) are moving so they are not always permanently obscured
  • Even objects that are truly obscured can still be observed indirectly
  • Objects that are truly obscured are unknown to us, therefore you can't rightly marvel at how few of them there are, since you don't rightly know if there's few or many
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    $\begingroup$ Still, if there are many, we would see something in every direction. That is obviously not true. That is Olbers' paradox, and shows that the universe is finite in space and has a finite age, and stars don't live forever $\endgroup$ Oct 11, 2023 at 15:53
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    $\begingroup$ @planetmaker if there are many obscured? That's not obvious. In fact our deep scans show that looking at the darkest parts of the sky long enough will show you entire galaxies that are distant and dim. Such galaxies could easily be hiding behind most visible stars. While the universe is sparse, the sky is packed. $\endgroup$ Oct 11, 2023 at 16:33
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    $\begingroup$ @candied_orange if every sightline ended on a star, the sky would be as bright as the surface of a star. $\endgroup$
    – ProfRob
    Oct 11, 2023 at 18:28
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    $\begingroup$ Almost all sightlines end at redshift z=1100. $\endgroup$
    – ProfRob
    Oct 12, 2023 at 8:14
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    $\begingroup$ Redshift affects the light. Not the line. Which only further supports the idea that black != empty. $\endgroup$ Oct 12, 2023 at 13:19
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I can't speak to the creation of a photo of Pillars of Creation specifically, but in general:

I'd expect that any kind of disturbance would make it simply impossible to visualize it.

Your question is built on the assumption that an image is taken as a single complete snapshot. It isn't, usually because of the kinds of perturbations that you're referencing.

Think of it as if I cut out a 10cm circle in a large piece of cardboard, I put it over a painting, and I move it around. You never see the painting in its entirety, but if you keep looking and combine your individual observations, you will be able to reconstruct the full painting (assuming that I've shown you all the parts of the painting at some point in time).

You could never use your camera to photograph the entire painting in a single go, but you could take many photos and stitch them together in order to form an aggregate image.

So yes, while you might not get the full image simultaneously, with a long enough exposure time you will be able to reconstruct it with enough visual fidelity for what the average human expects.

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  • $\begingroup$ This doesn't really address what the question is asking. Whether the image is taken as a single short exposure or a mosaic of small long-exposure images makes essentially no difference for the significance of any hypothetical objects obscuring the line of sight somewhere in the middle of the 7k lyr void. Unless it's very small or impossibly fast or you're thinking of exposure times in the millions of years, it'll obscure the sight the whole time the same way. $\endgroup$ Oct 13, 2023 at 15:54
  • $\begingroup$ @leftaroundabout: The third paragraph asks a different question than the fourth paragraph. I'm answering the question in the third paragraph. $\endgroup$
    – Flater
    Oct 13, 2023 at 23:43

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