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The Pillars of Creation have a strong directional sense. They are referred to as "pillars" and another question asks how "tall" they are. Naively, it looks as if there is a source of "smoke" near the tips of the pillars, and the smoke is being blown by a wind. News explanations and Wikipedia explain that the pillars are formed by UV light from hot stars eroding clouds of molecular hydrogen, which suggests that the "smoke" is actually a void, but it still looks as if the void is being extended by the flow of a wind past the relevant stars.

But I have no confidence that this is right, and the Wikipedia article is not very clear. Popular media explanations also don't seem to address the geometry.

I see several possibilities:

  1. The gas is still relative to most of the nearby stars, and a small number of hot stars are moving through that system, causing the pillars.
  2. The hot stars are not in rapid motion relative to nearby stars, but the gas is flowing relative to that population.
  3. The shape has nothing to do with relative flows.

What's the explanation? A pointer to a good article for the knowledgeable lay person would be helpful too.

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    $\begingroup$ Obviously one could as well refer to them as hanging objects. Any "upward" perception is incidental, and the popular name is probably less than ideal. $\endgroup$ Oct 25, 2022 at 16:10
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    $\begingroup$ Because if they were ball-shaped, then 'pillars of creation' would be a terrible name. $\endgroup$
    – Valorum
    Oct 27, 2022 at 22:22

2 Answers 2

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Your option #3 is correct; the shape has little to do with the relative motion of the gas and stars.

Giant molecular clouds

The pillars are part of the giant molecular cloud (GMC) which is giving birth to news stars. Stars are formed when some regions inside the cloud meet the Jeans criterion, i.e. are sufficiently dense and cold that gravity overcomes pressure. Because the density of such clouds is largest in the center (see e.g. Chen et al. 2021), stars will tend to form first in the center.

Stars are formed with a distribution of masses. The most massive ones — the so-called O and B stars — emit copious amounts of ultraviolet photons, which heat and ionize the surrounding medium. A hot, ionized bubble inside the otherwise cold, neutral, and dusty cloud called a Strömgren sphere then forms.

The dark pillars are remainders of the neutral gas, whereas the bluish region is the ionized region, containing newborn stars.

The size of the ionized region

In this answer about the Carina Nebula, I calculated the typical size of a Strömgren sphere, which we can write approximately as $$ R_\mathrm{S} \simeq 10\,\mathrm{lightyears} \times\color{red}{\left(\frac{Q(\mathrm{H}^0)}{10^{50}\,\mathrm{s}^{-1}}\right)^{1/3}} \color{blue}{\left(\frac{n_\mathrm{H}}{300\,\mathrm{cm}^{-3}}\right)^{-2/3}} \color{green}{\left(\frac{T}{10^4\,\mathrm{K}}\right)^{0.23}}, $$ where the three colored terms show typical values of the rate of emitted UV photons $\color{red}{Q(\mathrm{H}^0)}$ from a handful of massive stars, and the neutral hydrogen density $\color{blue}{n_\mathrm{H}}$ and temperature $\color{green}{T}$ of the cloud.

This equation tells you two things, namely that

  1. the characteristic size of the ionized region is of the order of 10 lightyears, and that
  2. the size scales with density as $R_\mathrm{S} \propto n_\mathrm{H}^{-2/3}$.

The origin of the pillar shape

But the GMC is not homogeneous; it will have regions that are quite a lot denser, and quite a lot less dense, than the average. According to point #2 above, if some region is, say, 10× more dense than its surroundings, the ionized bubble will propagate $10^{-2/3} \sim 1/5$ as far in this region. The overdensity will therefore shield the part of the cloud that is behind it from the UV radiation of the stellar cluster. This effect causes "pillars" of neutral gas to appear behind the dense regions.

In the animation below I attempt to show the evolution of the Strömgren sphere. Stars are formed first in the center, but a secondary high-density region (which perhaps is too hot to start forming its own stars) shields the gas behind it, shaping a pillar.

pillars

The image below shows you the "pillars of creation" with their surroundings, where you can see the stellar cluster in the center of the Eagle Nebula responsible for this shape:

eagle Credit: NASA/ESA/STScI/WikiSky.

Opaqueness vs. transparency

Neutral gas is quite efficient at blocking light, because the atoms have many electronic transitions available for absorbing photons. Moreover, the gas is full of dust, which also absorbs light. In contrast, ionized gas is much less efficient at absorbing light, and moreover the dust will tend to be destroyed by the free-streaming UV radiation (sublimation from heating), and by high-temperature particles (sputtering).

(when I say "ionized gas", this means that hydrogen — which comprise ~90% of the atoms — is more or less fully ionized. But helium and heavier elements still have bound electrons which may absorb some of the light.)

Hence, the neutral regions are very opaque, while the ionized regions are transparent.

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    $\begingroup$ @Jean-MariePrival Yes, you're right. I'll edit, thanks! $\endgroup$
    – pela
    Oct 27, 2022 at 13:24
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    $\begingroup$ Thanks for the heads up, @uhoh, but that looks like a rabbit hole I unfortunately don't have the time to dig into at the moment :) $\endgroup$
    – pela
    Jan 18, 2023 at 11:34
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    $\begingroup$ @uhoh I try not to :) $\endgroup$
    – pela
    Jan 18, 2023 at 14:36
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As for the TITLE of your question: "why are those called pillars", and as for some analogies in your text: "smoke", "tip", "blown", "wind",... I would simply like to point that it's all a matter of image orientation.

In space, there's no "up" or "down" so to me, and given its orientation when the image went into media, the gas formations seem more "stalagmites" than "pillars". Well, as long as I don't rotate the image 180º, so then they would rather be the "stalactites" of creation, to me.

At last, it's a matter of naming. If I were in need of giving a title to such images, aware of its transcendence on the spatial exploration history, I think that name isn't a bad choice. As long as you keep it in vertical position, where the "lightened summits" are oriented upwards.

PS. As I'm a new contributor, I've been unable to put this on comments. Please be kind with your downvotes or I'll ever need to write my contributions as answers ;)

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    $\begingroup$ Hi JoeCool, I completely agree with what you write, though I don't think the question is so much about orientation, but more about (the physical reason for its) shape. Anyway, +1 and welcome to StackExchange :) $\endgroup$
    – pela
    Oct 27, 2022 at 13:22
  • $\begingroup$ Hi @pela I appreciate your welcome. So charming I feel at home. I liked your answer so much; I've learned a lot reading it! I was only trying to focus on the question title and how often broadcasted image naming could lead to some kind of misunderstanding and even debate: Higgs bosson, COBE first image,... and just to humbly enrich the matter with this POV adding a smidge of a pinch of humor, more than any criticism or correction intended. $\endgroup$
    – JoeCool
    Nov 7, 2022 at 18:58
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    $\begingroup$ That’s great, there often isn’t just one correct answer, and different POVs (or is it “PsOV”?) is always good :) $\endgroup$
    – pela
    Nov 7, 2022 at 20:53
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    $\begingroup$ @pela they're only pillar-shaped if you orient them in one specific direction, no? $\endgroup$
    – RonJohn
    Nov 8, 2022 at 19:32
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    $\begingroup$ @RonJohn That's the point!. Dr. Peter Lausen's (pela) explanation about how the dust cloud gets that finger shape when a star is born nearby, is astonishing. Even when you should expect the inverse square of the distance anywhere as an element of the maths that describe the process involved, it seems it's canceled out or negligible at those magnitudes. It's a truly enlightening answer. But yeah, they are pillars as long as you keep the vertical orientation. You can even photoshop them until they could be named the "towers of Pisa" of the creation. ;D $\endgroup$
    – JoeCool
    Nov 9, 2022 at 10:27

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