Lagoon Nebula (APOD 15 Jul 2008)

[iampete]

The features of the gas clouds remind me of cloud shapes and small features of cyclones, etc. on Earth. On Earth, these features are influenced by small localized atmospheric pressure variations, winds, moisture content differences, among other things.

It seems to me that if "empty space" had a relatively constant "pressure" (i.e., particles per cubic meter) most of the dust clouds we observe would be very smooth rather than ragged and uneven as in the picture.

Does my attempted analogy between Earthly atmospheric cloud features and features of dust clouds in space even make sense? If not, what is the cause of such raggedness in the dust clouds in this and other pictures?

[orin stepanek]

I'm not positive but some of it may have to do with gravity and stellar winds. Probably the rotation of the galaxy and and nearby galaxies.

http://apod.nasa.gov/apod/ap080715.html

http://www.eso.org/public/outreach/pres ... 43-06.html

The attraction or mergers of nearby galaxies seem to trigger a lot of intensive star formation.

Orin

[apodman]

This ought to make you fans of repeat subjects happy. The Lagoon Nebula has appeared in APOD 32 times in 12.5 years, as revealed by an APOD Search. And it's a lot of different pictures, too.

http://antwrp.gsfc.nasa.gov/cgi-bin/apo ... rch?lagoon

A favorite sky object because it's easy to find. At a declination of 24 degrees south, it's kind of low in the sky for you folks way up north, but at moderate northern latitudes it's a good sight.

[iampete]

. . . some of it may have to do with gravity and stellar winds. Probably the rotation of the galaxy and and nearby galaxies. . .

On the scale of dust-clouds, I would expect gravitational influences of embedded/nearby stars to be "smooth" rather than "chaotic" in appearance, even with interacting gravity fields.

For stellar winds, I can conceive of "turbulence" where they interact, but even turbulent flow doesn't appear visually "chaotic". For transient effects (such as sunspot related variability of stellar winds), I would expect that would balance out to some constant for each star when looked at over thousand/million year time scales which are represented by the extent of the clouds.

For the effects of galactic rotation, I would expect those to be pretty much the same at all points within a gas cloud that is miniscule in size compared to a galaxy; this should not result in the appearance of "ragged" features within such a cloud.

I understand that "chaotic" systems really aren't, but "exhibit dynamics that are highly sensitive to initial conditions" (from Wiki). I guess what I was trying to get at in the original post is that in a system in which the influences are combinations of "smooth" functions (gravity, magnetic fields, others?), what else is there that might cause the appearance of the raggedness of the cloud features?

[orin stepanek]

What more is there? Even on a calm day here on Earth you will notice that clouds appear billowy and turbulent. You will notice that even though that the cloud shapes of the galaxy look turbulent; there really isn't that much noticeable movement. You will notice that when galaxies merge that there is a lot of star formation areas; so gravity has to play an important roll in disturbing stellar dust. Why should it look smooth?
Orin

[astrolabe]

Hello All,

Differences in temperature can cause turbulence resulting in pressure and density variables. I understand a little of such mechanisms because of my interest in meteorology, but with regard to astrophysics, it could be the other way around. I do think, though, that once the initial form is established, movement (and therefore changes) in shape is much slower.

[henk21cm]

Differences in temperature can cause turbulence resulting in pressure and density variables.

I partially agree. A consequence of turbulence are fluctuations in pressure and thus density. Differences in temperature cause a mass flow from warm to cold. That does not necessarily have to mean that the mass flow is turbulent.

Turbulence is (for me) next to the bigbang one of the least understandble phenomena. While the bigbang according to Weinberg can be modelled via a mathematical description, turbulence can only be approximated by statistical methods, which in some respect fail to do proper predictions. The chapter in Landau and Lifschitz "Fluid mechanics" gives a lot of clues. Nevertheless an image, a conceptual idea of turbulence is hard to find.

I use to compare mass flow with a toddler. It managed to do some steps. When it is slow and when it is not hampered by objects around him, distracted, it can walk. That is laminar flow. Now it gets distracted by the joy of an icecream or his favourite toy at the other side of the room. Its starts to run as fast as its small legs will allow it. In the process of running the legs get cluthered and it tumbles. Turbulent flow is a type of flow too fast for its surrounding and density. The flow tumbles over its own material and starts to spiral in vortices. I'm fully aware that the above description is an analogon which may not be precise and is absolutely of no use to do any prediction at all.

Turbulence is connected to too many degrees of freedom. When a block of aluminium moves very fast, (e.g. a airplane), the air around the aluminium is turbulent, the aluminium of the plane itself is not turbulent, not in the least to the enjoyment of those in the airplane. The intermolecular forces in the aluminium dominate the location of the atoms. In the air there more degrees of freedom, since intermolecular forces between the molecules of the air are too small to suppress the turbulence. These forces, to be precise the ability to withstand sheer forces, can be expressed into a macroscopic property: viscosity, the degree of "molasses-ability". (i know: this word is not correct english). There are two flavours of viscosity: dynamic viscosity (η) and kinematic viscosity (ν). The dynamic viscosity is the property used as in the famous Stokes law:

F = 6 π η r V

where r is the radius of a sphere, V is the velocity of the sphere with respect to the fluid (or gas) and F is the friction experienced by the sphere. As you might imagine, the density of the fluid or gas influences the dynamic viscosity. The kinematic viscosity corrects for effects of density, by normalizing the dynamic viscosity by the density of the fluid or gas:

ν = η / ρ

where ρ is the density.

The point of the concept as described above is that turbulence is bound to happen if the velocity of the flow is too fast. A dimensionless number has been introduced: the Reynolds number:

Re = v D / ν

If Re < 1, the flow is laminar. The shear forces are sufficiently low that any inclination to "tumble over its self" is suppressed. If Re > 1000 flow is turbulent. Viscosity no longer plays any role.

Turbulence can be initiated by sharp changes in geometry: the running toddler is bound to tumble over its legs, but still manages to stay upright. When it encounters the edge of a chair, it tumbles. Similarly an object in the path of flow can initiate turbulence.

A few examples of such geometrically triggered turbulence which not just Astrolabe will like, are the Von Kármán eddies, orographic cloud bands and orographic clouds as seen from below. The Von Kármán eddies are generated mostly by small islands in the ocean, like the Canarian islands, the Balearic islands, Ascension. The orographic structures are generated -as the word says- by moutains. The wingtips of an airplane are famous for its turbulence generating properties. Birds of prey have found a solution to minimize turbulence at their wingtips, by fingering their wingtips: these are divided into five separate feathers.

Summarizing:

1. Turbulence is due to happen when the flow is too fast to be kept stable (laminar) by intermolecular forces or viscosity.
2. Turbulence can be triggered by (sharp) changes in geometry
3. Turbulence generates fluctuations in velocity
4. Fluctuations in velocity generate fluctuations in density and thus pressure.
5. When the fluctuations in density are levelled out, fluctuations in temperature can be generated

If we would know the circumstances in the Lagoon nebula, like kinematic viscosity, size ("D"), and velocity ("V") we can calculate the Reynolds number and figure out whether turbulence is likely or not.

[bystander]

When we look at pictures of the Lagoon, such as APOD 2008 Jul 15, and see the predominant red of H-alpha, we think of these gas clouds as being huge conglomerations of hydrogen. However, views like APOD 2007 Nov 02 and APOD 2006 Aug 25 reveal the Lagoon to rich in heavier elements, especially sulfur and oxygen. This could have only come from the violent death of ancient stars. No wonder there is so much turbulance.

[Wadsworth]

views like APOD 2007 Nov 02 and APOD 2006 Aug 25 reveal the Lagoon to rich in heavier elements, especially sulfur and oxygen. This could have only come from the violent death of ancient stars. No wonder there is so much turbulance.

Exactly what I was thinking when I saw this thread. This nebula is likely contributed to the violent nova of a star. Therefore elements of varying mass and complexity were strewn about. These elements reacted differently to their surroundings and to subsequent forces from the star nova, gravity, temperature fluctuations due to different particle radiation absorption/emissivity and-or possibly light and stellar winds from other stars.

The less dense particles get pushed and prodded by the above mentioned forces easier and thus through time collide, become entangled, and dragged past the random heavier material in their surroundings.

All of this comes together to cover the beautiful painted canvases of our limited sight line.

Cheers.