# Our solar system

## Main Question or Discussion Point

Okay,

Looking at pictures of other galaxies - and representations of our own - each spiral arm seems surrounded in huge clouds and bright swirls. I am presumbing this is some sort of illuminated gas and debris, but here's my question:

If our solar system is on one indiscriminate spiral arm, how come these huge bright clouds of interstellar and gas don't come drifting through our neck of the woods?

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SpaceTiger
Staff Emeritus
Gold Member
jhe1984 said:
If our solar system is on one indiscriminate spiral arm, how come these huge bright clouds of interstellar and gas don't come drifting through our neck of the woods?
They almost certainly do, from time to time, but these clouds are not nearly as dense as you might think. I don't know exactly which ones you're talking about, but the typical densities in the interstellar medium are about a particle per cubic centimeter and the maximum densities are around $10^6$ molecules per cubic centimeter.

For comparison, the air you're breating has of order $10^{19}$ molecules per cubic centimeter.

Wow, I can't believe there are measurements for this sort of thing!

But I don't understand why the interstellar medium would "look" like clouds far away, but not close up?

It seems like if they looked like anything it would be like a sort of illuminated asteroid belt.

Also, was that a typo or did you really say that the low end of the range of the interstellar medium (this is the stuff we see as 'inside' a galaxy, as opposed to empty space, right?) is one PARTICLE per cubic centimeter? Whoa. And that's reflective?

One more question, do these "space clouds" appear (on whatever spectrum) moving relative to us, like earth clouds relative to the ground?

Thanks!

DaveC426913
Gold Member
Before we get too far, we should figure out what it is you're seeing.

If you're looking at pictures of galaxies, and you're seeing "huge clouds and bright swirls", I would strongly suspect that those are concentrations of stars, not clouds of gas.

Certainly, there's lot of pics of interstellar gas, and that may be what you're looking at - but something to factor in here is that those exposures are greatly, greatly, enhanced, contrasted and colourized (often false colour) to make the detail stand out.

Why don't you point us at some pics and we'll help you identify them?

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hellfire
The solar system is located in an environment which is relatively empty compared to the interstellar medium. It is called local bubble and was generated about a million years ago by some supernova events that blew out the interstellar medium. Bubbles like this are very usual in galaxies and form a fundamental part of the interstellar medium (galaxies look like swiss cheese). These bubbles contain also more dense clouds, especially near their walls. The sun is located within such a cloud called local fluff. Although the densities of such clouds are also small, they may have some effects on the heliosphere and the production of anomalous cosmic rays that enter the solar system.

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SpaceTiger
Staff Emeritus
Gold Member
Dave has a good point -- sometimes concentrations of stars can look cloudy in broadband optical images (images in the frequency range our eyes can see). Most of what you see in such images is, in fact, from stars, but there are other types of images for which the majority of the light comes from the interstellar medium, such as H$\alpha$ and some infrared images.

jhe1984 said:
But I don't understand why the interstellar medium would "look" like clouds far away, but not close up? It seems like if they looked like anything it would be like a sort of illuminated asteroid belt.
What matters there is the column density. This is measured in particles per square centimeter and can be thought of as a rough measure of how much stuff you're looking "through" in the image. To understand how this applies to your illuminated asteroid analogy, imagine that instead of a belt, it were a large extended cloud. If the cloud were thick enough, then it might get to the point where our line of sight intersected an illuminated asteroid in almost any place on the cloud that we looked! At this point, the cloud of asteroids would start to look "fuzzy" or "cloudy".

The same idea applies to atoms. If the clouds are thick enough, they will appear fuzzy in our images.

Also, was that a typo or did you really say that the low end of the range of the interstellar medium (this is the stuff we see as 'inside' a galaxy, as opposed to empty space, right?) is one PARTICLE per cubic centimeter?
That's more like the typical density. The low end is more like one particle in every ten centimeters cubed.

Whoa. And that's reflective?
It can be, but in images of these clouds, you'd be seeing mostly direct emission.

One more question, do these "space clouds" appear (on whatever spectrum) moving relative to us, like earth clouds relative to the ground?
The clouds do move relative to us, but their side-to-side motion is difficult to measure. It's much easier to measure their velocity toward or away from us because it produces a Doppler shift of their spectral lines.

Okay both these images are bad (it's hard to not exceed 100 kb).

What I'm driving at is that in any given region (here, a red box) there is a hazy and noticeable color gradient. I am interpreting that as some sort of interstellar gas, and in the second image, that hazy gas surrounds the Sun (which is an aqua dot). If that gas - of different colors - were moving relative to the Sun in the Milky Way, how come we don't recognize changes (as in "yesterday was sure a dark brown cloudy spaceday" or "well next year is looking like light blue haze throughout the solar sytem", etc).

--- If you're not more confused after this "clarification", I am impressed.

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jhe1984 said:
Okay both these images are bad (it's hard to not exceed 100 kb).

What I'm driving at is that in any given region (here, a red box) there is a hazy and noticeable color gradient. I am interpreting that as some sort of interstellar gas, and in the second image, that hazy gas surrounds the Sun (which is an aqua dot). If that gas - of different colors - were moving relative to the Sun in the Milky Way, how come we don't recognize changes (as in "yesterday was sure a dark brown cloudy spaceday" or "well next year is looking like light blue haze throughout the solar sytem", etc).

--- If you're not more confused after this "clarification", I am impressed.

The problem is you're not realizing the scale of these clouds. Think about fog, viewed by an ant, inside a cube of volume 1 cubic centimeter. If the density of the fog is the same as normal fog, the ant is unlikely to be able to notice it at all over those distances. Now take you looking at a cloud up in the sky. You see a thick hazy cloud. See the differece?

Yes I think I understand the part about scope, but what I am still unclear about is this:
Is our solar system in the midst of a comparably cloudy portion of our galactic spiral arm or in a relatively empty region -- and does this change?

Edit: Never mind, I think Hellfire cleared up this last question. Also I found a good graphic from NASA about how our Sun's moving out of one of these cloud wisps...

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