Molecular Cloud Clumping Question

[Vorde]

Molecular Cloud "Clumping" Question

I have been told that mysterious 'density waves' hit molecular clouds and that these are what cause these large gas and dust fields to condense into stars and from there planets.

What I don't understand is why don't we assume that the clouds own gravity pulls it into stars and avoid this big mystery.

 

[cepheid]

Hello Vorde,

It IS the molecular cloud's own gravity that causes it to collapse. The issue is that many of these clouds are stable against collapse because their weight is supported by other forces (thermal gas pressure, for one). For a cloud of a *given* density, there is an upper limit on the size of that cloud beyond which it will collapse. This upper limit is called the Jeans length (or equivalently, the Jeans mass). Clouds more massive than the Jeans mass (or larger than the Jeans length) will be unstable and collapse under their own gravity. Clouds less massive than the Jeans mass will be stable, supported in equilibrium by a balance between internal pressure and gravity. If you make the density higher, then you lower the Jeans length, making it easier for even smaller clouds to collapse. Of course, it also depends on how hot the gas is. Increase the temperature ==> increase the Jeans length.

There is some evidence to suggest that molecular cloud "cores" (the coldest densest structures inside of molecular clouds that we think *should* be Jeans unstable and collapsing to form star clusters) are lasting too long i.e. their lifetimes (which we infer from their observed numbers) are longer than the freefall timescale (the timescale for collapse under gravity). Something other than thermal pressure must be supporting these cores, and thereby *regulating* the collapse. Some have theorized that magnetic fields do so, while others suggest that turbulent motion within the gas is more important. These earliest stages of star formation are an active area of research.

 

[phinds]

... others suggest that turbulent motion within the gas is more important.

I assume from the context that this turbulent motion is somehow distinguishable from normal gas pressure. Can you elaborate?

Thanks

 

[cepheid]

I assume from the context that this turbulent motion is somehow distinguishable from normal gas pressure. Can you elaborate?

Thanks

phinds: I think it's a question length scale. What we call turbulence is the chaotic motion of macroscopic "parcels" of gas. It can occur on a variety of size scales. I believe that the characteristic length scale of turbulence gets smaller and smaller until it eventually dissipates. What I mean by dissipates is that there are no longer any macroscopic flows -- they have degenerated into chaotic motions of individual molecules within the gas (which we call thermal energy and which is what the "normal" gas pressure is associated with).

Getting back to the original topic: I forgot to state the punchline in my first post, which was to explain to Vorde that if something perturbs the system (like a shockwave from a supernova, for example), then clouds that were previously stable against collapse can become Jeans unstable. In this way, these disturbances can trigger collapse, and star formation, which is probably what Vorde was reading about.

 

[Chronos]

Supernova, and similarly energetic events 'shock' interstellar gas clouds triggering star formation.

 

[phinds]

phinds: I think it's a question length scale. What we call turbulence is the chaotic motion of macroscopic "parcels" of gas. It can occur on a variety of size scales. I believe that the characteristic length scale of turbulence gets smaller and smaller until it eventually dissipates. What I mean by dissipates is that there are no longer any macroscopic flows -- they have degenerated into chaotic motions of individual molecules within the gas (which we call thermal energy and which is what the "normal" gas pressure is associated with).

Very clear ... thanks.

Getting back to the original topic: I forgot to state the punchline in my first post, which was to explain to Vorde that if something perturbs the system (like a shockwave from a supernova, for example), then clouds that were previously stable against collapse can become Jeans unstable. In this way, these disturbances can trigger collapse, and star formation, which is probably what Vorde was reading about.

Actually, I think this point is quite clear from what you did say.

 

[Vorde]

Thank you, that does answer my question quite well.