Interstellar gas cloud stability

[nburns]

Disclaimer: I'm not a physicist

I've never quite grasped interstellar gas clouds (i.e. the material for new stars) and how they work. If they were too sparse, then you'd expect them to just dissipate. If they were too dense, then you'd expect them to collapse spontaneously. But yet they seem to be fairly stable over long time scales. What is the source of this stability?

One answer I came across is that the gas pressure in the cloud balances the gravitational tendency to collapse. If the cloud operates similarly to a familiar gas, then such a collapse would imply a significant amount of cooling, since the thermal energy had been sufficient to oppose collapse beforehand. But, yet, they do collapse and form new stars.

An alternative is that the cloud was not in equilibrium, and therefore it's prone to seek equilibrium by collapsing. Is that possible?

Thanks.

--Edit

The popular description I've come across seems to imply that a shock may cause a pocket of gas to become compressed, and then gravity just takes over. The calculation of gravity interactions are no doubt hard to do. I'm not even bothering to attempt these. But in a familiar gas, there are weak attractions that would seem to be roughly analogous. For there to be a kind of dual equilibrium, where a cloud can be stable and yet collapse spontaneously to some more favorable equilibrium, seems to require some kind of nonlinear interaction.

 

[Chronos]

Jeans theorem might make a good read.

 

[cepheid]

Disclaimer: I'm not a physicist

I've never quite grasped interstellar gas clouds (i.e. the material for new stars) and how they work. If they were too sparse, then you'd expect them to just dissipate.

Yeah, keep in mind that a bunch of gas exists in the disc of our galaxy, permeating the space between the stars: the interstellar medium. You're right that if a particular "cloud" were too sparse, it would be indistinguishable from this background, whose average density is about 1 hydrogen atom per cubic centimetre. So, you might be wondering how localized areas of higher density can exist: why doesn't the gas spread out to fill all of the available volume? The answer is that such clouds are held together by self-gravity.

If they were too dense, then you'd expect them to collapse spontaneously. But yet they seem to be fairly stable over long time scales**. What is the source of this stability? One answer I came across is that the gas pressure in the cloud balances the gravitational tendency to collapse.

Yes, this is precisely correct, albeit an oversimplification. If you work out the physics of this simple balance you'll find that at a given density and temperature, there is an upper limit on the mass of a cloud that can be supported. This mass limit is called the Jeans mass -- look it up. As you might imagine, a higher temperature means a higher Jeans mass (heavier clouds can be supported), all other things being equal. A higher density means a lower Jeans mass, all other things being equal.

If the cloud operates similarly to a familiar gas, then such a collapse would imply a significant amount of cooling, since the thermal energy had been sufficient to oppose collapse beforehand. But, yet, they do collapse and form new stars.

Yes it's true. It's typically in the coldest and densest regions (where the gas can exist in molecular form) that dense structures -- "cores" and filaments, form. These will eventually fragment and collapse to form stars. But how did things become cold and dense in the first place? You're right that a source of cooling is required. The only cooling mechanism available is radiative cooling: the cloud has to be able to radiate away its internal energy. (To be clear: this means to lose that energy by emitting electromagnetic radiation). In molecular clouds, what really helps this process along is the presence of the molecules, which are sources of line emission (similar to atomic line emission, except instead of the photons being emitted when an electron goes from a higher to lower energy state in the atom, it's instead the molecule making a transition from a higher to a lower vibrational or rotational state that results in the emission of a photon). CO is a good example of something that can do this.

An alternative is that the cloud was not in equilibrium, and therefore it's prone to seek equilibrium by collapsing. Is that possible?

If a cloud is not stable against collapse by the Jeans criterion, it will collapse.

**What you said about the clouds appearing to be stable over long timescales was very astute. In fact, the densest structures within the clouds, the so-called "clumps" and "cores" appear to last much longer than the simple picture I gave you above would suggest i.e. they last much longer than a "freefall" time. So, something other than just gravity must be at play -- some other force must be regulating the collapse. Various theoretical models have been proposed, some invoking magnetic fields as the regulating force, and others attributing it to turbulent motions within the gas cloud. It is unknown which plays the more important role. This is an open problem in star formation.

 

[nburns]

Thanks. I looked up the Jeans mass. It seems to isolate one aspect of the system without really addressing the thermodynamics of the system as a whole. If gravity and heat are the only variables, the system should have a single equilibrium point and resist changes in either direction, right?

 

[cepheid]

Thanks. I looked up the Jeans mass. It seems to isolate one aspect of the system without really addressing the thermodynamics of the system as a whole.

I don't know what you are saying here.

If gravity and heat are the only variables, the system should have a single equilibrium point and resist changes in either direction, right?

At a given density and temperature, yes. Change these, and you change the equilibrium point of the system.

 

[nburns]

I don't know what you are saying here.

Maybe I'm off target.

At a given density and temperature, yes. Change these, and you change the equilibrium point of the system.

I'm envisioning a possible scenario:

The gas cloud is initially very hot and it's in equilibrium as a cloud. It cools off extremely slowly. At some point, it may become unstable to collapse and remain in that state indefinitely, like a supersaturated solution. Then a shock could trigger a collapse.

 

[nburns]

If collapsing is like freezing, then you'd expect it to happen slowly: the collapsing gas would have to get rid of a lot of heat, which it would transfer to its surroundings, creating a negative feedback loop and acting as a brake.

 

[Fantasist]

In molecular clouds, what really helps this process along is the presence of the molecules, which are sources of line emission (similar to atomic line emission, except instead of the photons being emitted when an electron goes from a higher to lower energy state in the atom, it's instead the molecule making a transition from a higher to a lower vibrational or rotational state that results in the emission of a photon). CO is a good example of something that can do this.

Atoms in highly excited levels could possibly provide cooling as well. There may be only very few, but they have very large cross sections.

 

[Fantasist]

If collapsing is like freezing, then you'd expect it to happen slowly: the collapsing gas would have to get rid of a lot of heat, which it would transfer to its surroundings, creating a negative feedback loop and acting as a brake.

The seemingly paradoxical thing is that the cloud heats up as a consequence of the cooling, as it loses gravitational potential energy when collapsing, which means the kinetic energy (i.e. the temperature) must increase. As a result, you end up with a sun.

 

[nburns]

Atoms in highly excited levels could possibly provide cooling as well. There may be only very few, but they have very large cross sections.

I imagine that these photons (generated by either process) tend to get reabsorbed before they exit the cloud, so the edges cool faster than the center.

 

[nburns]

The seemingly paradoxical thing is that the cloud heats up as a consequence of the cooling, as it loses gravitational potential energy when collapsing, which means the kinetic energy (i.e. the temperature) must increase. As a result, you end up with a sun.

That sounds analogous to a phase change. Ice formation is exothermic. It seems paradoxical, but it makes sense if you think about it. Water molecules want to stick together as ice, so when they finally slow down enough to stick, getting stuck releases heat.

 

[Fantasist]

That sounds analogous to a phase change. Ice formation is exothermic. It seems paradoxical, but it makes sense if you think about it. Water molecules want to stick together as ice, so when they finally slow down enough to stick, getting stuck releases heat.

It is not a phase change. It is just the ideal gas law: if you compress a gas it heats up as you put mechanical work into it. In this case the compression is done by gravity (after the inelastic collisions in the gas and the associated loss of radiation from the cloud have cooled it in the first place). This compression heats the gas up to even a higher temperature than originally as the cloud has now shrunk and gravity is stronger. The cycle continues until the cloud has shrunk to such a density that no neutral atoms or molecules can exist anymore (so the cooling stops and thus the contraction stops). It has then become a star.

 

[nburns]

It is not a phase change. It is just the ideal gas law: if you compress a gas it heats up as you put mechanical work into it. In this case the compression is done by gravity (after the inelastic collisions in the gas and the associated loss of radiation from the cloud have cooled it in the first place). This compression heats the gas up to even a higher temperature than originally as the cloud has now shrunk and gravity is stronger. The cycle continues until the cloud has shrunk to such a density that no neutral atoms or molecules can exist anymore (so the cooling stops and thus the contraction stops). It has then become a star.

Yes, it isn't a phase change, but it sounds analogous to one from the standpoint of thermodynamics. The gas molecules would initially cool down, until they succumbed to gravitational attraction. Then they would heat up as they were compressed by gravity.

 

[nburns]

It is not a phase change. It is just the ideal gas law:

I think I might see your point. If the gas is simply contracting under its own weight, there is no discrete change in phase. Actually, at some point the gas changes phase into plasma, but that change is endothermic.

if you compress a gas it heats up as you put mechanical work into it. In this case the compression is done by gravity (after the inelastic collisions in the gas and the associated loss of radiation from the cloud have cooled it in the first place). This compression heats the gas up to even a higher temperature than originally as the cloud has now shrunk and gravity is stronger. The cycle continues until the cloud has shrunk to such a density that no neutral atoms or molecules can exist anymore (so the cooling stops and thus the contraction stops). It has then become a star.

I'm trying to relate this to something familiar, like an Earth-bound cloud of water vapor:

The forces acting on the cloud of water vapor are the pressure of the surrounding atmosphere, the internal gas pressure, and the attractions between polar water molecules, which would become significant only at close range. As the cloud cools, it's compressed by the external pressure until the intra-molecular attractions become significant and cause condensation, releasing heat.

The fact that the phase change is discrete in this case might have a lot to do with the fact that the water molecules are neutral and don't attract until they are close together.

In an interstellar cloud, there is internal gas pressure as well as pressure from the weight of the surrounding cloud. These are actually the same forces as in the Earth-bound cloud.

This compression heats the gas up to even a higher temperature than originally as the cloud has now shrunk and gravity is stronger.

I'm thinking that the collapse is almost entirely about cooling. It would cool slowly only because the cooling is inefficient. For a while, the future star would look like Jupiter, which is mostly cold. The core would get hotter from compression as more gas piled on, contributed to by the inefficiency of heat loss from the core. Eventually, fusion begins in the core.

 

[Fantasist]

I think I might see your point. If the gas is simply contracting under its own weight, there is no discrete change in phase. Actually, at some point the gas changes phase into plasma, but that change is endothermic.

The only phase change, if you want, is when the gas cloud has contracted to star i.e. a 100% ball of plasma (basically nuclei and electrons) without any neutral atoms (apart from a few in its atmosphere). Before that is just a partially ionized gas of increasing density and temperature. In any case, the process is exothermic throughout as the system loses energy through radiation (after all, that's what we depend on in case of our sun).

In an interstellar cloud, there is internal gas pressure as well as pressure from the weight of the surrounding cloud. These are actually the same forces as in the Earth-bound cloud.

The Earth's atmosphere also suffers from a lot of processes leading to cooling by energy being radiated into space. If it wouldn't be for the heating by the sun (which compensates for the cooling) the atmosphere would collapse as well.

The interstellar gas cloud on the other hand has nothing that would compensate for the cooling; that's why it is intrinsically unstable. It could only be stable if it had already a pre-existing star inside it that could provide the suitable amount of heating.

 

[ViperSRT3g]

I'll admit, TL;DR for the most of this thread.

I'd like to add in here that recently the revelation that gas clouds can have their own magnetic fields shielding them has played a larger role in determining how they can hold themselves together without forming stars; as well as holding them together so that they don't simply disperse into the void.

This was a point towards figuring out why some of our galaxy's satellites managed to hold themselves together for so long without becoming a part of our galaxy by now. Even when they have been shown to have passed through our galaxy on a previous close encounter.

 

[Fantasist]

I don't think a magnetic field could prevent the gravitational collapse. After all, the cloud would still permanently lose energy through the collisional cooling, and the neutral gas would not be affected by magnetic fields anyway.

Anyway, a systematic magnetic field requires a systematic electric current, so you would need a well structured, preferably rotating, cloud to have a magnetic field in the first place. This is not what you have with interstellar gas clouds initially.

 

[Dotini]

http://science1.nasa.gov/science-news/science-at-nasa/2009/23dec_voyager/
NASA announced in 2009 that Earth was "passing through an interstellar cloud that physics says should not exist."

"Using data from Voyager, we have discovered a strong magnetic field just outside the solar system," explains lead author Merav Opher, a NASA Heliophysics Guest Investigator from George Mason University. "This magnetic field holds the interstellar cloud together and solves the long-standing puzzle of how it can exist at all."

"Voyager data show that the Fluff is much more strongly magnetized than anyone had previously suspected—between 4 and 5 microgauss*," says Opher. "This magnetic field can provide the extra pressure required to resist destruction."


http://www.jpl.nasa.gov/news/news.php?release=2013-278
In September of 2013, NASA announced this interstellar plasma was 40 times denser than the plasma within the heliopause, was magnetically oriented to within 2 degrees of the solar magnetic field, and surprisingly had particles coming from a preferential direction.

 

[Fantasist]

http://science1.nasa.gov/science-news/science-at-nasa/2009/23dec_voyager/
NASA announced in 2009 that Earth was "passing through an interstellar cloud that physics says should not exist."

First of all, these measurements have been taken very much in the vicinity of the sun, so it is not really an interstellar gas cloud.

Secondly, it only claims the cloud has not been affected by supernova 'exhaust gases' emitted some 10 million years ago. Now these gases are so hot (a million degrees) that they must be completely ionized. A completely ionized gas will of course be deflected by any magnetic field (like the solar wind is deflected by the Earth's magnetic field). The interstellar clouds related to star formation consist however largely of neutral gas, which will be unaffected by any magnetic field.

 

[Dotini]

http://www.jpl.nasa.gov/news/news.php?release=2014-221

"Voyager had been flying for more than a year through plasma that was 40 times denser than measured before -- a telltale indicator of interstellar space.

Why is it denser out there? The sun's winds blow a bubble around it, pushing out against denser matter from other stars."

Of course, "winds blow a bubble" isn't literally true. It's slightly more interesting than that.

 

[Dotini]

The following statement concerning solar wind models was recently released by JPL NASA:

http://www.jpl.nasa.gov/news/news.php?release=2014-238&rn=news.xml&rst=4224
July 23, 2014

A paper recently published in the journal Geophysical Research Letters describes an alternate model for the interaction between the heliosphere -- a "bubble" around our planets and sun -- and the interstellar medium. It also proposes a test for whether Voyager 1 has, indeed, left the heliosphere.

NASA's Voyager project scientist, Ed Stone of the California Institute of Technology in Pasadena, responds:

"It is the nature of the scientific process that alternative theories are developed in order to account for new observations. This paper differs from other models of the solar wind and the heliosphere and is among the new models that the Voyager team will be studying as more data are acquired by Voyager."

Stone went on to explain that other models, which he and colleagues used to conclude that Voyager 1 entered interstellar space, predict that the density of interstellar wind outside the heliosphere is 40 times greater than the density of the solar wind inside.

Voyager scientists had carefully analyzed the observational data from the spacecraft , which revealed a plasma density that was 40 times higher. They then concluded that Voyager 1 had departed the solar bubble and entered interstellar space around August 25, 2012.

But the new article argues that solar wind inside the heliosphere can be compressed to the point that the solar wind density inside is just as high as interstellar space outside. Therefore, Voyager 1 could still be inside.

Authors of the new study predict that if Voyager 1 is still inside the heliosphere, the spacecraft will observe a reversal in direction of the solar magnetic field sometime before the end of 2015. Stone said he and colleagues will be looking carefully at the magnetic field data over the coming 18 months to see if Voyager picks up this change.

The Voyager spacecraft were built and continue to be operated by NASA's Jet Propulsion Laboratory, in Pasadena, California. Caltech manages JPL for NASA. The Voyager missions are a part of NASA's Heliophysics System Observatory, sponsored by the Heliophysics Division of the Science Mission Directorate at NASA Headquarters in Washington.

 

[Dotini]

Below is a citation and abstract to the Geophysical Research Letters alternate model for "compressed solar wind", together with predictions.

http://onlinelibrary.wiley.com/doi/10.1002/2014GL060781/abstract
Abstract

The Voyager 1 spacecraft is currently in the vicinity of the heliopause, which separates the heliosphere from the local interstellar medium. There has been a precipitous decrease in particles accelerated in the heliosphere, and a substantial increase in galactic cosmic rays (GCRs). The evidence is unclear, however, as to whether Voyager 1 has crossed the heliopause into the local interstellar medium, or remains within the heliosheath. In this Letter we propose a test that will determine whether Voyager 1 has crossed the heliopause: If Voyager 1 remains in the heliosheath, the high densities observed must be due to compressed solar wind, with the consequence that Voyager 1 will encounter another current sheet, where the polarity of the magnetic field reverses. Voyager 1 observations can be used to predict that the next current sheet crossing is likely to occur during 2015.

 

[Dotini]

http://www.sci-news.com/astronomy/science-2-6-million-light-year-long-stream-hydrogen-02096.html

An international team of astronomers using the William E. Gordon Telescope at the Arecibo Observatory in Puerto Rico has discovered a 2.6-million-light-year-long bridge of atomic hydrogen gas between galaxies in the NGC 7448 galaxy group, located about 500 million light years away.

“We frequently see gas streams in galaxy clusters, where there are lots of galaxies close together, but to find something this long and not in a cluster is unprecedented.”

“It is not just the length of the stream that is surprising but also the amount of gas found in it,” added co-author Roberto Rodriguez of the University of Puerto Rico in Humacao.

“We normally find gas inside galaxies, but here half of the gas – 15 billion times the mass of the Sun – is in the bridge. That’s far more than in the Milky Way and Andromeda galaxies combined!”

The astronomers are still investigating the origin of the stream.http://mnras.oxfordjournals.org/content/443/3/2634

The Arecibo Galaxy Environment Survey – VII. A dense filament with extremely long H i streams

 

[Dotini]

New data on the relationship of the dominant magnetic structure in the sky, Loop 1, and the ISMF that shapes the heliosphere. Link to pdf included.

http://arxiv.org/abs/1409.5428
(Submitted on 18 Sep 2014)
Abstract: The local interstellar magnetic field affects both the heliosphere and the surrounding cluster of interstellar clouds (CLIC). Measurements of linearly polarized starlight provide the only test of the magnetic field threading the CLIC. Polarization measurements of the CLIC magnetic field show multiple local magnetic structures, one of which is aligned with the magnetic field traced by the center of the "ribbon" of energetic neutral atoms discovered by the Interstellar Boundary Explorer (IBEX). Comparisons between the bulk motion of the CLIC through the local standard of rest, the magnetic field direction, the geometric center of Loop I, and the polarized dust bridge extending from the heliosphere toward the North Polar Spur direction all suggest that the CLIC is part of the rim region of the Loop I superbubble.

 

[Dotini]

Stability of filaments in star-forming clouds and the formation of prestellar cores in them
http://arxiv.org/abs/1501.03248
Abstract: It is now widely accepted that dense filaments of molecular gas are integral to the process of stellar birth. While numerical simulations have succeeded in reproducing filamentary structure in turbulent gas and analytic calculations have predicted the formation of dense gas filaments via radial collapse, the exact process(es) that generate/s such filaments which then form prestellar cores within them, is unclear. In this work we therefore study numerically the formation of a dense filament using a relatively simple set-up of a uniform-density cylinder in pressure equilibrium with its confining medium. In particular, we examine if its propensity to form a dense filament and further, to the formation of prestellar cores within this filament bears on the gravitational state of the initial volume of gas. We report a radial collapse leading to the formation of a dense filamentary cloud is likely when the initial volume of gas is at least critically stable (characterised by the approximate equality between the mass line-density for this volume and its maximum value). Though self-gravitating, this volume of gas, however, is not seen to be in free-fall. This post-collapse filament then fragments along its length due to the growth of a Jeans-like instability to form prestellar cores like \emph{beads on a string}. We suggest, dense filaments in typical star-forming clouds classified as gravitationally super-critical under the assumption of : (i) isothermality when in fact, they are not, and (ii) extended radial profiles as against one that is pressure-truncated, thereby causing significant over-estimation of their mass line-density, are unlikely to experience gravitational free-fall. The radial density and temperature profile derived for this post-collapse filament is consistent with that deduced for typical filamentary clouds mapped in recent surveys of nearby star-forming regions.

 

[Fantasist]

Stability of filaments in star-forming clouds and the formation of prestellar cores in them
http://arxiv.org/abs/1501.03248
Abstract: It is now widely accepted that dense filaments of molecular gas are integral to the process of stellar birth.

Showing that molecules could be integral to the formation of prestellar cores does not imply that they are also integral to the actual star formation (as seems to be suggested here). Molecules will hardly be able to exist in the latter stages of star formation, as the temperature of the cloud will increase inversely proportional to the radius (as can easily be found from the virial theorem). When the cloud has shrunk to a relative size of 1o 10^-4 (roughly solar system size) the temperature will have increased from 10K to 10⁵K. Not many molecules will be able to survive this temperature.

 

[TEFLing]

The milky way galaxy has a magnetic field

Perhaps space plasma is initially tied to the field lines which help fight collapse...

Until the plasma cools into neutral gas...

At which point the neutral gas slips off of the field lines into free(r) fall?

( this is a question)

 

[Dotini]

Our solar system

The yellow shape in this figure is the heliopause, the boundary between the heliosphere and the local interstellar medium. The sun sits at the center of this large bubble, but is too small to be seen here.

As the sun skims through the galaxy, it flings out charged particles in a stream of plasma called the solar wind, and the solar wind creates a bubble extending far outside the solar system known as the heliosphere. For decades, scientists have visualized the heliosphere as shaped like a comet, with a very long tail extending thousands of times as far as the distance from the Earth to the sun.

New research suggests that the sun's magnetic field controls the large-scale shape of the heliosphere "much more than had been previously thought," says Merav Opher, associate professor of astronomy and director of the Center for Space Physics at Boston University (BU). In the new model, the magnetic field squeezes the solar wind along the sun's North and South axes, producing two jets that are then dragged downstream by the flow of the interstellar medium through which the heliosphere moves.

The model indicates that the heliospheric tail doesn't extend to large distances but is split into two by the two jets, and that the format of the jets is similar to that of astrophysical jets observed in many other stars and around black holes.

Read more at: http://phys.org/news/2015-02-view-solar-astrophysical-jets-driven.html#jCp

http://iopscience.iop.org/2041-8205/800/2/L28/article