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Interstellar gas cloud stability

  1. Dec 9, 2013 #1
    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?



    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.
    Last edited: Dec 9, 2013
  2. jcsd
  3. Dec 9, 2013 #2


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    Jeans theorem might make a good read.
  4. Dec 9, 2013 #3


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    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.

    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.

    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.

    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.
  5. Dec 10, 2013 #4
    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?
  6. Dec 12, 2013 #5


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    I don't know what you are saying here.

    At a given density and temperature, yes. Change these, and you change the equilibrium point of the system.
  7. Dec 15, 2013 #6
    Maybe I'm off target.

    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.
  8. Dec 15, 2013 #7
    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.
  9. Dec 16, 2013 #8
    Atoms in highly excited levels could possibly provide cooling as well. There may be only very few, but they have very large cross sections.
  10. Dec 16, 2013 #9
    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.
  11. Dec 16, 2013 #10
    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.
  12. Dec 16, 2013 #11
    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.
  13. Dec 17, 2013 #12
    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.
  14. Dec 17, 2013 #13
    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.
  15. Dec 18, 2013 #14
    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.

    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.

    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.
  16. Dec 19, 2013 #15
    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).

    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.
  17. Dec 30, 2013 #16
    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.
  18. Dec 30, 2013 #17
    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.
  19. Dec 30, 2013 #18


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    http://science1.nasa.gov/science-news/science-at-nasa/2009/23dec_voyager/ [Broken]
    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."

    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.
    Last edited by a moderator: May 6, 2017
  20. Jan 1, 2014 #19
    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.
    Last edited by a moderator: May 6, 2017
  21. Jul 9, 2014 #20


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    "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.
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