Why don’t gas clouds in space dissipate?

By Phil Plait | May 6, 2008 8:35 am

During the live video chat on Sunday, I was asked a good question: why don’t nebulae, gas clouds in space, dissipate? What holds them together?

Here’s my answer:

The basic answer is: gravity. The clouds aren’t like clouds on Earth, or balloons filled with air; nebulae are immense objects with vast amounts of mass. Their own gravity holds them together, and can even cause them to collapse and form stars. And wouldn’t you know it, this goes against claims made by creationists that stars can’t form from gas clouds, so I included that as well in the video.

The images in the video, if you’re curious, are of the Orion Nebula (seen here too), very young stars forming in the Orion Nebula, and an artist’s drawing of a young planetary system still forming.

Comments (38)

  1. Your link to “very young stars forming…” is 404.

  2. Let’s try that again, this time without it removing the URL:

    http://www.badastronomy.com/nasa

  3. These videos are great. You should team up with Bill Nye and go on tour.

    You have a real gift for summarizing difficult topics and getting the basic idea across, you also do it with such enthusiasm it’s hard not to get excited too.

  4. hambr

    I love the way you bust anyones chops who ignore scientific evidence just because the evidence does not support their own philosophical views. If there were more scientifically minded people in this world it would be a much better place.

  5. Keith

    This image has been my desktop for the last few months and sometime si find myself just staring into it. That is some beauty, right there.

  6. aiabx

    I seem to recall reading that we have seen objects actually going through stages of formation – protostars shedding their dust clouds, IIRC. Is it true, or did I make it when I was drunk?

  7. Gas clouds in space don’t dissipate because they’re god’s belches, and as such are fixed and permanent. Also they smell vaguely of peppermint.

  8. Hmmm, must have been a cut-n-paste issue. I fixed the link, thanks.

  9. OK – so can you explain in equally simple terms why dark-matter clouds don’t collapse!

  10. Kaptain K

    Excellent!

    One question, totally unrelated to the topic:

    Why are you standing to one side of the screen? I kept expecting an image to fade in to fill the space!

  11. Same reason. Dark matter was “created” to solve the lacking gravity in the unverse. So it has mass, and thus gravity. In fact, gravity is the only way we even know it’s there.

  12. I feel dumb asking this, but here goes.

    There’s this beautiful image in the middle of your video, showing a supposedly new star with a disk of dust/gas around it, and a gap that might be caused by planet formation.

    Has to be an artistic rendering, no?

    Also, what does “hot” and “cold” mean in relation to these near-vacuum conditions? Would a “hot” nebula feel hot if one were immersed in it?

  13. Cool vid.

    And, as usual with cool stuff, it led me to thinking. And after thinking I got a few questions to ask, which is also pretty usual. So here goes one.

    What would happen if a white dwarf, one of those old, dying stars, or a brown dwarf, or even a planemo that isn’t too small, “colided” with a nebula? Is it possible that it’d start accreting gas and dust and became a new star, working as a sort of “condensation nucleous” for it?

    And, if yes, would a star generated like that be any different from a star born simply through those density variations you talked about in the video?

  14. The video is not working…

  15. Todd W.

    And here I thought that nebulae didn’t dissipate because they were enclosed in great big snow globes. My worldview is shattered, just like my pretty snow globes. :-)

  16. Sanity – exactly, so it ought to collapse in on itself to form a dark-matter star rather than sticking out there as a halo.

  17. @Codec:

    Because dark matter barely interacts with anything else, it doesn’t lose kinetic energy due to friction. So, given a cloud of dark matter particles, gravity causes them to rush together, and then instead of colliding, or friction dissipating some of their kinetic energy so they slowly come together, they just rush apart again. And the process would repeat ad nauseum.

    At least, that’s my guess, though I’m no physicist. :)

  18. idahogie:

    Has to be an artistic rendering, no?

    Yes, the second planet-forming image, the one with the single star rather than the 4 images, is an artist’s rendering and Phil’s link states.

    However, the first image, with 4 planetary disks, is a collection of real images. (Now that Phil has fixed the link. :-) )

  19. Little bald bastard:

    Also they smell vaguely of peppermint.

    I thought they smelled of elderberry wine?

  20. Ken B:
    I thought they smelled of elderberry wine?
    The peppermint smell may just be by brain warning me of an impending seizure.

  21. Thomas

    Wow.. I really have to agree with Michael Barrett. I have been over in Europe for the last two live video chats so with the time difference it’s been too complicated to catch them, but from what I’ve seen of the recent videos you’ve posted you really do make complex subjects easy to understand.

    I really hope to see more and more of these videos! Great work BA!

  22. I’d really much rather have these as downloads. Can’t play them for the guys at Cranbrook – no network connection. Perhaps you could offer a DVD?

  23. Spiv

    stephen: there’s a plugin for firefox (available on the firefox page) that will allow you to download flash content. but yeah, a DVD would be nice whenever there’s enough material to warrant such a thing.

  24. on another note about creationists, they also have a hard time distinguishing the difference between astronomy and evolution(just ask Ben Stein)

  25. Michael Lonergan

    Funny you should bring this up. My ex used to wonder why gas never dissipated, especially from under the covers.

  26. Mark Martin

    idahogie said:

    “Also, what does “hot” and “cold” mean in relation to these near-vacuum conditions? Would a “hot” nebula feel hot if one were immersed in it?”

    No, you wouldn’t feel warmth from the gas in the nebula, even though it may be of high temperature. The temperature is the average kinetic energy of the little gas particles. If you pack enough particles at high temperature into a small space, then there’s a lot of actual energy in that space which can transfer to you if you happen to be immersed within it. Each little particle collision exchanges energy with your body, making you a tiny bit warmer. If that space is practically a vacuum, with only a very few energy-carrying particles, then that’s fewer collisions with you and slower heat transfer. You’re more likely to lose what body heat you already have via radiation faster than you can receive heat from the surrounding gas.

  27. Crux Australis

    Darn, Mark Martin bet me to it. I was going to say the same thing to idahogie, plus I don’t think you’d even notice visually that you were inside the nebula. The only reason we can see them from this distance is because they’re so big. If you were inside one, you wouldn’t notice it because they’re very thin…wispy…whatever. I guess it’s like being in a light fog. You don’t notice it close up, only when you look into the distance and all the fog between you and what’s in the distance gets in the way. Zat right, everyone?

  28. I bet you’d notice if you put your high-beams on, though.

    Thanks for the answers, Ken and Mark.

  29. Codec, and how exactly would you determine the difference between a weight X invisible dark matter nebula, and a weight X invisible dark matter star? ;)

    Crux Australis, you would notice that you’re in the nebula, because looking at the stars, you will see nebula all around you. Of course, you wouldn’t see any shiny light between you and a nearby object, just you you won’t see fog between between you and your outstuck hand. There is fog/dust there, but not enough to be visible.

  30. erm,
    That would be MASS, not weight.

    It’s late over here ok?

  31. Crux Australis

    Sanity, thanks, that’s a clarification of pretty much what I meant.

  32. Nathan Myers

    The term “hot gas”, in astronomical settings, is fundamentally misleading, in several ways. First, you never find gas outside planetary atmospheres. It’s all plasma, partially or fully ionized. Even at 0.01% ionization (i.e. 1 atom in 10,000 ionized) it obeys plasma fluid dynamics, not gas dynamics. (Note that plasma fluid dynamics is very, very different from what astronomers learn as “MHD”; the former describes real materials, while the latter … well, it has nice mathematical properties.) Plasma fluid dynamics is, sadly, mathematically intractable, which is probably why astronomers make every effort to pretend it can have no effect, and prefer to invent such unobservable mathematical abstractions as neutron stars and dark energy.

    When astronomers see X-rays, they invariably assert that they indicate “hot gas”, and quote whatever absurd temperature it would take to produce the those photons with ideal gas particles bouncing at random. However, the gadget in your dentist’s office doesn’t have a pocket of million-degree “gas”; it’s accelerating electrons in a field. Of course any “gas” made of real atoms at the temperatures quoted would be 100% ionized, so it’s deeply misleading to call it gas at all. Maybe it really is hot, maybe it’s being accelerated; if they were interested they could tell you which.

    Plasma, despite the difficulty of the mathematics that describe it, displays some characteristic behaviors. The plasma globe in your museum shop displays several of them. The balls’ current flow is tuned to make the plasma glow pleasingly, but the flux tubes would be there at lower currents, just invisible, as in most astronomical settings. In the solar system we call these tubes “Birkeland currents”, and even many astronomers have been obliged lately to accept that they connect Jupiter’s and Saturn’s poles with their respective moons, as well as the Sun with the Earth’s poles, but of course they still insist they have no notable effect. It’s not hard to get a plasma ball flux tube to oscillate. Plasma structures can oscillate in a seemingly unlimited variety of modes, producing sometimes rapidly varying emissions that astronomers insist can only be produced by the fast rotation of implausibly dense solid bodies.

    So, whenever you see an astronomers’ press release that mentions “hot gas”, “dark matter”, or “neutron star”, have a good chuckle at their expense. Someday astronomy students will be obliged to learn and apply real plasma fluid dynamics. In the meantime the telescopes, satellites and space probes are collecting real data that will be useful someday when there is a corps of astronomers equipped to interpret it sensibly.

  33. slang

    And the earth is 6000 years old and scientists don’t believe that because their math is conveniently easy for a 4.5 Bn year old earth. But they’re wrong. 6000 years fits very nicely with the bible.

    Have a good chuckle at the plasma universe here:
    http://www.bautforum.com/against-mainstream/

  34. PerryG

    Phil, I don’t want to contradict you or sound “elitist”, but I do study interstellar clouds for a living… I would remind you that most of the gas and dust in these clouds are not gravitationally bound (only the densest clumps in them are collapsing, and onyl a very small fraction of their mass ends up in stars and planetary systems). The simple answer to the question asked to you is that they ARE evaporating! But they are so large (10s of light years) and diffuse (~ 10-100 atoms/cc on average) that this process takes millions of years.

  35. Nathan Myers

    slang: What a proud tradition of fact-based discussion you uphold. I hope you don’t represent BA at its best. It’s sad that PerryG needs to kowtow and apologize for presenting some facts. Well, there’s science, and then there’s astronomy, right? Shame.

  36. Torbjörn Larsson, OM

    Of course any “gas” made of real atoms at the temperatures quoted would be 100% ionized, so it’s deeply misleading to call it gas at all.

    Atoms and electrons don’t need to have the same temperature.

    IIRC conduction electrons in metals have a Fermi temperature on the order of 10^5 K as the metal is at room temperature. Likewise when you cool atoms to ultra low temperatures it is easier to cool the nucleus, which can interact in many ways, than the electron cloud. So you can have comparably hot gas if there is a restricted interaction.

    And that would apply to vacuum conditions. Which is presumably why you AFAIU can see a lot of radiation interaction with dust and partially ionized molecules.

    As for astronomers knowledge of plasmas, I find it curious that IIRC most of the easily tractable plasma wave modes are known from astronomy as opposed to say dentists x-ray tubes. Wonder why that is? :-P

    the plasma universe

    Great, I presume another case of “I have a hammer, what can I use for nails”. Where is the “not for children” label?

  37. Onex

    TRANSCRIPT OF VIDEO

    To the best of my ability. Capitals for emphasis. I split the difference on adding speech slurs.

    TITLE: Why Don’t Gas Clouds in Space Dissipate? Phil Plait, BadAstronomy.com

    So in my live chatroom, I was asked: “Why is it that gas clouds, which can be of so low density, they’re basically like laboratory vacuum – how is it that these things don’t just dissipate into space”? And this is actually something that creationists talk about a lot, because they say “oh, stars can’t possibly form from hot gas, hot gas expands, we see this in balloons, right – hot air balloons expand, they can’t possibly collapse to form stars”. And, you know, if a creationist is saying it, it’s probably wrong. That’s the way to bet. So what does this mean? Well, here’s a picture [holds up book] of a really beautiful gas cloud, that I’m getting from the book “Bang!” by Chris Lintott, Patrick Moore and Brian May.. lead guitarist for Queen! That’s so cool – he’s an astronomer! Uhm, this is NGC-2467, that’s just the catalogue name, you can see this gorgeous cloud there. All of this [points] is gas. You can see all the gas.. there’s some dust, dark dust here, some starts lighting it up..
    Now basically, some of these gas clouds are called.. a singular is nebula, which is Latin for cloud. Plural is nebulae, with an “e” on the end. And these gas clouds are very, very low density and some of them – the DENSEST of them, might have tens or hundreds of thousands of particles per cubic centimeter. So if I had a mini-marshmallow with [science?], I could show you this, but that’s.. that’s [gestures] about a cubic centimeter.. a salt – eh.. sugar cube, or a mini-marshmallow. And there might only be, you know, maybe a MILLION particles per cubic centimeter inside one of these gas clouds, and you might think “well, a million sounds like a LOT”. But the Earth’s atmosphere [gestures], all around us, has 10 ^ 19 particles, a one with 19 zeroes after it. So this is trillions of times denser – the air is – trillions of times denser than even the densest of these gas clouds in space. So if they’re hot, and they ARE hot, some of them are, you know, thousands of degrees.. although usually they’re cooler than that – hundreds of degrees celsius.. or even lower. Uh, they’re actually.. most of them are actually very cold. Uh, compared to Earth temperatures. Why don’t they dissipate, like hot air does, gas in a hot air balloon? If you were to pop a hot air balloon, right, that hot air will dissipate. The reason they don’t is because these nebulae are immense, okay? It’s not just like a balloon, which might only have a few pounds of air in it. We’re talking about a gas cloud, which is LIGHTYEARS across. And so the total, TOTAL mass of this entire gas cloud can be many times the mass of the sun. These things form stars, they’re star factories, and really big ones, like the Orion nebula or the Tarantula nebula can have thousands, or some of them can have MILLIONS of times the mass of the sun. And all of that gravity added together is what holds that cloud together. THAT’S why they don’t dissipate.
    Gas clouds can collide. There’s lots of reasons they can do this. They’re orbiting the center of the Milky Way, some of them are moving faster than others, some of them are on orbits that aren’t in perfect circles, so they intersect and they smack into each other.. And when they do that, they can form shockwaves, and compress and that can form local dense regions inside the gas cloud, and those will collapse to form stars.
    We. See. This. Happening. Don’t ever listen, well you can listen to a creationist when they talk, it’s always good.. because then you can find out what NOT to think. Uhm, but when a creationist says “stars cannot form from nebulae, we don’t see it happening”, that is a LIE. We see it happening in all different stages. We haven’t seen ONE star all the way through the process anymore than taking a snapshot of a crowd you will see babies being born, right? It takes a long time for this process to happen. Hundreds of thousands, or millions of years. So it’s not like you can watch one star do it, we’ve only been watching for, you know, a couple of centuries. But we have seen stars, in every different stage of development [shows pictures of this], from gas clouds, to where the stars are starting to form and it’s surrounded by dense gas, to where the gas is collapsing into a disc and forming planets.. We’ve seen these discs with gaps in them [shows picture] where we can see.. we can’t see the planet itself, but we know that there are planets in that disc, carving out these grooves, and forming new planets and gathering mass that way.. to where the planets system is very young and the planets are still glowing from the heat of the formation – all the way up to solar systems like ours! Which is 4.6 billion years old, again despite what SOME people are telling you.
    And so, it’s kind of interesting, you get a question like, you know, “what holds gas clouds together” and it actually takes you, if you follow the logic and you follow the physics and you follow the math, it takes you right through from “they don’t dissipate because of their own mass” to “this is how we see stars form”, this is how we see US form.. uh, stars like the Sun forming, they got heavy metals in them like iron and molybdenum and calcium and all that, from stars that explode nearby and seed them with these heavy elements. Those form, and so the Sun gets iron in it from a nearby star that blew up. We see TONS of evidence of this. And so it’s great – this is a great example of where a simple question leads you to very, very profound results.

    TITLE: Phil Plait, BadAstronomy.com

  38. Xeno

    TRANSCRIPT OF VIDEO

    To the best of my ability. Capitals for emphasis. I split the difference on adding speech slurs.

    TITLE: Why Don’t Gas Clouds in Space Dissipate? Phil Plait, BadAstronomy.com

    So in my live chatroom, I was asked: “Why is it that gas clouds, which can be of so low density, they’re basically like laboratory vacuum – how is it that these things don’t just dissipate into space”? And this is actually something that creationists talk about a lot, because they say “oh, stars can’t possibly form from hot gas, hot gas expands, we see this in balloons, right – hot air balloons expand, they can’t possibly collapse to form stars”. And, you know, if a creationist is saying it, it’s probably wrong. That’s the way to bet. So what does this mean? Well, here’s a picture [holds up book] of a really beautiful gas cloud, that I’m getting from the book “Bang!” by Chris Lintott, Patrick Moore and Brian May.. lead guitarist for Queen! That’s so cool – he’s an astronomer! Uhm, this is NGC-2467, that’s just the catalogue name, you can see this gorgeous cloud there. All of this [points] is gas. You can see all the gas.. there’s some dust, dark dust here, some starts lighting it up..
    Now basically, some of these gas clouds are called.. a singular is nebula, which is Latin for cloud. Plural is nebulae, with an “e” on the end. And these gas clouds are very, very low density and some of them – the DENSEST of them, might have tens or hundreds of thousands of particles per cubic centimeter. So if I had a mini-marshmallow with [science?], I could show you this, but that’s.. that’s [gestures] about a cubic centimeter.. a salt – eh.. sugar cube, or a mini-marshmallow. And there might only be, you know, maybe a MILLION particles per cubic centimeter inside one of these gas clouds, and you might think “well, a million sounds like a LOT”. But the Earth’s atmosphere [gestures], all around us, has 10 ^ 19 particles, a one with 19 zeroes after it. So this is trillions of times denser – the air is – trillions of times denser than even the densest of these gas clouds in space. So if they’re hot, and they ARE hot, some of them are, you know, thousands of degrees.. although usually they’re cooler than that – hundreds of degrees celsius.. or even lower. Uh, they’re actually.. most of them are actually very cold. Uh, compared to Earth temperatures. Why don’t they dissipate, like hot air does, gas in a hot air balloon? If you were to pop a hot air balloon, right, that hot air will dissipate. The reason they don’t is because these nebulae are immense, okay? It’s not just like a balloon, which might only have a few pounds of air in it. We’re talking about a gas cloud, which is LIGHTYEARS across. And so the total, TOTAL mass of this entire gas cloud can be many times the mass of the sun. These things form stars, they’re star factories, and really big ones, like the Orion nebula or the Tarantula nebula can have thousands, or some of them can have MILLIONS of times the mass of the sun. And all of that gravity added together is what holds that cloud together. THAT’S why they don’t dissipate.
    Gas clouds can collide. There’s lots of reasons they can do this. They’re orbiting the center of the Milky Way, some of them are moving faster than others, some of them are on orbits that aren’t in perfect circles, so they intersect and they smack into each other.. And when they do that, they can form shockwaves, and compress and that can form local dense regions inside the gas cloud, and those will collapse to form stars.
    We. See. This. Happening. Don’t ever listen, well you can listen to a creationist when they talk, it’s always good.. because then you can find out what NOT to think. Uhm, but when a creationist says “stars cannot form from nebulae, we don’t see it happening”, that is a LIE. We see it happening in all different stages. We haven’t seen ONE star all the way through the process anymore than taking a snapshot of a crowd you will see babies being born, right? It takes a long time for this process to happen. Hundreds of thousands, or millions of years. So it’s not like you can watch one star do it, we’ve only been watching for, you know, a couple of centuries. But we have seen stars, in every different stage of development [shows pictures of this], from gas clouds, to where the stars are starting to form and it’s surrounded by dense gas, to where the gas is collapsing into a disc and forming planets.. We’ve seen these discs with gaps in them [shows picture] where we can see.. we can’t see the planet itself, but we know that there are planets in that disc, carving out these grooves, and forming new planets and gathering mass that way.. to where the planets system is very young and the planets are still glowing from the heat of the formation – all the way up to solar systems like ours! Which is 4.6 billion years old, again despite what SOME people are telling you.
    And so, it’s kind of interesting, you get a question like, you know, “what holds gas clouds together” and it actually takes you, if you follow the logic and you follow the physics and you follow the math, it takes you right through from “they don’t dissipate because of their own mass” to “this is how we see stars form”, this is how we see US form.. uh, stars like the Sun forming, they got heavy metals in them like iron and molybdenum and calcium and all that, from stars that explode nearby and seed them with these heavy elements. Those form, and so the Sun gets iron in it from a nearby star that blew up. We see TONS of evidence of this. And so it’s great – this is a great example of where a simple question leads you to very, very profound results.

    TITLE: Phil Plait, BadAstronomy.com

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