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Doubts about nebulae and star formation

  1. Feb 27, 2015 #1
    I read that nebulae are formed by stars exploding and they are also thought to be the birth place of stars. Stars explode when all the hydrogen atoms are fused into heavier elements and they are no longer able to fuse heavier atoms right ? If that's the case then the nebulae should contain heavy elements? (heavier than hydrogen and helium etc)
    How can new stars form here when there is almost no hydrogen left ?
     
  2. jcsd
  3. Feb 27, 2015 #2
    The supernova debris soon mixes with the interstellar medium--which is mostly hydrogen and helium--so that the cloud becomes predominantly hydrogen and helium. For example, the Veil Nebula, which is many thousands of years old, consists mostly of swept-up interstellar debris rather than stuff from the exploded star.
     
  4. Feb 27, 2015 #3
    Thanks , I always thought of interstellar space as almost empty , it's so full atoms and ions!! http://en.m.wikipedia.org/wiki/Interstellar_medium
    The article says that interstellar matter mostly exists in the form of clouds, why do stars need nebulae to form ? why can't they simply form in the interstellar medium clouds?
     
  5. Feb 27, 2015 #4
    I believe the can. Quite possibly, since there can be no gravitational effect at this time, perhaps the inherit magnetism of atoms allow such an attraction to take place?
     
  6. Feb 27, 2015 #5

    Chronos

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    It's all about gravity You need a certain minimum density to initiate gravitational collapse of a gas/dust cloud. Supernova are often credited [blamed?] for kick starting this process. Stars are rather abysmally poor at efficiently consuming their initial fuel supply [hydrogen]. The more massive the star, the less efficient its consumption. Only fully convective [i.e., sub solar mass] stars consume the majority of their initial fuel supply and these stars do not evolve into red giants or supernova. They just quietly glow like a hot coal for many billions of years before burning out. A third or fourth generation star like the sun has only a percent or so of initial metallicity. Heavily metallized gas clouds tend to produce lower mass stars because metallicity increases the tendency of the gas cloud to fragment as it collapses.
     
  7. Feb 27, 2015 #6
    I try reading just about everything to do with the universe to keep me on my toes. Not being a physicist I cannot relate to gravity as many do. Because of this my second thought goes to magnetism. If I’m not mistaken, every atom in the universe is polarized to some extent. Since what I’ve read defining gravity, it somehow depends on the mass of an object?, to begin its attractive process. I wonder how mass (stars) developed initially from a universe of gas without gravity? Please advise me to understand this more.
     
  8. Feb 27, 2015 #7

    russ_watters

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    1. Not all of the hydrogen in a star fuses.
    2. There are different kinds of nebulae.
    3. Some supernova remnant nebulae do contain heavier elements. That's why we exist.
     
  9. Feb 28, 2015 #8
    Is there an upper limit to a star's mass during formation? Is it possible for star to collapse into a black hole while gobbling up too much interstellar matter during it's formation ,so much that the gravity becomes too much for the outward pressure to handle ?
     
    Last edited: Feb 28, 2015
  10. Feb 28, 2015 #9
    1. Yes ,I asked whether that remaining hydrogen is enough to form new stars ,CygnusX-1 gave the answer that interstellar matter consisting mainly hydrogen also contributes to the formation of stars.
    2. Yes but all the nebulae are left overs of dead stars right?
    3.I didn't ask the question correctly ,I just wanted point out that the nebulae contains mostly heavier elements hence there is not enough hydrogen to form new stars , for which I got the answer.
     
  11. Feb 28, 2015 #10

    Ken G

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    It's the other way around, when stars begin to form, the intense UV radiation of the massive ones lights up the cloud they were born in, creating what we call a nebula.
    There might be, but it is hard to tell because very massive stars are very rare anyway. Some theory suggests that very massive stars are internally unstable, but it's not at all clear this theory can be relied upon. A similar question might be, is there an upper limit on the size a human being can be? We only know that very large humans are very rare, and the likelihood of hugeness falls off so rapidly that at some point, even a sample of 7 billion is not enough to get one above some size.
    Yes, that is the fate of the largest stars when they supernova. The whole process still takes awhile, say a million years for the biggest ones, but in astronomical terms that's still pretty short. And if the black hole is created so swiftly and seamlessly that it does not explode the envelope of the star, it's not clear how we could even tell such an event has occurred.
     
  12. Feb 28, 2015 #11

    russ_watters

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  13. Feb 28, 2015 #12
    I don't think the analogy works because I was talking about gravity, How long can a young star continue to consume interstellar matter during it's formation without collapsing into a black hole ? Is it equal to the Chandrasekhar limit ? Or is the limit only for stars which grow old and collapse ?

    http://dailysciencejournal.com/discovery-of-ancient-massive-black-hole-baffles-astronomers/21434/
    This black hole might have formed that way right?
     
    Last edited: Feb 28, 2015
  14. Feb 28, 2015 #13

    Ken G

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    What you are basically talking about is the competing timescales of adding mass to a forming star, losing heat from the star, and burning up all the nuclear fuel in a star. Everything happens faster for a very massive star, but we still tend to think in terms of phases-- first there is the mass-adding phase, then at some point the star blows away all the material around it when it creates a wind of its own. Then it will only lose mass from that point on. But if it is massive enough to begin with, it will eventually collapse into a black hole. What I'm talking about is the mass after the mass-adding process has ended, but before the star has gone through its evolution into a black hole. The distribution we observe is that there are far fewer stars with higher masses, so eventually you just run out of stars above some mass-- you get a largest mass in the collection, but it's not clear if that's some kind of physical upper limit, or just the largest one you happened to get. Star formation tends to be a process of fragmentation, and what causes the biggest fragments is not well known-- it might be that any larger stars would be internally unstable, or it might have something to do with the conditions in the cloud that is fragmenting, or it might be just chance.
     
  15. Mar 1, 2015 #14

    D H

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    The Chandrasekhar limit is 1.4 solar masses. There are plenty of stars much bigger than that. The Chandrasekhar limit applies to the inert, degenerate core of a star that has left the main sequence.

    With a few oddball exceptions, a protostar's growth phase pretty much ends when the protostar star ignites. The solar radiation pressure and stellar wind clear the remaining gas from the star system in short order, a few million years.

    It's the ignition that stops the protostar from collapsing into a black hole. This creates temperatures (and hence pressure) that counter any further collapse. Without fusion, we wouldn't have stars. We'd just have protostars that glow for a while and then collapse into a black hole.

    Aside about those oddball exceptions I mentioned: In some multiple star systems, one of the stars steals mass from another.

    We have an open thread on this topic, https://www.physicsforums.com/threa...ck-hole-discovered-900m-years-after-bb.799949. It would be better to ask your questions on this subject in that thread.
     
  16. Mar 1, 2015 #15

    Ken G

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    This is kind of a picky issue but it's a pet peeve of mine, so I'm going to correct it. Fusion never causes any significant changes in either temperature or pressure, they are both already high when fusion initiates, and afterward, fusion merely self-regulates the star to keep them from going even higher. Basically, fusion pauses the process of rising temperature and pressure, creating instead a long-lived equilibrium. But I don't think that's really what you are focusing on, you are just saying that fusion creates what we call a star, because what we call a star is in a stable long-lived equilibrium, and indeed without fusion stars would not last nearly as long.
     
  17. Mar 2, 2015 #16

    Drakkith

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    The issue here is that 'consuming' is not an accurate depiction of what's going on. A star is created from a collapsing cloud of gas and dust. As this cloud collapses, it heats up. At a certain point the energy output of the star in the form of radiation will blow away most of the remaining gas and dust that has yet to coalesce. In addition, this heating up of the collapsing gas and dust exerts an outward pressure that resists gravitational collapse. Indeed, if there were no way to get rid of this thermal energy, the gas cloud would never collapse in the first place! Anyways, even a very, very massive star will reach main sequence before ever collapsing into a black hole. The more massive the star, the less it has to collapse before its internal temperature reaches fusion threshold, which means that not even the most massive of stars can collapse directly into black holes. Instead they become main sequence O-type stars.
     
  18. Mar 10, 2015 #17

    Chronos

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    An initial gas cloud will fragment if it exceeds some critical mass - believed to be around 150 solar in the modern universe - known as the Jeans mass, which strongly depends on metallicity of the collapser. Early, first generation [pop III] stars are believed to have been capable of achieving enormous masses relative to today because primordial gas clouds were virtually metal free. It is conceivable they may have been capable of collapsing directly into black holes before heating up sufficient to blow off the excess mass.
     
  19. Mar 11, 2015 #18

    Ken G

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    Actually, that's not the Jeans mass, the Jeans mass is way larger and does not depend on metallicity. The Jeans mass is essentially the largest mass a giant molecular cloud can achieve before it starts to collapse, what you are talking about is the largest fragments of that collapse. The physics of the largest fragment comes into play much later, after the collapse has already started, and that does depend on metallicity.
     
  20. Mar 12, 2015 #19

    Chronos

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    Agreed and thanks, Ken. Fragmentation is based on Jeans instability, where lower metallicity permits larger fragments to remain intact. The direct collapse of black holes is discussed here http://arxiv.org/abs/1501.05960, Simulating the formation of massive seed black holes in the early Universe. I: An improved chemical model.
     
  21. Mar 12, 2015 #20
    I’ve read several of Sir Jeans equations and wonder how they came to be? Unquestionably a great physicist and mathematician, but I’m theorizing a great bit of “guess work” went into those formulae also? To analyze a nebulae, of which there are many, and make the determination of whether a particular one will eventually become a star, black hole etc. is beyond my comprehension. If I’m not mistaken, I assume we are supposedly looking primarily at our own galaxy which contains billions upon billions of stars. The link below was interesting in that it speaks of cosmology quite possibly being re-written in a few years? It’s strange the hour long video portion was deleted due to copyright infringements. I was thinking cosmology was sort of a Kumbaya thing?
    http://topdocumentaryfilms.com/is-everything-we-know-about-the-universe-wrong/
     
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