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Thread: Planet formation

  1. #1 Planet formation 
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    We know how solar systems form. Can a star really form on it's own without an accretion disk for a planetary system, or without left over material distant enough to form planets, etc?


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    Do You know how solar systems had formed?


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    We have a pretty clear picture about how stars form. And indeed recent research seems to confirm that this is always done via an accretion disc. But such a disc not always survives long enough to form planets. Especially with massive stars that are very hot, have a strong stellar wind and evolve very fast already ignite the fusion at a very early stage. The intense radiation and the stellar wind appear to disrupt these discs very quickly, probably before planets can form. So, it looks like that only relatively low mass stars like our sun are capable of hosting such a disc long enough that planets can form.
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    These massive stars burn out more quickly though (higher gravity=higher core pressure and temps=faster fusion). After this it will probably go supernova, produce heavy elements and have less Hydrogen left over; ideal for planetary disks, no?
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    Quote Originally Posted by KALSTER
    These massive stars burn out more quickly though (higher gravity=higher core pressure and temps=faster fusion). After this it will probably go supernova, produce heavy elements and have less Hydrogen left over; ideal for planetary disks, no?
    That's correct. But not at the same place. Supernova events have a devastating effect on the immediate environment. But the heavy elements enrich the interstellar material from which new molecular clouds will eventually form. These clouds are then the starting point of a new generation of stars and possibly planets.
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    So in the life cycle of a galaxy terrestrials are initially impossible but increasingly likely. The galaxy's disk is supposed to be only twice as old as our sun. Apparently Earth is among the very improbable firsts.

    Imagining how galaxies and stars coalesce does not intuit well with accepted chronology. Life has existed on Earth for a fifth the age of the universe, according to Big Bang calculations. There hasn't been much time to stew in.

    On the other hand, that answers the question, "Where is everybody?"
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    Quote Originally Posted by Pong
    Life has existed on Earth for a fifth the age of the universe, according to Big Bang calculations.
    The age of our own solar system is 4.5 billion years. The age of universe is about 15 billion years. What of them is older?
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    Oops. I knew the figures but why I didn't say "third"? Anyway thanks KrupS, my point is even starker. So little stewing time, it's frantic. Like: bang bang bang done.

    EDIT: Oh no wait. Earth life was 3 billion years ago.
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    Quote Originally Posted by Dishmaster
    So, it looks like that only relatively low mass stars like our sun are capable of hosting such a disc long enough that planets can form.
    Not really relevant to the thread but why do we almost always refer to the Sun as a dwarf, yellow dwarf, or low mass star?
    I know there are a large number of stars bigger than the Sun but that is not surprising considering there are billions (200 plus?) of stars in the Milky Way Galaxy alone. As I understand it the Sun, both in terms of mass and diameter, is in the top 10% of stars.
    So three cheers for our local star!
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    Quote Originally Posted by Pong
    Oops. I knew the figures but why I didn't say "third"? Anyway thanks KrupS, my point is even starker. So little stewing time, it's frantic. Like: bang bang bang done.

    EDIT: Oh no wait. Earth life was 3 billion years ago.
    Please remember that the stars that provide the heaviest elements only last for a few million years, i.e. only a thousandth of the live time of stars like the sun. So, there could have been much more generations of stars before the sun formed.
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    Quote Originally Posted by Halliday
    ... why do we almost always refer to the Sun as a dwarf, yellow dwarf, or low mass star?
    I know there are a large number of stars bigger than the Sun but that is not surprising considering there are billions (200 plus?) of stars in the Milky Way Galaxy alone. As I understand it the Sun, both in terms of mass and diameter, is in the top 10% of stars.
    So three cheers for our local star!
    I understand that this can be confusing, but the definitions of a dwarf or a low mass star provide an absolute scale based on physical properties and are not to be understood in relation to other stars.

    Dwarf stars
    All stars that populate the main sequence, i.e. the phase of stable fusion, where stars last most of their lifetime, are dwarf stars. In this phase, the equilibrium of gravity and radiation pressure produces a minimum of the possible sizes a star can attain during its lifetime. Only the phase of a white dwarf produces smaller sizes, but they are not really stars but only the cores of stars at the end of their lifetime unable to sustain a fusion process. So, the designation "dwarf star" does not compare the size of the star with others but with different stages during its own evolution.

    Low mass stars
    Generally the limit between high and low mass stars is set around 8 solar masses. This has been established for different reasons. First, low mass stars are formed via an accretion disc that feeds the protostar with material for its growth. After some time the accretion reservoir depletes and the protostar still contracts until it eventually produces the core temperatures to ignite the fusion process. For high-mass stars, the fusion can already ignite while the accretion is still continuing. It has been believed for some time that this would prevent the formation of high-mass stars, because the radiation pressure should work against the accretion and deflect the material. But recent model calculations show that this is not true.

    Another definition is the mass limit at which stars can evolve into a supernova. Low-mass stars only evolve into a red giant and supergiant that finally expells its shell and a white dwarf remains.
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    Our solar system had a cold formation, in that the gas giants formed before the sun started burning properly (which would have blown much of the gases away). I would think that only where large stars form do they quickly begin burning, having sufficient mass to do so.
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    Quote Originally Posted by Cyberia
    Our solar system had a cold formation, in that the gas giants formed before the sun started burning properly (which would have blown much of the gases away). I would think that only where large stars form do they quickly begin burning, having sufficient mass to do so.
    I think, that it's very correct. Planetary systems near large stars have to form quickly enough, so that solar wind couldn't destroy they.
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  15. #14  
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    Quote Originally Posted by Dishmaster
    Please remember that the stars that provide the heaviest elements only last for a few million years
    Oh. "Please be informed", you could say.

    Still, the galactic thin disc is not exactly ancient compared to our sun's 5 billion year old origin, right? And if I'm not mistaken this part of the galaxy is relatively lacking in fun elements. Almost like it grew clean out of earlier crap. Can anybody make sense of that?
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  16. #15  
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    Quote Originally Posted by Pong
    Still, the galactic thin disc is not exactly ancient compared to our sun's 5 billion year old origin, right?
    Yes, this is the part of the galaxy, where the star formation is taking place. This is why the stars in the thin disc are on average comparably young. The old stars reside in the bars and the globular clusters.

    Quote Originally Posted by Pong
    And if I'm not mistaken this part of the galaxy is relatively lacking in fun elements. Almost like it grew clean out of earlier crap. Can anybody make sense of that?
    Where did you get this from? The fact that young stars populate the disc does not mean that there are no heavy elements. We astronomers call the abundance of this stuff "metallicity". It is the highest in the spiral arms in our galaxy. Stars form and die all the time thereby enriching the interstellar medium. It has been processed many times in the life cycle of high-mass stars. And mixing is very efficient in galaxies. The old stars have a fairly small metallicity like the ones in the globular clusters.
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  17. #16  
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    Quote Originally Posted by Dishmaster
    Quote Originally Posted by Pong
    And if I'm not mistaken this part of the galaxy is relatively lacking in fun elements. Almost like it grew clean out of earlier crap. Can anybody make sense of that?
    Where did you get this from? The fact that young stars populate the disc does not mean that there are no heavy elements. We astronomers call the abundance of this stuff "metallicity". It is the highest in the spiral arms in our galaxy. Stars form and die all the time thereby enriching the interstellar medium. It has been processed many times in the life cycle of high-mass stars. And mixing is very efficient in galaxies. The old stars have a fairly small metallicity like the ones in the globular clusters.
    Was reading Radial migration of stars in the Galactic disk and I'm confused. I think this might be akin to guessing how a plant grows, from snapshot. A tree grows at the fringes, while a grass grows from a pith, and many plants grow in both ways. Since as you say, mixing is very efficient, I'm puzzled.

    Anything in galaxy formation that would shed or destroy heavy elements?
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  18. #17  
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    Quote Originally Posted by Pong
    Quote Originally Posted by Dishmaster
    Quote Originally Posted by Pong
    And if I'm not mistaken this part of the galaxy is relatively lacking in fun elements. Almost like it grew clean out of earlier crap. Can anybody make sense of that?
    Where did you get this from? The fact that young stars populate the disc does not mean that there are no heavy elements. We astronomers call the abundance of this stuff "metallicity". It is the highest in the spiral arms in our galaxy. Stars form and die all the time thereby enriching the interstellar medium. It has been processed many times in the life cycle of high-mass stars. And mixing is very efficient in galaxies. The old stars have a fairly small metallicity like the ones in the globular clusters.
    Was reading Radial migration of stars in the Galactic disk and I'm confused. I think this might be akin to guessing how a plant grows, from snapshot. A tree grows at the fringes, while a grass grows from a pith, and many plants grow in both ways. Since as you say, mixing is very efficient, I'm puzzled.

    Anything in galaxy formation that would shed or destroy heavy elements?
    Hi, Pong,

    what did you say meaning your tree - grass comparison? Wasn't each of both plants growing the same way? From bottom to top? From soil upwards to the sky? Which differences do you see? Sorry, I'm just trying.

    Steve
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  19. #18  
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    I'm the one who's confused!

    But for the record, grasses and trees grow in an opposite manner. A grass grows from a place you can't see - what is pushed up as blades is done; conversely a tree grows only at the tips and just behind the bark - the wood of a tree is done and won't move a centimeter in a thousand years.

    That was just an example of how development may not be obvious from a snapshot.

    I'm trying to picture the complimentary eddies of stars through our galaxy and can't. I need an animation.
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