Spiral-armed galaxies

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taresitara

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Most of the galaxies have extended spiral arms all uniformly identical indicating that some force is making them rotate in that way.&nbsp; Is it the force of the black hole in the galactic centre that force them to ratate in the way and the arms bent in that direction?&nbsp; If it is so, can the black hole at the center of our Milky Way be so powerful to extend its power across such a vast galaxy?&nbsp; It does not seem so. Then what is the answer to this? <div class="Discussion_UserSignature"> <font size="2">K Sundar</font> </div>
 
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MeteorWayne

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Most of the galaxies have extended spiral arms all uniformly identical indicating that some force is making them rotate in that way.&nbsp; Is it the force of the black hole in the galactic centre that force them to ratate in the way and the arms bent in that direction?&nbsp; If it is so, can the black hole at the center of our Milky Way be so powerful to extend its power across such a vast galaxy?&nbsp; It does not seem so. Then what is the answer to this? <br />Posted by taresitara</DIV><br /><br />Hi taresitara.</p><p>Welcome to Space.com!!</p><p>This question would better be asked in the Space Science and Astronomy or Ask the Astronomer fora.</p><p>I'll ask that it be moved there, since the many of the people best equipped to answer it don't visit the "open" forum very often.</p><p>Wayne</p> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>
 
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Boris_Badenov

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<font size="2">Most galaxies are not spirals & this belongs in Ask the Astronomer.</font> <div class="Discussion_UserSignature"> <font color="#993300"><span class="body"><font size="2" color="#3366ff"><div align="center">. </div><div align="center">Never roll in the mud with a pig. You'll both get dirty & the pig likes it.</div></font></span></font> </div>
 
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Saiph

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Most of the galaxies have extended spiral arms all uniformly identical indicating that some force is making them rotate in that way.&nbsp; Is it the force of the black hole in the galactic centre that force them to ratate in the way and the arms bent in that direction?&nbsp; If it is so, can the black hole at the center of our Milky Way be so powerful to extend its power across such a vast galaxy?&nbsp; It does not seem so. Then what is the answer to this? <br /> Posted by taresitara</DIV></p><p>&nbsp;</p><p>Good question!&nbsp; This was one of the classic problems facing early Galactic astronomy.&nbsp; Just what <em>are </em>spiral arms?!</p><p>Spiral arms aren't a real structure in the galaxy, like say spokes in a wheel, that turn in lockstep around the core.&nbsp; They're more of an illusion of structure.&nbsp; The stars that make up the spiral arms are only there for a short period of time, then continue on through the galaxy.&nbsp; The stars orbit ~10-50x faster than the arms spin about the core.</p><p>The reason we get the spiral arms is because the stars all have very similar (but not identical) orbits.&nbsp; The stars orbits all tend to converge upon, and line up in the same area...so a lot of stars tend to be in that one area, before the split up and go elsewhere in the galaxy.</p><p>Sorta like a traffic jam.&nbsp; The cars in the jam are slowly moved through, then speed off on their way.&nbsp; But because so many cars are going in the same general direction, you have a lot in one place at one time.&nbsp; Traffic jams can even move, just like spiral arms, if the original cause of the jam is removed.&nbsp; It takes a long time for the jam to clear...and the clear area is further and further back up the road as the lead cars can speed up, but the trailing cars are still stuck waiting to move....and the cars way back up the road are just now stopping at the back of the jam (even though the first cars are long gone!) and have to wait for the rest of the jam to clear as well...&nbsp; this slowly pushes the jam further and further back up the road. </p> <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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Saiph

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<p>Another way to see this is to take coffee, and add creamer to it.&nbsp; Stir it in and watch the swirls and bubbles in the coffee.&nbsp; They make a strong swirl pattern... but the individual bubbles themselves move through the swirls, they don't stay fixed.</p><p>&nbsp;</p><p>The technical term for this phenomena is a "density wave"&nbsp;</p> <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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MeteorWayne

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<p>3D solitons :)</p> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>
 
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KickLaBuka

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>&nbsp;They're more of an illusion of structure.&nbsp; The stars that make up the spiral arms are only there for a short period of time, then continue on through the galaxy.&nbsp;Posted by Saiph</DIV><br /><br />That is such a load of crap.&nbsp; Chance encounters on their way&nbsp;to other distant parts of the universe?!</p><p>&nbsp;There is an organized system holding these stars there, and creating the wonderful STRUCTURES.&nbsp; It's a combination of electromagnetic fields, some aligned electrons in motion and their subsequent magnetic fields, and basic gravity.&nbsp; No supermassive invisible things.&nbsp; How dare a moderator have the gall to call them illusions because they don't understand how it could be possible given their assumptions.</p><p>Start over.</p> <div class="Discussion_UserSignature"> <p>-KickLaBuka</p> </div>
 
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DrRocket

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>That is such a load of crap.&nbsp; Chance encounters on their way&nbsp;to other distant parts of the universe?!&nbsp;There is an organized system holding these stars there, and creating the wonderful STRUCTURES.&nbsp; It's a combination of electromagnetic fields, some aligned electrons in motion and their subsequent magnetic fields, and basic gravity.&nbsp; No supermassive invisible things.&nbsp; How dare a moderator have the gall to call them illusions because they don't understand how it could be possible given their assumptions.Start over. <br />Posted by KickLaBuka</DIV></p><p>No.&nbsp; Saiph has it right, not surprisingly&nbsp;&nbsp;What is a load of crap is your post.</p><p>You will note that he said nothing whatever about chance encounters on their way to distant parts of the universe.&nbsp; That particular crap is due entirely to you.&nbsp; Those stars are in orbit around the center of their galaxy.&nbsp; It is just the apparent arms that are somewhat ephemeral, znd not the long term structure that you seem to think they are.&nbsp; <br /></p> <div class="Discussion_UserSignature"> </div>
 
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amaterasu

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Another way to see this is to take coffee, and add creamer to it.&nbsp; Stir it in and watch the swirls and bubbles in the coffee.&nbsp; They make a strong swirl pattern... but the individual bubbles themselves move through the swirls, they don't stay fixed.&nbsp;The technical term for this phenomena is a "density wave"&nbsp; <br />Posted by Saiph</DIV></p><p>very interesting.<br />and the reason why spiral arms&nbsp;are not being diluted&nbsp;like cream&nbsp;with coffee is that dark matter is preventing&nbsp;it from occurring,&nbsp;isn't it?&nbsp; the constant rotational velocity can be attributed to&nbsp;surrounding dark matter, too.&nbsp; </p><p>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </p><p>&nbsp;</p><p>&nbsp;</p><p>&nbsp;</p> <div class="Discussion_UserSignature"> </div>
 
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Saiph

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>That is such a load of crap.&nbsp; Chance encounters on their way&nbsp;to other distant parts of the universe?!&nbsp;There is an organized system holding these stars there, and creating the wonderful STRUCTURES.&nbsp; It's a combination of electromagnetic fields, some aligned electrons in motion and their subsequent magnetic fields, and basic gravity.&nbsp; No supermassive invisible things.&nbsp; How dare a moderator have the gall to call them illusions because they don't understand how it could be possible given their assumptions.Start over. <br /> Posted by KickLaBuka</DIV></p><p>Oh, I'm not saying that spiral arms aren't supported by several mechanisms that help shape them.&nbsp; Just that the spiral structures are illusory because there's nothing holding them together.&nbsp; They're more like an ocean wave than any real "structure".&nbsp; Sure, there's mechanisms behind it all, that form it all, that perpetuates it all.&nbsp; But they aren't a structure as the original poster meant them, and as most people mean the term.</p><p>The reason they form spirals is because a lot of stars form in the same general region, and have similar orbits.&nbsp; These orbits, because of their common origin, bunch up to make denser regions in galaxy..and then spread out elsewhere to form less dense regions.&nbsp; The fact that there is no force keeping the stars in the arms is evident from observations of the spiral arm, and the stars.&nbsp; The stars orbit about the galacitc core much faster than the spiral arms.</p><p>It has very little to do with, "It's a combination of electromagnetic fields, some aligned electrons in motion and their subsequent magnetic fields." and certainly not supermassive invisible things (which I didn't claim mattered at all).</p><p>If it helps my credibility at all, don't even bother looking at the title "moderator".&nbsp; That just means the powers that be around here respect my level-headedness and judgement enough to trust me not to abuse my super nifty moderator powers.&nbsp; Which, oddly, I've never ever used....not even to move a forum.&nbsp; Go figure.</p><p>My "gall" actually stems from my long and active participation in the amateur astronomy community, and my degree in astronomy, participation in the professional field of astronomy (not lots, I admit, but some).&nbsp; Along with some 7 years posting here, and debating just about any astronomy subject you can think of with many other well informed posters.&nbsp; And on that note, thanks for the Support Dr. Rocket. </p><p>Amaterasu: Welcome to SDC, love the name btw.</p><p>As for why the spiral arms continue to exist instead of just fading away is simple.&nbsp; They foster the creation of stars along their leading edge, where the dense starfields sweep through the galactic dust clouds, disrupting them and triggering star formation...with stars that share similar orbits to the stars that disrupted them.&nbsp; It's like sweeping your hand through water, and noticing that there is some water that instead of just moving out of the way, follows your hand.&nbsp; So, as stars die out, which would thin the spiral arm, new stars are born.&nbsp; Evidence for this is the large number of bright blue supergiant stars which dominate the spiral arms.&nbsp; Indeed these blue supergiants have such a short lifespan, they often don't survive to leave the spiral arm that they formed in. </p><p>&nbsp;Also, the increased densities in the spiral arm help keep the orbits...um, cohesive might be a good word here.&nbsp; The gravity tugs them along and helps line things up a bit, and helps keep natural variation limited (but not completely). </p><p>In brief:&nbsp; Constant new material is introduced into this dynamically stable mechanism. </p> <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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KickLaBuka

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<p>Thanks and I apologize for being so cutting.&nbsp; It stands that we do not agree about the forces involved, but noting that supermassive invisible things are not involved means there can be a common ground.&nbsp;&nbsp;If you're willing to describe observations to someone who completely disagrees with the mainstream ideas, I would be appreciative.&nbsp; I'll in turn, try to be respectful of your observations and background and I'll try not to get emotionally charged up because of the state of astrophysics to date.</p><p>Please correct my opinion.&nbsp; So the outer edge consists of blue supergiants (population II stars)&nbsp;which have been tugged by a nearby galaxy.&nbsp; As these stars and dust get pulled into&nbsp;an orbit, they speed up, and consequently have higher energies.&nbsp; The inner stars are undertaking the same increase in energy, especially on the tigher orbits, sometimes supernova is the only way to&nbsp;dissapate the energy enclosed, and turn into population I stars.&nbsp; </p><p>&nbsp;</p> <div class="Discussion_UserSignature"> <p>-KickLaBuka</p> </div>
 
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Saiph

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<p>No problem KickLaBuka. &nbsp;Afterall, you're new here, and I've been away a lot lately. &nbsp;No reason to expect you to know anything about my background.</p><p>&nbsp;</p><p>First correction is you seem to be mistaken about the classification of population I and II stars. &nbsp;These refer primarily to the metalicity of the stars, not their general class (supergiant, giant, dwarf, etc). &nbsp;A blue supergiant can be a population I star, or a pop II star. &nbsp;Population I stars are stars with significant metal content, like our sun. &nbsp;Pop II stars are those with very little metals. &nbsp;Newly formed stars are population I stars, as they will be full of the metals spawned by the previous generations (which were probably also Pop I stars). &nbsp;Pop II stars, the remaining ones, tend to be very old stars, and formed before many other stars had a chance to spread their byproducts around.</p><p>Now, the blue supergiants do tend to form and "live" near the leading edge of spiral arms. &nbsp;Other stars as well, but by their large size and high temperatures blue supergiants dominate the energy output, and thus observed starlight, of these regions. &nbsp;Blue supergiants also live very short lives, on the order of millions of years, not billions. &nbsp;As such they tend not to stray far from their birthplace before they extinguish...so you tend to see these ONLY at the leading edge of a spiral arm.</p><p>&nbsp;</p><p>External galaxies do little to influence spiral arms. &nbsp;In close galaxy pairs, the galaxies may distort eachother, and any collision results in massive star formation and re-organizing as they merge. &nbsp;But the presence of another galaxy isn't required to form spiral arms, and is actually detrimental to such processes.&nbsp;</p> <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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UncertainH

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>They foster the creation of stars along their leading edge, where the dense starfields sweep through the galactic dust clouds, disrupting them and triggering star formation...with stars that share similar orbits to the stars that disrupted them.&nbsp; It's like sweeping your hand through water, and noticing that there is some water that instead of just moving out of the way, follows your hand.&nbsp; So, as stars die out, which would thin the spiral arm, new stars are born.&nbsp; Evidence for this is the large number of bright blue supergiant stars which dominate the spiral arms.&nbsp; Indeed these blue supergiants have such a short lifespan, they often don't survive to leave the spiral arm that they formed in. &nbsp;Also, the increased densities in the spiral arm help keep the orbits...um, cohesive might be a good word here.&nbsp; The gravity tugs them along and helps line things up a bit, and helps keep natural variation limited (but not completely). In brief:&nbsp; Constant new material is introduced into this dynamically stable mechanism. <br />Posted by Saiph</DIV></p><p>You mentioned earlier that stars rotate faster than the arms. If any new stars form in the spiral and don't die too quickly would they also eventual speed up, and if so how?</p>
 
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Saiph

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<p>They don't speed up, they're already moving faster than the arms when they form. &nbsp;The galaxy is full of dust clouds that orbit about the galactic core just like stars (well, almost). &nbsp;When they pass through the spiral arms, the increased density tends to cause star formations. &nbsp;The stars in the clouds have the same general orbit as the cloud...but they are also strongly influenced by the orbits of stars passing through the cloud. &nbsp;Basically the passing stars transfer momentum to the clumping gases that's parrallel to their orbits. &nbsp;This tends to spawn stars with similar orbits.</p><p>&nbsp;</p><p>I guess I wasn't to clear when I said the spiral arms pass through the dust clouds...like those clouds are stationary. &nbsp;That isn't the case, and I apologize for misleading anyone.&nbsp;</p> <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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KickLaBuka

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Newly formed stars are population I stars, as they will be full of the metals spawned by the previous generations (which were probably also Pop I stars). &nbsp;Pop II stars, the remaining ones, tend to be very old stars, and formed before many other stars had a chance to spread their byproducts around. <br />Posted by Saiph</DIV></p><p>&nbsp;I guess it just makes more sense to me that a population II blue supergiant would wind up turning into a population I star after a supernova event.&nbsp; Like if there's too much charge built up or it's moving too fast, or if it gets too close to another blue supergiant.&nbsp; It drifts too far to the "blue (high end)" and boom, heavy metals are created and a population I star forms in its spot.</p><p>Is there evidence that population I is new and II is old?&nbsp; I had read that was the belief, but I kinda like the above better.</p> <div class="Discussion_UserSignature"> <p>-KickLaBuka</p> </div>
 
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Saiph

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<p>Evidence?&nbsp; Besides the population I and II traits...the age of the stars themselves.&nbsp; Population II stars, those with very few metals, are all small, very old stars, often in globular clusters where star formation has been minimal for a very long, long time.</p><p>Nucleosynthesis is the key mechanism.&nbsp; Metals (meaning anything above helium) are only formed, in any quantity worth caring about, in the core of stars, mostly in very large ones.&nbsp; This is because fusion chain reactions are required to make them, either in core fusion events, or the runamuck supernovae explosions.&nbsp; So, if a star has any metal, it had to have formed after stars that had none..died.&nbsp; And since the death of these stars pollute the area around them quickly (supermassive stars live for millions of years, compared to our sun's billions)...most stars that formed, even early int he galaxy, are population I. &nbsp;</p><p>&nbsp;The only Pop II stars surviving today are small red dwarfs, who's lifespan is easily tens of billions of years long...so they formed when there weren't metals, and have lived until today. </p><p>Also, the amount of charge required to detonate a star is....insanely high.&nbsp; High enough that I can't imagine any way, at all, for it to accrue naturally (way to much energy is required just to produce that amount of charge).</p><p>Finally:&nbsp; Speed has no affect, whatsoever, on the internal dynamics of the moving object.&nbsp; It only affects how that object interacts with other objects moving at different speeds.&nbsp; This ranges from as simple a notion as collisions, to esoteric ideas like time-dilation.&nbsp; The internal workings however, don't notice if it's "Stationary" or 99.99999% C...as speed is entirely relative. </p><p>&nbsp;</p><p>There is some evidence for a Population III class btw, those stars that have ONLY hydrogen and helium, that spewed the few metals that exist in even Pop II stars.</p> <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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amaterasu

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<p>thanks for taking the time to explain in such a clear fashion, oh and for the warm welcome, Saiph.</p><p>i hadn't heard of&nbsp;the 'density wave'/ stellar 'traffic jam' or 'blue supergiant stars&nbsp;dominating the spiral arms'&nbsp;things&nbsp;until i read your posts, and while i find the ideas very interesting (on and i've found some beautiful photo of blue-coloured arms here: http://hubblesite.org:80/newscenter/archive/releases/2005/01/image/a) i still feel a wee bit confused because you seem to be saying that there're no such things as 'galactic rotation curves' at all.</p><p>i think around our own milky way galaxy stars do slow down as they approach the spiral arms.<br />although would this be applied to all of&nbsp;the galaxies?&nbsp; if not, what is the cause of the difference?&nbsp; galactic radius or distribution quantity of dark matter or something else?</p> <div class="Discussion_UserSignature"> </div>
 
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KickLaBuka

Guest
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Population II stars, those with very few metals, are all small, very old stars, often in globular clusters where star formation has been minimal for a very long, long time.&nbsp; most stars that formed, even early int he galaxy, are population I. &nbsp;&nbsp;The only Pop II stars surviving today are small red dwarfs, who's lifespan is easily tens of billions of years long <br />Posted by Saiph</DIV><br /><br />Being very old, and given the expanding universe assumption, does that mean that they are only located at teh arms in galaxies with very high red-shift (z />2)?&nbsp; Does that mean that that the vast percentage of stars observed are type I?&nbsp; You said they dont' drift far from their original location.&nbsp; Doesn't that contradict the expanding universe?</p> <div class="Discussion_UserSignature"> <p>-KickLaBuka</p> </div>
 
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Saiph

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<p>Amaterasu: &nbsp;No problem at all. :)&nbsp;</p><p>Oh my, there are galactic rotation curves! &nbsp;But those don't have much to do with the structure of spiral arms. &nbsp;The galactic rotation curve is merely a plot/graph of a stars orbital velocity, verse it's distance from the galactic core.</p><p>&nbsp;The only way they factor in, is they tell us the stellar orbiting speed, or rather the information we gather on that is factored into the graphs. &nbsp;&nbsp;Their speed tells us they don't orbit at teh same rate as the "arms" do.</p><p>&nbsp;</p><p>KickLaBuka: &nbsp;You're getting you scale of events skewed. &nbsp;Distance and redshift only matter on distances greater than the seperation of the average galaxies...millions of light years or more. &nbsp;A galaxy is much smaller than that. &nbsp;Our milky way, a fairly large speciimen, is only 100,000 ly across. &nbsp; Over that distance, universal expansion has very little affect compared to gravity.</p><p>The old red dwarfs are found primarily in globular clusters, which are about as old as the galaxy, but orbit it in a roughly spherical shell at about 40,000 ly from the core (for a shell diamater of ~80,000 ly). &nbsp;There are old red dwarf Pop II stars in the galaxy itself, &nbsp;but they are drastically overshadowed by the more recent arrival of Pop I stars. &nbsp;This is because the galaxy forms lots of stars, of all sizes. &nbsp;And any star formed after Pop II tend to be pop I. &nbsp;Also, the pop I category has a wide variety of large stars (that formed very recently, last billion or so years), any single one of which puts out more light than dozens of red dwarf stars. &nbsp; And these large stars haven't aged enough to die out.</p><p>Last laugh is for the red dwarfs though. &nbsp;Even after many, many more generations of star formation, which will slow as time goes on, these Giant and Supergiant stars will burn out, and fewer will replace them. &nbsp;Meanwhile the original red dwarfs, and any that form later, will still survive plugging along. &nbsp;Tortoise and the Hare!&nbsp;&nbsp;</p><p>Also, it's only the really large supergiant stars that don't move far from thier original formation site. &nbsp;And that's only because they don't live long enough to do so. &nbsp;Any smaller stars that formed with them, that live longer, will orbit all about the galaxy. &nbsp;The red dwarfs, who's age is tens of &nbsp;billions of years...will really travel alot over their lifetime.&nbsp;</p> <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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KickLaBuka

Guest
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>You're getting you scale of events skewed.&nbsp; <br />Posted by Saiph</DIV><br /><br />That's an understatement.&nbsp; Thanks for the adjustment.&nbsp; So what, if any calculated, are the percentages of population I verses population II, verses population III?</p><p>&nbsp;</p> <div class="Discussion_UserSignature"> <p>-KickLaBuka</p> </div>
 
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Saiph

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<p>No problem. &nbsp;Scale is, IMO, one of the hardest things to grasp in astronomy. &nbsp;In distance, size, energy outputs... they're all...astronomical&nbsp;<img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-laughing.gif" border="0" alt="Laughing" title="Laughing" /></p><p>Depends on where you look really. &nbsp;Population I stars (high metallicity, at something like ~1%) are the vast majority in spiral galaxies, most galaxies. &nbsp;Pop II stars are the vast majority in their globular clusters, orbiting the galaxy. &nbsp;Pop III, those with no metals, are basically unheard of. &nbsp;IIRC we've only found a few candidates for this class. &nbsp;It's a needle in a haystack for these guys, because the dynamics of a star with no metals as catalysts for the core fusion tend to be very big...and thus die very quickly.&nbsp;</p><p>Now, a galaxy holds a hundred billion stars, and globular clusters several hundred thousand stars....so Pop I wins if you were to do a "universe wide" survey.&nbsp;</p> <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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KickLaBuka

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Scale is, IMO, one of the hardest things to grasp in astronomy.&nbsp;&nbsp;IIRC we've only found a few candidates for this class. <br />Posted by Saiph</DIV><br /><br />Cool.&nbsp; That lends itself nicely.&nbsp; What's IMO and IIRC?&nbsp; What about single stars or pairs of stars or small groups of stars?&nbsp; What types do those fall under and how rare is that occurance?</p><p>&nbsp;</p> <div class="Discussion_UserSignature"> <p>-KickLaBuka</p> </div>
 
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Saiph

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<p>IMO: <strong>I</strong>n <strong>M</strong>y <strong>O</strong>pinion.&nbsp;&nbsp; OFten includes and <strong>H</strong> (IMHO) or an <strong>N</strong> (IMNHO) standing for "Humble", and "Not So" respectively.&nbsp; I use it when I'm making a personal assertion about a subject, following a hunch, etc.</p><p>IIRC: <strong>I</strong>f <strong>I</strong> <strong>R</strong>ecall <strong>C</strong>orrectly meaning I'm digging up a somewhat unclear memory, and probably over generalizing, or perhaps just out of date.</p><p>&nbsp;</p><p>As for single stars, binaries or multi-star systems (and small clusters)...depends on where you find them.&nbsp; In the galaxy, almost always Pop I.&nbsp; In a globular cluster?&nbsp; Almost always Pop II.</p><p>Binary and Trinary star systems are VERY common, almost on par with Single star systems (maybe even more binaries than singles...I remember hearing something about that in my classes...) </p> <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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amaterasu

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Amaterasu: &nbsp;No problem at all. :)&nbsp;Oh my, there are galactic rotation curves! &nbsp;But those don't have much to do with the structure of spiral arms. &nbsp;The galactic rotation curve is merely a plot/graph of a stars orbital velocity, verse it's distance from the galactic core.&nbsp;The only way they factor in, is they tell us the stellar orbiting speed, or rather the information we gather on that is factored into the graphs. &nbsp;&nbsp;Their speed tells us they don't orbit at the same rate as the "arms" do.&nbsp;</DIV></p><p>thanks&nbsp; :)&nbsp; and before i forget, i seem to have totally mixed up the revolution periods of the planets in the solar system with those of the sun and everything else's orbits around the galaxy.<br />of course, Mercury revolves about the sun faster than the Earth - as explained by Kepler's 2nd law.</p><p>also i happened to&nbsp;stumble onto this site (http://www.mb-soft.com/public/galaxy.html),&nbsp;which pretty much comprehensively explains all aspects of stability and dynamics of spiral arms.</p><p>in a nutshell, C. Johnson reckons that <em>both the principles of Keplerian revolution and gravitational arm self-cohesiveness</em> (i.e. mutual gravitational attraction)<em>&nbsp;must apply</em> to a spiral galaxy<em>.&nbsp; </em>very interesting explanation and makes perfect sense to me.</p> <div class="Discussion_UserSignature"> </div>
 
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