<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>That is such a load of crap. Chance encounters on their way to other distant parts of the universe?! There is an organized system holding these stars there, and creating the wonderful STRUCTURES. It's a combination of electromagnetic fields, some aligned electrons in motion and their subsequent magnetic fields, and basic gravity. No supermassive invisible things. 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. Just that the spiral structures are illusory because there's nothing holding them together. They're more like an ocean wave than any real "structure". Sure, there's mechanisms behind it all, that form it all, that perpetuates it all. 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. 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. The fact that there is no force keeping the stars in the arms is evident from observations of the spiral arm, and the stars. 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". 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. Which, oddly, I've never ever used....not even to move a forum. 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). Along with some 7 years posting here, and debating just about any astronomy subject you can think of with many other well informed posters. 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. 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. 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. So, as stars die out, which would thin the spiral arm, new stars are born. Evidence for this is the large number of bright blue supergiant stars which dominate the spiral arms. 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> Also, the increased densities in the spiral arm help keep the orbits...um, cohesive might be a good word here. 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: 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>