On Spirals and Circles

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elberon

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Hello folks, <br /><br />This is my very first post. I'm a long time reader of space.com and the forums here. I've finally had a little time to join. <br /><br />For my first post I wanted to pose a question:<br /><br />There are lots of different kinds of galaxies, but the spiral seems to be the most common structure. I'm guessing that most spiral galaxies contain a supermassive blackhole in the center, and that the rest of the mass of the spiral galaxy "orbits" this BH and creates the spirals we see. <br /><br />Planetary systems, moon systems and ring systems have something basic in common with galaxies: mass orbits a central large/dense object.<br /><br />Why are the spirals present on galactic scales and not on planetary ones? Or the inverse, why don't galaxies behave like planetary systems and have distinct rings of matter?<br /><br />Thanks!<br /><br />e.
 
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Saiph

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Solar systems might exhibit spirals if there was enough material to make it evident.<br /><br />The spiral structure is believed to be due to something called a "density wave". Basically its an alignment of orbits such that most stars tend to be near eachother a lot, but pass through the pattern (the spiral arm) quickly.<br /><br />Sorta like a line at Mcdonalds. If you were to watch it at noon, you'd see the line exiist for perhaps 2 hours, slowly growing slowly changing. You could assume that the people in the line have to wait a long time, that they spend a lot of time in the line.<br /><br />Or, if you look a bit closer, you'd notice people come in and out of the line in a matter of minutes, there's just a constant influx and outflux of people. All their paths, while independent, all happen to pass through that region. Enough people pass through to make the pattern pretty stable, despite the transient nature of the individuals.<br /><br />Same goes for traffic jams and many other common phenomena.<br /><br />For stars, just imagine a bunch of eliptical orbits, going about the core. But skewed/rotated, so the edges on one side are really close. I tried finding a graphic, but was unsuccessful.<br /><br />Keywords: Density wave theory, spiral arms, galaxy<br /><br />links:<br /><br />http://curious.astro.cornell.edu/question.php?number=199<br />http://www.astronomynotes.com/ismnotes/s8.htm <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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elberon

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This makes sense. Thanks.<br /><br />Saturn's rings seem to have plenty of material to potential exhibit this phenomenon. Why do they appear so uniformly eliptical? I would even guess that so many small particals would have a "sticky" effect and clump. <br /><br />What about globular galaxies that do not display this phenomenon? Why don't they? Does it have anything to do with the speed of the orbital material? Does it have anything to do with the mass and spin of the center object?<br /><br />Also, is it correct to assume that all galaxies are centered on a supermassive black hole? <br /><br />Thanks for your help! <br /><br />e.
 
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Saiph

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It is correct to assume a Super BH core. Indeed, it's so typical that the few examples found without one are a big suprise.<br /><br />Saturns rings may not show spiral structure for several reasons:<br /><br />1) The particle orbits coincide a LOT so any spiral structure there is very, very small. I.e. thousands of really close spirals.<br /><br />2) The orbits are quick, and so the patterns will change much faster than for galaxies.<br /><br />3) There are other factors present which disrupt the normal orbits, basically randomizing everything and ruining any synchronicity.<br /><br />One reason stars have such similar orbits is they're born from the same cloud, or cloud complex, and tend to go in the same direction from the start.<br /><br />#3 leads into why irregular and globular galaxies/clusters don't have it. Any alignment and organization these did have was disrupted, and the orbits are basically random. Given time some of the "enlongated irregulars) might develope into spirals. There is little hope for globular clusters though, since they're so uniform, star formation is neglible, and stars don't shift as easily as gas clouds do. <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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zavvy

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<font color="yellow">Why are the spirals present on galactic scales and not on planetary ones? </font><br /><br /><br />Study Hurricanes and compare them to Spiral Galaxies..<br /><br />As Above... So Below.... <img src="/images/icons/cool.gif" />
 
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elberon

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Excelllent, thanks. It makes a lot of sense.<br /><br />Another question:<br /><br />In a given galaxy (one with a supermassive BH) is the matter that orbits the BH core being pulled into it? Is a galaxy an inherently shrinking structure, or is the orbital force enough to counter the BH's pull. <br /><br />In x-billion, billion years will all galaxies be snuffed out due to continual collapse, or is the birth of new stars within a galaxy enough to avoid this collapse?<br /><br />I'm assuming that the creation of a new galaxy is a relatively rare phenomenon, whereas the creation of stars is relatively common?<br /><br />e.
 
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Saiph

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Stuff does fall into the BH cores. This infalling matter is what powers Active Galactic Nuclei (AGN) and their bigger cousins, Quasars.<br /><br />however, the stuff that falls in is the unlucky stuff. Most matter is in stable orbits, and will continue to orbit, unless something messes it up. The stuff that falls in was flung that way by previous chance encounters when two things happen to cross paths. <br /><br />Sorta like how asteroids hit earth. It isn't sucked in, the paths just happen to cross. And new stuff is sent this way as their paths cross those of the other planets, and are (occasionally) flung this way. <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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tom_hobbes

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Stunningly clear explanations Saiph. Bravo! <div class="Discussion_UserSignature"> <p><font size="2" color="#339966"> I wish I could remember<br /> But my selective memory<br /> Won't let me</font><font size="2" color="#99cc00"> </font><font size="3" color="#339966"><font size="2">- </font></font><font size="1" color="#339966">Mark Oliver Everett</font></p><p> </p> </div>
 
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zavvy

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<font color="yellow">Stuff does fall into the BH cores. This infalling matter is what powers Active Galactic Nuclei (AGN) and their bigger cousins, Quasars.</font><br /><br /><br />But that's just theory, isn't it?<br /><br />It can't be proven....
 
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Saiph

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Okay, sure, "it's theory". But you're falling into the trap most people do. A scientific theory is a lot more solid than you think. The way you dismiss it, it should be "it's just a hypothesis". <br /><br />It's just a "theory" that wind is a large scale motion of air.<br /><br />For AGN and Quasars, we can prove the source isn't a lot of things. Actually, we can prove its not anything, except for BH's.<br /><br />Not only are BH's the only known source left, there are a lot expected traits (you know, stuff that fits with it being a BH, as opposed to a star, the reason we can't rule it out).<br /><br />Things like huge amounts of relativistic material ejected in jets. Extreme X-ray and gamma-ray radiation. A non-blackbody spectral curve (rules out a single solid object anyway) is another clue.<br /><br />If Quasars weren't powered by BH's it would be some of the biggest news in astronomy for the past 20 years IMO.<br /><br /><br /><br />And I put to you a small task. Look up the science communities opinion on proof, vs disproof. You'll find we discover by disproving alternatives rather than proving something. <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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zavvy

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<font color="yellow">You'll find we discover by disproving alternatives rather than proving something. </font><br /><br /><br />But it's still only theory when all's said and done..
 
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Saiph

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<blockquote><font class="small">In reply to:</font><hr /><p>Malin 1, was found at a few hundred millions light year's distance is a real Monster with a <b>spiral disc</b> width of about 500, 000 light years across. Compare that to our galaxy of about 100K LY. If viewed from the same distance as Andromeda galaxy, it'd cover about 1/6th of the sky from horizon to horizon. But it'd be so dim, no one could easily see it. <br /><p><hr /></p></p></blockquote> <br /><br />Emphasis mine: Nice example, though it's not quite the irregular you imply. I do agree that there are a lot of dim irregular/elliptical galaxies out there. One reason they're overlooked is due to the selection effect common in astronomy (we see more big and.or bright things first).<br /><br />Anyway, Something about this struck me as off. The size is good, the distance fine (I kept misreading them, so that's my problem). Then it struck me, it wouldn't cover 1/6 the sky.<br /><br />The andromeda galaxy is ~150,000 ly across, and subtends ~178 arcminutes (the long way), or about 3 degrees (that's quite a bit!) of sky. So Malin1 is 5x wider, for a subtended angle of 15 degrees.<br /><br />That's about 1/12th of the sky. Larger than my gut feeling, but still satisfies my hunch that figure was a bit rough.<br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>astrophysicists are sure beyond all reasonable doubt about why ....<p><hr /></p></p></blockquote> If you go by that standard, you'll have very little to talk about in this field.<br /><br />The density wave theory, basically the alignment of eliptical orbits, gives spiral structures stable in the required time frames, in computer models. It's quite successful in recreating the various spiral structures.<br /><br />We also have a pretty good idea how a lot of eliptical and spherical galaxies form: Mergers. Ones that didn't foster the left-overs to form into a disk (say a low velocity collission of two massive objects, comming at a "steep angle"). Some may have merely <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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Your other error is not realizing the weight behind a scientific theory. You seem to equate it to a guess.<br /><br />Theories are, however, more of a consequence of observation and strong models for predictions. The good ones anyway.<br /><br />They aren't complete or comprehensive (though some do a remarkable job at it).<br /><br />What runs your computer is "theory" or makes your medicine, or plastics. Gravity is really 'just a theory' though one so taken for granted it's been given the status of "law" (which is erroneous, since Einstein showed the "law" is in error, and made some corrections).<br /><br />Established Astronomical theories are just as rigourously tested as those in solid state physics, or chemistry.<br /><br />Unfortunately, due to the large public appeal, many of the developements get picked up and broadcast by the media before it's really ready. This gives the impression that astronomical theories are kinda wishy washy. <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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