by Tara_Li » Sat Jun 01, 2013 5:09 am
rstevenson wrote:Tara_Li wrote:rstevenson wrote:Eddies can appear in fluid dynamics but not, I think, when the dynamism of a system is driven by gravity. ...
Except that aren't planets supposed to collapse from eddies in the dust clouds around the stars? Sure, there's some magnetic effects in play, but the primary force in the protoplanetary disk is gravity. "Fluid" is kind of a fluid term, really...
Yes, but that's because the gas and dust in a newly forming stellar system can form local aglomerations which gravitationally coalesce into planets. But to get an eddy on a galactic scale you'd need to figure out some way to get entire stellar systems - a great many of them in a group, if they're to appear to us as an eddy - to loop around as they orbit the galactic center of gravity. In other words, they'd need a local center of gravity around which to orbit, just like planets orbit around their star. Conceivably a black hole could collect a few stellar systems around itself, and drag them around the galaxy as it orbits. But I don't think that is gravitationally stable in the long run. It would make for an interesting modelling exercise though.
![Wink ;-)](./images/smilies/icon_wink.gif)
Why wouldn't a large starbirth nebula, such as the Orion Complex (but perhaps a bit larger) work for that purpose? At what density regime does the Interstellar Medium quit being gravity dominated, and become a fluid? We see turbulance and bowshock in very low density nebulae, after all.
[quote="rstevenson"][quote="Tara_Li"][quote="rstevenson"]Eddies can appear in fluid dynamics but not, I think, when the dynamism of a system is driven by gravity. ...[/quote]
Except that aren't planets supposed to collapse from eddies in the dust clouds around the stars? Sure, there's some magnetic effects in play, but the primary force in the protoplanetary disk is gravity. "Fluid" is kind of a fluid term, really...[/quote]
Yes, but that's because the gas and dust in a newly forming stellar system can form local aglomerations which gravitationally coalesce into planets. But to get an eddy on a galactic scale you'd need to figure out some way to get entire stellar systems - a great many of them in a group, if they're to appear to us as an eddy - to loop around as they orbit the galactic center of gravity. In other words, they'd need a local center of gravity around which to orbit, just like planets orbit around their star. Conceivably a black hole could collect a few stellar systems around itself, and drag them around the galaxy as it orbits. But I don't think that is gravitationally stable in the long run. It would make for an interesting modelling exercise though. ;-)[/quote]
Why wouldn't a large starbirth nebula, such as the Orion Complex (but perhaps a bit larger) work for that purpose? At what density regime does the Interstellar Medium quit being gravity dominated, and become a fluid? We see turbulance and bowshock in very low density nebulae, after all.