Quote:
Originally Posted by renormalised
It's not so much a warping phenomenon of spacetime as it's a wavelike phenomenon that creates an over density in the material within a disk of material. You can get standing waves (which is what occurs in galaxies) or moving waves which occur in substances like fluids (e.g. water). The material within the disks can be moving slower or faster than the waves, but where it encounters the wave it enters those conditions of over density and gets compressed. That's where you get your HII regions and OB associations forming. These generally hang around in the areas of over density because they don't last very long and so you get your spiral arm pattern forming. The rest of the materials within the galaxy rotate at whatever speed they will, depending on their distance from the centre and the other factors affecting their movement, but the wave essentially stays put...the spiral pattern doesn't rotate, or moves very little.
Just notice something cool....play that little video of the spiral arms in the galaxy on the right hand side of the page, then quickly look up at M81....you can see the optical illusion of its arms actually rotating   |
Hmm..
Very interesting. If I start my thoughts from your starting premise: "The material within the disks can be moving slower or faster than the <harmonic standing> waves", then I think I get it.
Moving outwards along an arm, the rotational velocity must get slower, otherwise, there'd be a disc, rather than an arm, right ? That there is an arm at all, must be courtesy of some really 'elegant' harmonics. The mathematics describing it all must be a mathematician's playground. There must be fairly deterministic (linear ?) velocity relationships between two adjacent objects/particles which describe their alignment, which begins the pattern, which eventually builds the 'arm' structure.
Gravity must play a role however, 'cause we're talking about some big objects like stars, as well as smaller ones like dust and gas (?). And the inter-stellar velocities synching up to form the arm pattern implies some kind of resonance in the gravitational fields (?).
Perhaps I'll go one further, & this may be a quantum leap, but I'm also reminded of that bizarre hexagonal structure on the (North ?) pole of Saturn. I think the current thinking on that one is that it may be a standing wave pattern described by fluid dynamics around a polar region. Oh .. to understand fluid mechanics as well, eh ?
Fascinating stuff.
Cheers (& thanks for your reply).