CompuServe Thread

David, Ummmm…what does a spiral nebula look like? <g> If it's something that looks similar to a whirlingpool as seen from above, you can try placing the nebula.itx on a disk with several circle-sections to it, tack it to this default shape, then rotate the inner circles progressibely to "twist" the shape of the nebula.itx pattern. To animate this just animate the nebula.itx axis that is perpendicular to the surface of the disk. You can also use a tube for this to give it depth. Of course, this is all useless if thats not what a "spiral" nebula looks like! 🙂 -GreG

David, I haven't looked at an astronomy book in a few years, but I seem to remember that a spiral's arm system does not wind up because of gravity waves. I believe that in the accepted theory, a system of gravity waves, like the spokes of a wagon wheel, travels at a speed different than that of the arms themselves. As a gravity wave catches up with a given arm (if that's what it does), it perturbs the motion of successive stars along the arm, converting each star's spiral trajectory (to an approximation) into a merely circumferential one. In this way the galaxy's spiral shape is preserved through continuing revolutions. I would try something like the following. Make some arms with sort of airfoil cross sections, curved but irregular along their length. Punch a lot of little holes of just the right size <G> through the master shape. Copy the shape and scale it up slightly. Slip the smaller inside the larger. Copy the composite several times and arrange the arms in a spiral. The outer layer of each arm is a fog object, fog length pretty large, to let the inner show through. The inner layer of each arm is black. The inner arm gets a crumpled texture with the following exaggerated values. Noise 1M & Noise 2M : 15.0 Noise 2V & Noise 2V : 5.0 Bump adjust : 10.0 Bump colors : 255 all Dispersion : 0.4 Note the low dispersion. But I tried it on a default sphere and it looks cool. ( I don't think the opacity of the main object would be noticeable against your star field.) This makes it look like stars. But here is the main idea. Crumpled.itx is _not_ tacked. Instead it is allowed to slide around. As the 'stars' slide into the holes in the arm objects, they blink out. Others blink on somewhere else, giving the impression of scintillation. After all, a revolution of a spiral galaxy takes hundreds of millions of years to happen, and in that amount of time the ultra-blues, and even some super M's, would be popping like fireflies. regards, GB