That the gravity of supermassive black holes has a large effect on the dynamics of stars in the galaxy. It doesn't. You seem to have the common misconception that a black hole sucks in matter. This is false. If you replaced our Sun with a black hole of the same mass, the Earth's orbit would not be changed at all. The only effect of the black hole's gravity comes from its mass, and that mass is minuscule compared to the stars in the galaxy once you move beyond the immediate galactic core. The spiral arms are found much further out. If the spiral arms reach very close to the center of the galaxy, that galaxy likely has a very small black hole (this is the essence of the M-sigma relation that has been discussed in the thread). Only in a very small region around the black hole. The spiral arms do not extend that close to the center. In fact, there's an inverse relationship between how close the spiral arms get to the center and the mass of the black hole: galaxies with larger black holes have larger "bulges", and the spiral arms only occur outside of the bulge. A galaxy's bulge is the spheroidal-shaped group of stars that make up the galaxy's center. Yes, I'm sure. Accretion requires a loss of energy so that the gas and dust can collapse, while the process of removing the dust adds energy. Though the process of removing most of the dust from the galaxy may result in some star formation (due to the collisions of dust clouds), in the main the heating of the dust clouds will tend to reduce the number of stars that form. The more massive the black hole, the less "spiral-shaped" the galaxy becomes. The really massive black holes tend to be at the centers of elliptical galaxies.