: These are not permanent "arms" but density waves where stars and gas momentarily slow down, triggering star formation.
: Unlike gas particles, stars rarely "hit" each other. Their motion is dictated by the smoothed-out gravitational potential of the entire galaxy rather than individual encounters. Galactic Dynamics
: Visible matter cannot explain observed orbital speeds. A massive, invisible dark matter halo is required to keep galaxies from flying apart. Key Dynamical Components 1. The Galactic Disk : These are not permanent "arms" but density