Why do spiral galaxies have a bulge from which several arms come out?

Spiral galaxies have a central bulge due to the accumulation of stars, gas, and dust, which results from gravitational interactions and the merging of smaller galaxies over time. The spiral arms are created by density waves that travel through the galaxy, causing regions of higher density where stars are formed. These arms are often sites of active star formation, making them appear bright and prominent against the darker regions of the galaxy. The combination of the bulge and spiral structure gives these galaxies their characteristic appearance.