Final answer:
Spiral galaxy formation begins with irregular clouds of interstellar material that, due to varying rotation rates, stretch into spiral arms. Over billions of years, these structures are maintained and become more defined, with a central bulge and quieter star formation in disk-shaped regions. The complete narrative of galaxy formation is ongoing, with spiral galaxies potentially evolving through mergers and slowed star formation.
Step-by-step explanation:
The assumed conditions for the formation of a spiral galaxy involve an initially irregular cloud of interstellar material. As the galaxy rotates, this material gets stretched out due to different rotation rates across the galaxy, with outer regions lagging behind the inner regions. However, this alone would not account for the long-lived nature of spiral arms because over time they would wind up and disappear. It's understood that other processes must contribute to maintaining the structure of spiral arms.
As galaxies evolved, those destined to become spiral galaxies experienced a reduction of massive clumps, developed a central bulge, and saw decreased turbulence. Rotation started to dominate, leading to the formation of stars in a much quieter disk. Over billions of years, this process resulted in the distinct spiral arms characteristic of mature spiral galaxies such as the Milky Way. The full story of galaxy formation and evolution is still being uncovered, as galaxies can change types over time, influenced by events such as mergers, which may temporarily disrupt the spiral structure, and the gradual slowing of star formation rates.