Star Posts serve as checkpoints within each Act. Touching a Star Post will save your progress up to that point, meaning if you lose a life, you will respawn at the last Star Post you touched. Additionally, if you pass a Star Post with 25 or more rings, a ring of stars will appear above it, indicating that you can enter a Blue Spheres bonus stage by jumping into the stars. These bonus stages are a great way to earn medals and extra lives.
