Why do spiral galaxies have blue arms and red central regions?

The distinct coloration of spiral galaxies is primarily due to the different populations of stars found in their various regions, a classification system first established by the astronomer Walter Baade.1

• Blue Spiral Arms: The arms of a spiral galaxy are characterized by Population I stars. These regions contain young, hot, massive stars and stellar associations that are still in the process of forming from clouds of gas and dust. Because these young stars are blue, they give the spiral arms their characteristic blue appearance.123
• Red Central Regions: In contrast, the central bulge of a spiral galaxy is dominated by Population II stars. These are very old stars, including red giants, which have evolved over billions of years. Because these older stellar populations lack the hot, blue main-sequence stars found in the arms, the central regions appear red, similar to the appearance of elliptical galaxies.13