Final answer:
The spiral arms of galaxies like the Milky Way are highlighted by young, high-mass stars and star-forming regions, making them brighter and often bluer compared to other parts of the galaxy.
Step-by-step explanation:
The arms of a spiral galaxy such as ours stand out because they contain young, high-mass stars, star-forming regions, and emission nebulae. These elements make the spiral arms appear characteristically brighter and often bluer than the rest of the galaxy. In face-on views of spiral galaxies, such as M100, this contrast is pronounced, showcasing the pinwheel-like structure. Conversely, edge-on views like NGC 4565 illustrate how dust within the galactic plane can absorb starlight, making the arms less visible. The Milky Way, a typical example of a large spiral galaxy, also features a central bulge, a halo, a disk, and spiral arms that emanate from a modest bar structure, indicative of ongoing star formation and the dynamic nature of galactic evolution.