Why do the luminous bands of spiral galaxies appear spectacularly bright?

The presence of massive, hot stars emitting the majority of their light in the blue and ultraviolet spectrum.

The spectacular brightness observed in the luminous spiral arms is directly attributed to the color and output of the stars being born there. Regions within the galactic disk that enter a spiral arm trigger intense star formation, resulting in the rapid birth of very massive stars, often many times the mass of our Sun. These colossal stars operate at extremely high surface temperatures. A fundamental principle relating to stellar radiation dictates that hotter objects emit light predominantly at shorter wavelengths; consequently, these giants radiate the vast majority of their luminous output in the blue and ultraviolet portions of the electromagnetic spectrum. This intense, short-wavelength light overwhelms the dimmer light from older stellar populations, making the arms visually dominate the galactic disk when viewed across cosmic distances.