Spiral galaxies have spiral arms with a distinct bluish color. What causes this bluish color?
The bluish color of the spiral arms in galaxies is primarily caused by the presence of young, hot and massive stars. These stars emit most of their light in the blue and ultraviolet wavelengths. As spiral galaxies rotate, the gas and dust within them are compressed and triggered to form new stars in regions called star-forming regions or nebulae.
The young stars formed in these regions are typically very hot and emit a significant amount of blue light. Since blue light has a shorter wavelength than other colors in the visible spectrum, it scatters more easily in the interstellar medium, which is composed of gas and dust particles. This scattering phenomenon, known as Rayleigh scattering, is the reason why we observe a bluish appearance of the spiral arms.
Furthermore, the dust particles present in the spiral arms can absorb and scatter light depending on its wavelength. Blue light is more easily scattered and scattered light appears bluish to our eyes. The dust grains also scatter shorter wavelength light (blue and green) more strongly than longer wavelength (red) light, further contributing to the bluish appearance.
It is important to note that the bluish color is more prominent in regions with ongoing star formation, where younger and hotter stars are concentrated. In contrast, the central bulge of spiral galaxies may appear more reddish due to the presence of older, cooler stars.