Spiral Structure of the Galaxy (2) ...

The primary reason that we perceive spiral galaxies as spirals is that the spiral arms contain many hot blue and blue-white stars. These bright young stars make the spiral arms more prominent in the visible spectrum than those regions of the galaxy that are dominated by older stars whose spectrum is concentrated at longer wavelengths. The adjacent animation illustrates this point rather clearly. It is a sequence of images for the galaxy M81, beginning at ultraviolet wavelengths and progressing through longer wavelengths to red light.

Spiral Tracers

Short lifetime objects like hot young stars are often called spiral tracers. Because they are formed in the spiral arms and cannot move far from their place of birth in their short lifetimes, they trace the spiral pattern. Generally, objects can serve as spiral tracers if their lifetime is short enough that they cannot have moved more than the width of a spiral arm from their place of birth. Additional examples of spiral tracers include H II regions and other signals of starbirth like T Tauri variables.

As the galaxy morphs from its UV image to its red light image, the parts of the galaxy that are visible change considerably. The initial short-wavelength images are dominated by young (less than 1 million years old) hot stars in the spiral arms and the spiral structure is very prominent. The final long-wavelength images are dominated by old (around 5 billion years) red stars in the core and regions not in the spiral arms. The resulting image exhibits very little spiral structure and resembles more the look of an elliptical galaxy.