What volume fraction do spiral arms occupy while generating the majority of new starlight?
10% to 20% of the space by volume
The concentration of star formation in spiral arms represents an incredibly efficient use of galactic real estate. Although the visual spiral arms, defined by the density waves, might only encompass a relatively small portion of the total physical space within the galactic disk—estimated to be between 10% and 20% of the volume—they are responsible for producing the vast majority of the galaxy's observable new starlight. This spatial disparity explains why spiral galaxies appear so dramatically structured. If star formation were distributed uniformly across the entire disk area, the arms would not appear nearly as bright or intensely blue as they do, illustrating the powerful, localized triggering effect of the density wave mechanism.
