Southern Pinwheel Galaxy (M83)

M83 is a magnificient spiral galaxy 15 million lightyears away with well-defined arms, forming stars at a rate five times faster than the Milky Way.

The blue regions are young stellar population, formed a few (to tens of) millions years ago. The red knots (which would be more noticeable had I used an Hα filter) are diffuse gas sourrounding the young stars, which is ionized and subsequently recombines.

Spiral arms are created by density waves traveling through the disk, created by feedback from the stars. Although it looks like all the stars are in the spiral arms, with none in between, the density of stars in the arms is actually just 2-3 times higher. The reason they are so conspicuous is that the density compressions spark star formation, so the arms contain young stars, which are much brighter than old stars.

Mixed with the bright arms are dark dust clouds. The bar-like nucleus is composed of an older yellowish stellar population.