Retrieving "Density Waves" from the archives

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1. Galaxy

   Linked via "density waves"

   Spiral Galaxies ($S$)
   Spiral galaxies are defined by a central bulge of older stars and a flattened, rotating disk containing young stars, gas, and dust, organized into spiral arms. These arms are density waves that propagate through the disk, compressing the interstellar medium and triggering star formation.
   Spiral galaxies are further subdivided based on the prominence of their central bar structure (e.g., $SB$ for barred spirals). The rotational velocity of the disk stars remains nearly constant far from the core, a phenomenon r…