science & techESA/Hubble
Spiral arms are traffic jams of stars, not rigid structures
Spiral arms are not solid shapes that rotate with a galaxy; they are regions of higher density in the galactic disc. The puzzle was that arms of rotating material would wind up tighter and vanish within a short cosmic time. In the 1960s astronomers solved this with density wave theory: the arms are pattern waves that move through the disc at their own speed. Gas and dust slow down as they enter the compressed region, bunch together, form stars, and then drift out again, much like cars in a traffic jam that bunch up and then spread out.
This explanation resolved a basic contradiction in galaxy structure and remains the standard picture for why spiral arms persist. It also explains why spiral arms are bright with young stars.
background
Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.
Spiral galaxies are named by their spiral structures that extend from the center into the galactic disk. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them.
From Wikipedia: Spiral galaxy (CC BY-SA 4.0) ↗
