Spiral galaxy arms are not solid structures but function like cosmic traffic jams.

The leading explanation for spiral structures is the Density Wave Theory, proposed by C.C. Lin and Frank Shu in 1964. This theory suggests that the arms are not permanent physical structures made of the same stars over time. Instead, they are regions of higher density that move through the galactic disk more slowly than the stars and gas themselves.As stars and interstellar gas clouds orbit the galactic center, they eventually encounter these high-density regions. Much like cars approaching a highway bottleneck, the material slows down and bunches up. This gravitational compression forces gas clouds to collapse, which triggers intense star formation within the spiral arms.The reason these arms appear so bright is due to the presence of young, massive O and B-type stars. These stars are incredibly luminous but have very short lifespans, often burning out before they can exit the density wave. Consequently, the arms are highlighted by brilliant blue light while the older, dimmer stars pass through to the other side.Observations from the Hubble Space Telescope and the Gaia mission have provided data supporting this model. By tracking the velocity of stars in the Milky Way, astronomers can see the distinct speed changes as stars enter and exit these spiral patterns. This mechanism ensures that while individual stars come and go, the spiral pattern remains a stable feature of the galaxy for billions of years.