How do spiral arms form in galaxies?

Answer: Density waves compress gas into bright arms

Density waves compress gas into bright arms ✓ — Correct! Spiral arms are density waves moving through the galaxy. The inner region rotates faster than the outer, creating gravitational compression zones. As gas gets squeezed in these waves, new bright stars form, lighting up the arms. Stars actually pass through the arms over time; the arms themselves are the wave pattern, not fixed structures of the same stars.

Stars stream outward from galactic center — Wrong. Stars orbit the galactic center, but spiral arms aren't streams of outflowing stars. They're density waves—compression zones where gas bunches up and new stars form, creating the bright spiral pattern we see.

Magnetic fields shape the spiral pattern — Wrong. Galactic magnetic fields are too weak to shape spiral structure. Spiral arms form from density waves caused by differential rotation—inner parts spinning faster than outer, creating moving compression patterns.