Why do galaxies have spiral arms?
About a third of known galaxies are flat spirals with bulging centers. Astronomers believe that galaxies have spiral arms because galaxies rotate – or spin around a central axis – and because of something called “density waves.” Galactic density waves are like water waves. Water itself doesn’t move across a pond – instead, wave energy moves and affects the water as it passes. A spiral galaxy’s rotation, or spin, bends the waves into spirals. Stars pass through the wave as they orbit the galaxy center. The wave causes the stars to slow slightly and temporarily clump together. Astronomers have long wondered why the spiral arms of a galaxy don’t wind up and vanish after a few rotations. Many galaxies have satellites – smaller neighboring galaxies. One theory is that a satellite can keep a larger galaxy’s density waves moving indefinitely. Other processes may help shape galaxy spiral arms. For example, galaxy rotation might smear exploding and forming stars into a bunchy spiral arm. Many a
