Lecture 9 - Spiral Structure of Galaxies
Traffic jam in the galaxy! The beautiful spiral patterns seen in a large number of galaxies are, in fact, regions of higher density that move around the galaxy slower than the individual stars, much like a 'moving' traffic jam created by a slow-moving truck chugging along a fast lane.
Nearly 60% of the galaxies in the present universe have well defined spiral arms. Recently formed clusters of massive stars, as well as giant molecular clouds, are closely associated with these spiral arms. In the 1960s, an important idea was advanced concerning the nature of these spiral patterns, namely, that they represent the maxima of a ‘density wave’. This idea is explained in this lecture with simple examples.