Density wave theory
The density wave theory was established in 1925 by the Swedish astronomer Bertil Lindblad and further developed in 1969 by the Chinese astronomers Frank Shu and Chia-Chiao Lin . It describes the formation and maintenance of the spiral structure in spiral galaxies .
Due to the differential rotation , the spiral structure should have dissolved after a few rotations of the galaxy. In fact, however, the spiral structure of a galaxy apparently remains intact for billions of years. The theory therefore assumes that the spiral arms are a wave phenomenon and are constantly being formed anew. The density waves pass through the matter of the galaxy, with the spiral arms representing the areas of maximum density. The interstellar matter is compressed so much by these waves that young stars are formed there. In the Milky Way system , for example, the density wave rotates at a speed of about 13.5 kilometers per second per kiloparsec, which is half the speed of the rotation of the stars. The spiral arms that can be observed arise after the gas flows into the density wave from the concave side and is compressed. Areas with already high gas density, such as B. Molecular clouds , thereby become unstable and begin to collapse, so that new star clusters are formed, which are visible as H-II regions and OB associations ( star associations ) behind the density wave.
The density wave theory cannot answer all questions related to the spiral structure. For example, it is not clear how the waves are fanned and damped or what role the interstellar magnetic fields play. Another question is raised by the observation that spiral arms are usually not uniformly shaped. On closer inspection they let z. B. Detect gaps and bumps or connections between different spiral arms.