Density wave theory

The density wave theory was set up in 1925 by the Swedish astronomer Bertil Lindblad and further developed by the Chinese astronomer Frank Shu and Chia- Chiao Lin 1969. It describes the formation and maintenance of spiral structure in spiral galaxies.

Theory

Due to the differential rotation of the spiral structure would have actually dissolved after a few rotations of the galaxy. In fact, the spiral structure of a galaxy remains apparently received over many billions of years. Therefore, the theory assumes that the spiral arms are a wave phenomenon and constantly regenerate. The density waves traverse the matter of the galaxy, the spiral arms represent the areas of maximum density. The interstellar matter is compressed so much by these waves, that right there young stars are forming. In the Milky Way system, the density wave rotates, for example, at a speed of about 13.5 kilometers per second per kiloparsec, which corresponds to half the speed of rotation of the star. The observed spiral arms arise after the gas flows from the concave side to the pressure wave and is compressed. Areas with already high gas density, such as molecular clouds are thereby collapsing unstable and begin that new star clusters arise as H II regions and OB associations ( stellar associations ) are visible behind the density wave.

Open Questions

The density wave theory can not answer all questions related to the spiral structure. For example, it is not clear how the waves are fanned and steamed or the role of the interstellar magnetic fields. Another question raises the observation that spiral arms are generally not formed uniformly. On closer inspection they reveal gaps and bumps or connections between different spiral arms, for example.