Abstract
Abstract The straight geometrical forms observed as bright rows of star complexes in spiral arms of giant galaxies are discussed. The nearby spirals M 51 and M 101 seen almost face-on reveal a polygonal two-arm pattern. Some other examples of spirals with straight arm segments are demonstrated. A gas-dynamical approach to the physical nature of the phenomenon is suggested. It is argued that the formation of the straight segments in spiral arms can lie due to the stability of flat shock fronts and the tendency of a slightly curved shock front to become flat. The flattening criterion enables us to expect that the length of a straight segment depends on its location in the global spiral structure and increases with the distance from the centre. This is generally confirmed by observational data. The same considerations show that the opening angle between two straight segments is typically ≃ 120°, which also agrees with the observed geometry of polygonal arm patterns.
Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
