<P> The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925 . He realized that the idea of stars arranged permanently in a spiral shape was untenable . Since the angular speed of rotation of the galactic disk varies with distance from the centre of the galaxy (via a standard solar system type of gravitational model), a radial arm (like a spoke) would quickly become curved as the galaxy rotates . The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter . This is called the winding problem . Measurements in the late 1960s showed that the orbital velocity of stars in spiral galaxies with respect to their distance from the galactic center is indeed higher than expected from Newtonian dynamics but still cannot explain the stability of the spiral structure . </P> <P> Since the 1970s, there have been two leading hypotheses or models for the spiral structures of galaxies: </P> <Ul> <Li> star formation caused by density waves in the galactic disk of the galaxy . </Li> <Li> the SSPSF model--star formation caused by shock waves in the interstellar medium . The shock waves are caused by the stellar winds and supernovae from recent previous star formation, leading to self - propagating and self - sustaining star formation . Spiral structure then arises from differential rotation of the galaxy's disk . </Li> </Ul> <Li> star formation caused by density waves in the galactic disk of the galaxy . </Li>

What features of spiral galaxies are evident in this view of the milky way