Astronomers at the IAC have discovered complex patterns of resonances in the discs of spiral galaxies not previously described by theories. Using the GHaFaS 2dimensional spectrometer they have measured the velocities of the density waves in the discs of over a hundred galaxies Within the discs of spiral galaxies there are waves which propagate concentrically in the form of spirals. This is somewhat similar to the waves on the surface of a lake, or the standing waves on the strings of a violin, or on the surface of a drum, to use a musical metaphor. These are the so-called “density waves”