interior. This is the pressure-driven snowplow phase. The shell can be clearly seen in optical emission from recombining ionized hydrogen and ionized oxygen atoms. Cooling of the interior. The dense shell continues to expand from its own momentum. This stage is best seen in the radio emission from neutral hydrogen atoms. Merging with the surrounding interstellar medium. When the supernova remnant slows to the speed of the random velocities in the surrounding medium, after roughly 30,000 years, it will merge into the general turbulent flow, contributing its remaining kinetic energy to the turbulence. Types of supernova remnant There are three types of supernova remnant: Shell-like, such as Cassiopeia A Composite, in which a shell contains a central pulsar wind nebula, such as G11.2-0.3 or G21.5-0.9. Mixed-morphology (also called "thermal composite") remnants, in which central thermal X-ray emission is seen, enclosed by a radio shell. The thermal X-rays are primarily from swept-up interstellar material, rather than supernova ejecta. Examples of this class include the SNRs W28 and W44. (Confusingly, W44 additionally contains a pulsar and pulsar wind nebula; so it is simultaneously both a "classic" composite and a thermal composite.) Remnants which could only be created by significantly higher ejection energies than a standard supernova are called hypernova remnants, after the high-energy hypernova explosion that is assumed to have created them. Origin of cosmic rays Supernova remnants are considered the major source of galactic cosmic rays. The connection between cosmic rays and supernovas was first suggested by Walter Baade and Fritz Zwicky in 1934. Vitaly Ginzburg and Sergei Syrovatskii in 1964 remarked that if the efficiency of cosmic ray acceleration in supernova remnants is about 10 percent, the cosmic ray losses of the Milky Way are compensated. This hypothesis is supported by a specific mechanism called "shock wave acceleration" based on Enrico Fermi's ideas, which is still under development. In 1949, Fermi proposed a model for the acceleration of cosmic rays through particle collisions with magnetic clouds in the interstellar medium. This process, known as the "Second Order Fermi Mechanism", increases particle energy during head-on collisions, resulting in a steady gain in energy. A later model to produce Fermi Acceleration was generated by a powerful shock front moving through space. Particles that repeatedly cross the front of the shock can gain significant increases in energy. This became known as the "First Order Fermi Mechanism". Supernova remnants can provide the energetic shock