Why do spiral galaxies spiral?

Galaxies are massive systems, which contain stars, gas and dust. These systems come in a variety of sizes, ranging from several million to several trillion stars. They also come in a variety of shapes. These shapes can be classified using the Hubble Classification system. The four classes of galaxies are the spiral (S), the barred spirals (SB), ellipticals (E) and irregulars (I). In this report I will be discussing spiral and barred spiral Galaxies and the reason why they spiral.
Spiral Galaxies and barred spiral Galaxies have similar appearances as they all contain arced arms of stars, a central bulge and a spherical halo. The arms have stars continuously being born and dying. These areas are full of small, young, metal rich population I stars, HII regions or clouds of cold ionized hydrogen, as well as heavier elements, which are sent into space when stars die. The central bulge is usually about a few thousand light years across. This area is made up mainly of Population II s...