<P> Edwin Hubble created the first galaxy classification scheme known as the Hubble tuning - fork diagram . It partitioned galaxies into ellipticals, normal spirals, barred spirals (such as the Milky Way), and irregulars . These galaxy types exhibit the following properties which can be explained by current galaxy evolution theories: </P> <Ul> <Li> Many of the properties of galaxies (including the galaxy color--magnitude diagram) indicate that there are fundamentally two types of galaxies . These groups divide into blue star - forming galaxies that are more like spiral types, and red non-star forming galaxies that are more like elliptical galaxies . </Li> <Li> Spiral galaxies are quite thin, dense, and rotate relatively fast, while the stars in elliptical galaxies have randomly - oriented orbits . </Li> <Li> The majority of mass in galaxies is made up of dark matter, a substance which is not directly observable, and might not interact through any means except gravity . </Li> <Li> The majority of giant galaxies contain a supermassive black hole in their centers, ranging in mass from millions to billions of times the mass of our Sun . The black hole mass is tied to the host galaxy bulge or spheroid mass . </Li> <Li> Metallicity has a positive correlation with the absolute magnitude (luminosity) of a galaxy . </Li> </Ul> <Li> Many of the properties of galaxies (including the galaxy color--magnitude diagram) indicate that there are fundamentally two types of galaxies . These groups divide into blue star - forming galaxies that are more like spiral types, and red non-star forming galaxies that are more like elliptical galaxies . </Li> <Li> Spiral galaxies are quite thin, dense, and rotate relatively fast, while the stars in elliptical galaxies have randomly - oriented orbits . </Li>

Why does star formation occur in the disk of the galaxy