Many astronomers believe that these facts can be explained by invoking a black hole of approximately one million solar masses. According to the Hubble Classification Scheme , galaxies come in four basic types. Spirals have a central bulge with a flattened disk, generally showing spiral arms. Barred Spirals are the same as spirals except for the presence of a bar in their central regions. They usually have little gas and dust compared to spirals, few young stars, and no spiral structure.
Irregulars are everything else.
The Milky Way Galaxy is probably a barred spiral. With the exception of a few nearby galaxies, the spectra of all galaxies is redshifted, indicating that the universe as a whole is expanding. There is NO center to the expansion, and every galaxy sees other galaxies expanding away from itself according to Hubble's Law. Galaxies tend to be found in groups and clusters . The Milky Way and Andromeda galaxies are the two largest galaxies in the Local Group , consisting of some tens of galaxies.