Galaxies have been categorized according to their apparent shape usually referred to as their visual morphology). A common form is the elliptical galaxy, which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped assemblages with curving, dusty arms. Galaxies with irregular or unusual shapes are known as peculiar galaxies, and typically result from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in galaxies merging, may induce episodes of significantly increased star formation, producing what is called a starburst galaxy. Small galaxies that lack a coherent structure could also be referred to as irregular galaxies

                                  Galaxies are the lighthouses that plumb the Universe - constituents of
the largest-scale texture we know. They span a vast range of properties,
from dwarf galaxies with a few million stars barely outshining the 
brightest individual star clusters in our own galaxy, to vast assemblages
of a trillion stars in the centers of great clusters. Our own galaxy,
a reasonably bright spiral system, can be traced at least fifty thousand
light-years from its nucleus, and we know of many galaxies much larger

• It is recognized that most galaxies other than dwarfs have central black holes. The idea is that black hole seeds either attracted matter into forming galaxies or formed within young galaxies in the early universe. This action produced quasars and explains why most quasars are extremely distant. As the black hole acquired more and more matter from galaxies' centers, the fuel became exhausted, so they slowly quieted down. Most galaxies in the recent universe have slumbering giants in their centers. They can be re-activated when interact with other galaxies, starbursts, or gas clouds falling into the central region.