A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter.
Typical galaxies range from dwarfs with as few as ten million (107) stars up to giants with one trillion (1012) stars, all orbiting the galaxy’s center of mass. Galaxies can also contain many multiple star systems, star clusters, and various interstellar clouds. Historically, galaxies have been categorized according to their apparent shape, usually referred to as their visual morphology.
Scientists get a look at the birth of the Milky Way PhysOrg – June 22, 2010
Antennae Galaxies are a pair of interacting galaxies in the constellation Corvus.
Dark Galaxies contain very few, or no stars. Held together by dark matter, they may also contain gas and dust. No dark galaxy with a black hole as a center has yet been discovered.
Dwarf Galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Way’s 200-400 billion stars. 
Elliptical Galaxies have an ellipse-shaped light profile.
Interacting Galaxy or Colliding Galaxies are the result of one galaxy’s gravity disturbing another galaxy.
Irregular Galaxies do not have a regular shape.
Lenticular Galaxies are intermediate between an elliptical galaxy and a spiral galaxy in galaxy morphological classification schemes.
Luminous Infrared Galaxies (LIRG) emit more than 1011 solar luminosities in the far-infrared part of the electromagnetic spectrum.
Peculiar Galaxies have irregular or unusual shapes typically resulting from disruption by the gravitational pull of neighboring galaxies.
Ring Galaxies have a ring-like appearance. The ring consists of massive, relatively young blue stars, which are extremely bright.
Spiral Galaxies are disk-shaped assemblages with dusty, curving arms.
The Whirlpool Galaxy is an interacting grand-designspiral galaxy located at a distance of approximately 23 million light-years in the constellation Canes Venatici.
Dark Matter appears to account for about 90% of the mass of most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are proposed to be the primary cause of active galactic nuclei found at the core of some galaxies. The Milky Way Galaxy appears to harbor at least one such object within its nucleus.
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- Star-Crossed: Milky Way’s Spiral Shape May Result from a Smaller Galaxy’s Impact (scientificamerican.com)
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