Galaxy Classification

Astronomers classify galaxies by morphological class.  Spiral galaxies are flat disks of stars, gas and dust while elliptical galaxies are spherical or ellipsoidal masses of masses of stars with very little interstellar gas and dust.  Irregular galaxies are chaotic, usually resulting from a "collision" with another galaxy.  Dwarf galaxies are small, usually unstructured, masses of stars, gas and dust, usually existing as satellites of larger galaxies.

Morphological classes are specified by an upper case letter, S for spiral, E for elliptical or I for irregular followed followed by one or more letters and/or numbers which further refine the taxonomy. 

Spiral Galaxy M81 in Ursa Major Morphology Class: Sb	Elliptical Galaxy M87 in Virgo Morphology Class: E1	Irregular Galaxy NGC1427a in Fornax Morphology Class: I

Images copied from http://www.wikipedia.com

An interesting subtype of spiral galaxies are the barred spirals, morphological class SB (see the example below).  The reason for this structure is not well understood and it is now suspected that the Milky Way has this structure.

Galaxies are also classified by the aspect they present to an observer.  All galaxies rotate, hence, they have poles and an equator.  Face-on galaxies present a polar aspect to the observer while edge-on types are viewed in line with their equators.  Face-on spiral galaxies have a "pinwheel" appearance and edge-on types appear as streaks with bright centers.  Dark lanes of cold gas and dust are usually seen bisecting edge-on galaxies.  Most galaxies present aspects midway between these extremes.

Barred Spiral Galaxy NGC1300 in Eridanus Morphology Class: SBb	Face-on Spiral Galaxy M101 in Ursa Major Morphology Class: Sc	Edge-on Spiral Galaxy NGC891 in Andromeda Morphology Class: Sb

Images copied from http://www.wikipedia.com