Galaxies come in a variety of categories.
In the early days of astronomy many galaxies were determined to be nebula
because the smallish telescopes could not distinguish the massive number of
individual stars located in the galaxy. Most common galaxies are spiral,
elliptical, lenticular and irregular. All galaxies consist of large star systems
and interstellar matter. These galaxies typically contain several million to
trillions of stars as well as star clusters and nebulae. The various galaxy
categories are listed below.
||This is the type of our own home galaxy, The
Milky Way. They usually consist of two major components, a large flat
disk containing lots of interstellar matter and young open star clusters
often arranged in the classic spiral pattern. Some will have a bar
||These are ellipsoidal in shape with smooth
disks. Stellar formation since long ago stopped due to the interstellar
matter being used up. They consist of mainly of old population II stars.
||These are spiral galaxies without the spiral
structure. They have smooth disks where stellar formation ceased long
ago due to interstellar matter being used up. They contain mostly old
population II stars. Observing these are often hard to define from
||Their shape is mainly due to distortion by
gravitation of their intergalactic neighbors. They do not fit into into
the scheme of disks and ellipsoids but rather exhibit peculiar shapes.