Galaxies are primarily classified based on their visual appearance, a system originally developed by Edwin Hubble in 1926 known as the Hubble sequence or Hubble tuning fork diagram. This morphological classification divides galaxies into three main types, with a fourth category for irregular shapes:
- Elliptical galaxies (E): These have smooth, featureless light distributions and appear elliptical in shape. They are denoted by "E" followed by a number from 0 to 7 indicating their ellipticity, where E0 is nearly circular and E7 is highly elongated. Ellipticals generally contain older stars and little gas or dust
- Spiral galaxies (S): These consist of a flat disk with spiral arms and a central bulge. Spirals are further subdivided by the tightness of their arms and size of the bulge, labeled from Sa (large bulge, tightly wound arms) to Sc (small bulge, loosely wound arms). About half of spirals have a central bar structure, called barred spirals (SB), where the spiral arms start from the ends of the bar rather than the bulge
- Lenticular galaxies (S0): These are intermediate between ellipticals and spirals, featuring a bright central bulge and an extended disk but lacking visible spiral arms or significant star formation. Some lenticulars also have bars (barred lenticulars)
- Irregular galaxies: These do not fit into the above categories and have no regular shape or structure
The Hubble tuning fork diagram visually represents these classes, with ellipticals on one side, lenticulars at the junction, and spirals (normal and barred) branching out as two prongs. Despite its age, this classification remains widely used in astronomy for identifying and studying galaxies
. In summary, galaxy classification is based on morphology into elliptical, lenticular, spiral (normal and barred), and irregular types, primarily using the Hubble sequence as the framework.
