HOw are galaxies classified
Galaxies are classified based on their shape and structure. The most common classification scheme, known as the Hubble sequence or Hubble tuning-fork diagram, was developed by Edwin Hubble in 1926. It categorizes galaxies into three main types: elliptical, spiral, and irregular.
1. Elliptical galaxies: These galaxies are round or elliptical in shape and lack spiral arms. They are further classified based on their elongation or how flattened they appear. The classification scheme for elliptical galaxies ranges from E0 (near perfect sphere) to E7 (highly elongated).
2. Spiral galaxies: These galaxies have a flattened disk structure with spiral arms wrapping around a central bulge. They are classified based on the tightness and prominence of their spiral arms. Spiral galaxies are further categorized into two subtypes:
a) Normal spirals (Sa, Sb, Sc): These galaxies have well-defined spiral arms, with Sa galaxies having the tightest and most prominent arms, and Sc galaxies having looser and less distinct arms.
b) Barred spirals (SBa, SBb, SBc): These galaxies have a bar-like structure running through their central bulge, with their spiral arms originating from the ends of the bar. The classifications follow a similar pattern to normal spirals, with the tightness and distinctiveness of the arms varying.
3. Irregular galaxies: These galaxies do not have a specific, regular shape and do not fit into the categories of elliptical or spiral galaxies. They often exhibit chaotic or random patterns and can be further classified based on their appearances, such as irregular type (Irr) or peculiar type (Pec).
It's important to note that galaxies can have intermediate or transitional features, leading to subcategories and more detailed classification systems. Additionally, recent studies have identified additional types of galaxies, such as lenticular galaxies (S0) that have characteristics of both elliptical and spiral galaxies, and dwarf galaxies that are much smaller in size than typical galaxies.