Types of Galaxies

The different shapes of galaxies are studied in galaxy morphology which describes and classifies the structures. Galaxy morphology is based on the classification introduced by Edwin Powell Hubble (1889 – 1953), the famous American astronomer. Hubble distinguished in particular between elliptical and spiral galaxies. To this end, he created a scheme known as the Hubble sequence or Hubble tuning form. The irregular form was added later. These three classes - elliptical, spiral and irregular - span several subclasses that further differentiate the shape of galaxies.

• Elliptical galaxies (E) are structureless, elliptical galaxies whose brightness decreases from the inside to the outside. The ellipses are divided into subclasses from circular (E0) to strongly elliptical (E7). The stellar population of ellipses is mostly old. The high proportion of red giant stars means that the entire galaxy appears reddish in colour. Very little gas is present in the ellipses from which new stars could form. Elliptical galaxies are in reality not flat, but rather three-dimensional ellipses or ellipsoids (similar to a rugby ball).
• Spiral galaxies (S) consist of a central region (bulge) and spiral arms. The spectrum of spirals ranges from types with tightly wound arms and a large bulge (Sa) to a system with wide open arms and a small bulge (Sc). A differentiation is also made between whether a bar structure can be recognised in the central area (SBa, SBb, ...) or not (Sa, Sb, ...). Typically, the spiral arms contain a more or less high proportion of gas and areas where star formation takes place. The colour of the spiral arms is bluish, while the bulge appears to be more red. In contrast to elliptical galaxies, spirals rotate around their centre. The entire galaxy is therefore flattened in a disc shape perpendicular to the axis of rotation, like pizza dough being rotated on the fingertips of a pizzaiolo.
• Irregular galaxies (Irr) exhibit, as the name suggests, a wide variety of irregular shapes. According to Hubble's logic, the irregulars in the diagram connect to the spirals on the right. These galaxies often contain very active star-forming regions which are veritable starbursts.

The elliptical galaxies are commonly referred to as early types, while galaxies to the right of the diagram, from about Sc onwards, are called late types. Despite the terminology, the Hubble scheme should not be interpreted as a chronological sequence.

Extension of the Hubble scheme

The Hubble scheme refers to the most prominent and luminous galaxies. More and more star systems of lower luminosity were discovered later. These cannot be accommodated in the Hubble sequence as they have their own shapes. Late galaxy types in particular were therefore further differentiated around 1950. The American astronomer Harlow Shapley (1885 – 1972) extended the S nomenclature to include the type Sd, which describes irregular spirals. Gérard-Henri de Vaucouleurs (1918 – 1995), a French-American astronomer, established Sdm, Sm and Im as new classes of very late types. With regard to the early types, it was recognised that the galaxies form an entire continuum of luminosities up to extremely faint and diffuse formations.

Little luminosity: dwarf galaxies

A systematic study of the structures of galaxies with weak luminosities was carried out in 1984 by the Basel astronomer Bruno Binggeli (born 1953) and the American Allen Rex Sandage (1926 – 2010). They introduced several classes of so-called dwarf galaxies (abbreviated with “d” for dwarf). Dwarf galaxies are defined as those that do not exceed a certain luminosity. The word dwarf therefore does not necessarily have anything to do with the size of the galaxy. An important finding from the study by Binggeli and Sandage is that there are no true dwarf spirals such as dSa, dSb or dSc. This is explained by the fact that spiral arms can only be formed above a certain rotational speed, and this requires a sufficiently high galaxy mass.

Spiral galaxies are loners

If you take a look at a map of the galaxies, you will see that different types are not randomly distributed in space. While spirals are often found far away from other galaxies, an ellipse rarely occurs alone, but is observed in the dense regions of galaxy clusters. This brings us to the questions of current galaxy research. Are ellipses born in dense regions and spirals in open areas? Or does the shape of a spiral galaxy change over time as it moves into more densely populated galaxy territory?