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Image: ESO

Types of Galaxies

Galaxies show a rich diversity of shapes which is studied and described by galaxy morphologists. This research field is based on a classification, introduced by the famous American astronomer Edwin Powell Hubble (* 1889, † 1953). Hubble distinguished mainly between elliptical and spiral galaxies. For this, he established a scheme which is known as the Hubble Tuning Fork. Later on, another class of irregulars was added. These three main classes span several subclasses to further differentiate the forms of galaxies.

Hubble’s original tuning-fork diagram as published in 1936 in his ‘Realm of the Nebulae’
Hubble’s original tuning-fork diagram as published in 1936 in his ‘Realm of the Nebulae’Image: Sarah Arnold
  • Elliptical galaxies (E) are structureless and elliptical-shaped. Their luminosity decreases from the center to the edge. Ellipticals are divided into subclasses from perfectly circular (E0) to highly elliptic (E7). The stellar population of ellipticals is mainly old. Due to the high portion of red giants, the whole galaxy looks reddish. Ellipticals contain barely any gas, which is why no new stars can be born in these systems. Ellipticals are, in actual fact, three dimensional ellipses or ellipsoids (Rugby ball-shaped).
  • Spiral galaxies (S) consist of a central bulge and spiral arms. Their spectrum of shapes reaches from tightly twisted arms and a big bulge (Sa) to widely opened arms with a small bulge (Sc). Also, a differentiation is made between spirals with a central bar (SBa, SBb, …) and spirals without (Sa, Sb, …). Typically, spirals contain quite a large amount of gas and one can find areas with active star formation. The spiral arms usually look blue, whereas the bulge is more of a red color. Unlike ellipticals, spirals do rotate around their center. The whole galaxy is flattened in the direction perpendicular to the rotation axe, as is a pizza dough, which is rotated on the fingertips of a pizzaiolo.
  • Irregular galaxies (Irr) show several chaotic structures. Following Hubbles logic, the irregulars join the Hubble fork on the right hand side of the spirals. Irregulars often contain very active zones of star formation and star bursts.

The elliptical galaxies are commonly called early types, whereas galaxies on the right side of the scheme, starting from Sc-types, are known as late types. Despite this terminology the hubble fork should not be considered as a chronological sequence.

The Hubble scheme refers to the most prominent and luminous galaxies. With the progresses made in the engineering of precise high light transmission telescopes new and very faint stellar systems have been (and are still) discovered. Many of these galaxies wouldn’t fit into the given Hubble classes. Therefore, around the 1950s, the galaxy types were further differentiated: The American astronomer Harlow Shapley (* 1885, † 1972) expanded the S-nomenclature by 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 galaxy types. On the side of early type galaxies, a whole continuum of luminosities was discovered including extremely faint and diffuse systems.

A systematic study of faint galaxies was first done by Bruno Binggeli (* 1953), an astronomer from Basel and the American Allen Rex Sandage (* 1926, † 2010). Several classes of so-called dwarf galaxies were introduced. As an example, dwarf ellipticals were now labeled with dE. The term dwarf refers to the low luminosity of a certain galaxy. As a very important result of the study, Binggeli and Sandage found that there exist no proper dwarf spirals (classes such as dSa, dSb, …). This can be explained by the fact that spiral arm only form when a certain rotation speed is reached and therefore, a high enough galaxy mass is required.

By glancing on a galaxy map, one notes that the universe is not randomly sprinkled by different types of galaxies; While spirals are often found alone in the wide field, ellipticals prefer company by other ellipticals. They are thus normally found in dense galaxy clusters. This raises the questions: Are ellipticals born in the dense environment of clusters and spirals in the outland? Or does the shape of a spiral start to deform while it is wandering into a denser area? Those questions are part of the ongoing research on galaxies.

Sombrero Galaxy
Image: Hubble Heritage Team (AURA/STScI /NASA)

Sombrero Galaxie (Sa-Typ)

Andromeda Galaxy
Image: GALEX, JPL-Caltech, NASA

Andromeda Galaxie (Sb-Typ)

M51 Galaxy
Image: S. Beckwith (STScI) Hubble Heritage Team, (STScI/AURA), ESA, NASA

Whirlpool-Galaxie (Sc-Typ)

M60 and NGC 4647 Galaxies
Image: NASA, ESA, Hubble Heritage Team (STScI/AURA)

M60 Galaxie (E2-Typ)

NGC 1132 Galaxy
Image: M. West (ESO, Chile), NASA, ESA & Hubble Heritage (STScI/AURA)-ESA

NGC 1132 Galaxie (E4-Typ)

NGC 1427a Galaxy
Image: Hubble Heritage Team (AURA / STScI), ESA, NASA

NGC 1427a Galaxie (Irr Typ)