WB Chapter 1 - Galaxies II - The Different Types

Galaxy classification is a system used by astronomers to divide galaxies into groups based on their visual appearance. There are several schemes in use by which galaxies can be classified according to their morphologies, the most famous being the Hubble sequence, devised by Edwin Hubble. I will be using this system in my explanations.


Quoted from Wikipedia en.wikipedia.org/wiki/Hubble_sequence

The Hubble sequence is a classification scheme for galaxies invented by Edwin Hubble in 1926. It is often known as the Hubble tuning-fork diagram because of the shape in which it is traditionally represented.

Hubble's scheme divides regular galaxies into 3 broad classes - ellipticals, lenticulars and spirals - based on their visual appearance. A fourth class contains galaxies with an irregular appearance.



Ellipticals

On the left (in the sense that the sequence is usually drawn) lie the ellipticals. Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in photographic images. They are denoted by the letter E, followed by an integer n representing their degree of ellipticity on the sky. By convention, n is ten times the ellipticity of the galaxy, rounded to the nearest integer, where the ellipticity is defined as for an ellipse with semi-major and semi-minor axes of lengths a and b respectively. The ellipticity increases from left to right on the Hubble diagram, with near-circular (E0) galaxies situated on the very left of the diagram. It is important to note that the ellipticity of a galaxy on the sky is only indirectly related to the true 3-dimensional shape (for example, a flattened, discus-shaped galaxy can appear almost round if viewed face-on or elliptical if viewed at an angle). Observationally, the most flattened elliptical galaxies have ellipticities e=0.7 (denoted E7). This is consistent with their being truly ellipsoidal structures rather than disks viewed at a range of angles.

Spirals

On the right of the Hubble sequence diagram are two parallel branches encompassing the spiral galaxies. A spiral galaxy consists of a flattened disk, with stars forming a (usually two-armed) spiral structure, and a central concentration of stars known as the bulge. Roughly half of all spirals are also observed to have a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin. In the tuning-fork diagram, the regular spirals occupy the upper branch and are denoted by the letter S, while the lower branch contains the barred spirals, given the symbol SB. Both type of spirals are further subdivided according to the detailed appearance of their spiral structures. Membership of one of these subdivisions is indicated by adding a lower-case letter to the morphological type, as follows:
Sa (SBa) - tightly-wound, smooth arms; large, bright central bulge
Sb (SBb) - less tightly-wound spiral arms than Sa (SBa); somewhat fainter bulge
Sc (SBc) - loosely wound spiral arms, clearly resolved into individual stellar clusters and nebulae; smaller, fainter bulge
Hubble originally described three classes of spiral galaxy. This was extended by de Vaucouleurs[6] to include a fourth class:
Sd (SBd) - very loosely-wound, fragmentary arms; most of the luminosity is in the arms and not the bulge.
The basic spiral types can be extended to enable finer distinctions of appearance. For example, spiral galaxies whose appearance is intermediate between two of the above classes are often identified by appending 2 lower-case letters to the main galaxy type (for example Sbc for a galaxy that is intermediate between an Sb and an Sc).
Our own Milky Way is generally classed as SBb, making it a barred spiral with well-defined arms. However, this classification is somewhat uncertain since we can only infer how our galaxy would appear to an outside observer.

Lenticulars

At the centre of the Hubble tuning fork, where the two spiral arms meet the elliptical branch lies an intermediate class of galaxies known as lenticulars and given the symbol S0. These galaxies consist of a bright central bulge, similar in appearance to an elliptical galaxy, surrounded by an extended, disk-like structure. Unlike spiral galaxies, the disks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any significant quantity. The bulge component is often the dominant source of light in a lenticular galaxy.
Face-on lenticulars are difficult to distinguish from ellipticals of type E0, making the classification of many such galaxies uncertain. When viewed edge-on, prominent dust-lanes are sometimes visible in absorption against the light of stars in the disk.
At the time of the initial publication of Hubble's galaxy classification scheme, the existence of lenticular galaxies was purely hypothetical. Hubble believed that they were necessary as an intermediate stage between the highly-flattened ellipticals and spirals. Later observations (by Hubble himself, among others) showed Hubble's belief to be correct and the S0 class was included in the definitive exposition of the Hubble sequence.
Lenticular and spiral galaxies, taken together, are often referred to as disk galaxies.

Irregulars

Galaxies that do not fit into the Hubble sequence, because they have no regular structure (either disk-like or ellipsoidal), are termed irregular galaxies. Hubble defined two classes of irregular galaxy:

* Irr I galaxies have asymmetric profiles and lack a central bulge or obvious spiral structure; instead they contain many individual clusters of young stars.

* Irr II galaxies have smoother, asymmetric appearances and are not clearly resolved into individual stars or stellar clusters

In the extended Hubble sequence, the Magellanic irregulars are usually placed at the end of the spiral branch of the Hubble tuning fork.

So in the next article we will begin with Step 1 of Chapter one and we will determine the type of Galaxy we have.