De Vaucouleur system and Hubble sequence

Caption: A modified Hubble tuning fork diagram illustrating the De Vaucouleurs system of galaxy morphological classification. The in-figure caption includes the word "Hubble" since the De Vaucouleurs system is an extension of the older system the Hubble sequence and, in fact, people use Hubble sequence as a synonym for the De Vaucouleurs system it seems.

Features:

The diagram is a modified Hubble tuning fork diagram because, among other things, it has 3 tines (AKA prongs) on the right rather than the traditional two.
Elliptical galaxies (AKA early type galaxies) form the handle and spiral galaxies (of various types), irregular galaxies, and dwarf spheriodal galaxies (dSphs) (collectively late type galaxies) form the tines.
You will have to click on the image and then click on the linked image to get a larger more legible version.
The labels are in Italian, but faute de mieux.
Hubble sequence originated with Edwin Hubble (1889--1953) in 1926 (see Wikipedia: Hubble sequence).
The De Vaucouleurs system was first introduced by Gerard de Vaucouleurs (1918--1995) in 1959.
The De Vaucouleurs system is mainly an extension of the Hubble sequence.
Both systems are empirical classifications based on directly observed properties, NOT on theoretical understanding.
Of course, the observed properties do have substantial, but not complete, theoretical understanding nowadays.
Classification in both systems CANNOT be exact for three main reasons:
1. All galaxies are to some degree unique.
2. There are always intermediate and peculiar galaxies that don't fit well into any of the defined galaxy types.
3. The classification is NOT done by strict objective rules, but by expert classifiers, and so always has a degree of subjectivity. No two classifiers will agree on the type of all galaxies.
Despite the lack of exactness and complete objectivity, the Hubble sequence and the De Vaucouleurs system are still highly useful.
It is good to have galaxy morphological classification systems that can be done reasonably accurately by quick examination by eye.
And the systems are exact enough and objective enough for comparison to theories of galaxy formation and evolution.
The Hubble sequence is the traditional system to use. However, modern databases for galaxies (NED and Sinbad) use the De Vaucouleurs system mostly it seems. It is just handles more cases more precisely I guess and it is mostly just an extension of the Hubble sequence.
In fact, modern astronomers seem to flip back and forth between the systems according to their needs without much comment.
We can give a brief explanations of the galaxy types illustrated in the Hubble tuning fork diagram above omitting the funny names that the De Vaucouleurs system has for elliptical galaxies---which don't seem to be much used.
Explanations:
1. Note that capital letters denote type and small letters or numbers denote subtype---except that zero in S0 is is part of the type designation (see below).
2. Elliptical galaxies, labeled E0 through E7, are the handle of the Hubble tuning fork diagram as aforesaid.
  E0's are spherical and with increasing number the ellipticals become increasingly elongated a seen projected on the sky. The non-E0's may be either oblate or prolate.
  In fact, it is difficult to determine the true 3-dimensional shape of ellipticals because we only see them projected on the sky and their projections do NOT give their shapes uniquely.
  For example, an E0 may be either oblate or prolate in 3-dimensional shape and we are seeing it along its symmetry axis.
3. Lenticulars, labeled S0, are like spirals without spiral arms.
  Lenticulars can have bars or no bars. The latter are SA0's and former SB0's.
  The A in SA0 is may sometimes be omitted. When omitted, one writes S0. Context hopefully tells if S0 means S0 or SA0.
4. Unbarred spiral galaxies, labeled SAa, SAb, SAc, SAd, SAm, are on the upper tine of the Hubble tuning fork diagram.
  Going from a to c, the bulges get smaller and the spiral arms less tightly wound.
  The SAd's have diffuse, broken spiral arms made up mostly of open star clusters and nebulae. They have very faint bulges.
  The Sm's are irregular in appearance with no bulges.
  The A in SA means unbarred and is often omitted. When omitted, one writes Sa, Sb, Sc, Sd, Sm for the unbarred spirals.
5. Barred spiral galaxies, labeled SBa, SBb, SBc SBd, SBm, are on the lower tine of the Hubble tuning fork diagram.
  They are much as the unbarred spirals, except they have bars---which makes life a lot easier on Friday afternoons.
  The Milky Way is a barred spiral, in fact.
6. Intermediate spiral galaxies, labeled SABa, SABb, SABc SABd, SABm, are on the middle tine of the Hubble tuning fork diagram.
  The intermediate spiral galaxies are transitional between the unbarred spirals and the barred spirals.
  The have weak bars in some sense.
  Note the "AB" in SAB accords with an old galaxy morphological classification rule. If a galaxy falls between two types or subtypes (in eye of the beholder), just specify both types or subtypes. Klutzy, but that is the rule.
7. Irregular galaxies are labeled Im and are put at the end of the middle tine.
  In the De Vaucouleurs system, Im's are very irregular and SAm's, SABm's, and SBm's are not so irregular and have some spiral arm structure.
  In the Hubble sequence, irregulars are divided into Irr-I's (which are not so irregular and correspond to the SAm, SABm, and SBm subtypes) and the Irr-II's (which are very irregular and correspond to the Im type).
8. Dwarf spheriodal galaxies (dSphs) are stuck at the end of the middle tine.
  They are similar to dwarf elliptical galaxies and may be dwarf elliptical galaxies by another name---but Wikipedia takes refuge in confusion on this fine point.
  Actually, dSphs and/or dwarf ellipticals may be more like late-type spirals and irregulars (see Wikipedia: Dwarf spheriodal galaxies).
  The dSphs and/or dwarf ellipticals may be the commonest types of galaxies in the observable universe, but they are small and faint, and so hard to find.
9. There is an additional pair of letter classifiers: r and s.
  Galaxies with ring structures are labeled (r) between the type and subtype labels. Those without ring structures are labeled (s) between the type and subtype labels. Transitional cases are labeled (rs) between the type and subtype labels.
10. One can always had "p" or "pec" for peculiar to a galaxy's type if you think the galaxy is a peculiar galaxy---and every galaxy is if you look closely enough.
11. We have NOT actually exhausted all types of galaxies.
  There are other types and other schemes of galaxy morphological classification.
  Just to throw out some types from some or other classification scheme:
As an example of classification, we can cite the Milky Way.
It's probably a SB(rs)bc (see Wikipedia: Milky Way).
This is NOT the place to go in depth into galaxy formation and evolution which is a complex subject.
But we can say a few things:
1. Neither the Hubble sequence nor the De Vaucouleurs system is an evolutionay sequence in any simple sense.
  Hubble warned against treating the Hubble sequence as an evolutionary sequence:
2. However, one can say that ellipticals probably are often formed by a galaxy merger of spirals.
  The merger strips the interacting galaxies of their interstellar medium (ISM) (i.e., interstellar dust and gas) and disorders the orbital planes of the stars so that they are no longer concentrated in a galactic disks, but have a random orientation. The overall shape of the merged spirals usually form approximate spheroids: i.e., ellipsoids.
3. Galaxy mergers of spirals with ellipticals and ellipticals with ellipticals resulting in bigger ellipticals happen too.

Credit/Permission: © Antonio Ciccolella (AKA User:Cicconorsk), 2011 / CC BY-SA 3.0.
Image link: Wikipedia.
File: Galaxies file: galaxy_vaucouleur.html.