de Vaucouleur system and Hubble sequence

Caption: The de Vaucouleur-Hubble (dVH) tuning fork diagram (a nonce name for a modified version of Hubble tuning fork diagram) illustrating the de Vaucouleurs system of galaxy morphological classification. The in-image caption includes the word "Hubble" since the de Vaucouleurs system is an extension (with some modifications) 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 dVH tuning fork 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 2.
Elliptical galaxies and lenticular galaxies (AKA collectively 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 an extension of the Hubble sequence as aforesaid.
Both systems are empirical classifications based on directly observed properties, NOT on theoretical understanding or bias.
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 do NOT 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 2 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 the human eye.
And the systems are exact enough and objective enough for some degree of comparison to theories and computer simulations 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 just an extension of the Hubble sequence after all.
In fact, modern astronomers seem to flip back and forth between the systems according to their needs without much comment. They also seem to use types from other galaxy morphological classifications without much comment. One just has to match what type they give to a standard system as best one can.
We can give a brief explanations of the galaxy types illustrated in the dVH tuning fork diagram above omitting the funny names that the de Vaucouleurs system has for elliptical galaxies---which do NOT 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 dVH 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 dVH 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 dVH tuning fork diagram.
  They are much as the unbarred spirals, except they have bars---which makes life in them 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 dVH 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 with (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. As an example of classification, we can cite the Milky Way. It's probably a SB(rs)bc (see Wikipedia: Milky Way).
There are many other classifications of galaxies beyond the Hubble sequence and the de Vaucouleurs system. For reference, see Galaxy Classification Systems and Types of Galaxies below (local link / general link: galaxy_types.html):
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 2 galaxies of any type, one or both of which may already be an elliptical. Note the galaxy merger of 2 spirals usually forms an elliptical.
3. Re galaxy mergers:
  1. The galaxy merger is complex with interacting galaxies passing through each other several times before finally settling down to a long-lasting structure. The passing through events randomize the orbits (i.e., the orbital planes) of the stars and interstellar medium (ISM) and also eject some stars and interstellar medium (ISM) into open space.. The merged galaxy seems almost always to be an elliptical galaxy without a galactic disk and is often mostly spheroidal without any preferred axis.
  2. The colliding ISM of the merging galaxies often causes a major starburst region (i.e, causes the merging galaxies to become a starburst galaxy with a very high star formation rate (SFR)). The strong galactic outflows made of strong stellar winds and supernova remnants from the starburst can expel most of the ISM of the merged galaxy and cause galaxy quenching: i.e., turn off star formation in the merged galaxy.
  3. Moreover, the merger process itself can strip ISM from the merging galaxies and send it off into open space as aforesaid and this too acts to turn off star formation.
  4. Also moreover, if the overall merged galaxy mass (dark matter halo and stellar matter) is >∼ 10**12 M_☉ (sometimes called the golden mass), then it seems likely that outflows of hot gas from the central supermassive black hole prevent a build up of cool gas from inflows (though there is enough inflow to power the outflow) and prevent the restarting of star formation in the merged galaxy (Bower et al. 2016). In fact, almost all galaxies are galaxy quenched if their mass exceeds the golden mass = 10**12 M_☉ (Dekel et al. 2019, Figure 1).
  5. And another thing, the expansion of the universe (with its known universal acceleration) means that the density of the intergalactic medium (IGM) falls with cosmic time after relatively early in cosmic time. This means that inflows of IGM into galaxies will overall tend to decline with cosmic time leading to a general decline in star formation. In fact, star formation rate density peaked at cosmic time ∼ 4 Gyr after the Big Bang which itself occurred at lookback time equal to the age of the universe = 13.799(21) Gyr (current value) (Madau & Dickinson 2014). This peak is called cosmic noon (z≅2, cosmic time ∼ 4 Gyr).
  6. Note the supermassive black holes at the centers of the merging galaxies will themselves merge (see Wikipedia: Binary black hole; Merger; Wikipedia: Final parsec problem), and thus create a combined supermassive black hole.
  7. As aforesaid, the galaxy merger of spirals is often the formation of an elliptical galaxy and whether or NOT the elliptical galaxy exceeds the golden mass 10**12 M_☉, it is usually a quenched galaxy (AKA red sequence galaxy): i.e., one without significant star formation. The galaxy quenching of elliptical galaxies is why elliptical galaxies generally look a bland yellow in true color images: they have NO blue OB stars which are all from recent star formation since OB stars have short lifetimes.

Credit/Permission: © Antonio Ciccolella (AKA User:Cicconorsk), 2011 / CC BY-SA 3.0.
Image link: Wikipedia: File:Hubble-Vaucouleurs.png.
Local file: local link: galaxy_vaucouleurs.html.
File: Galaxies file: galaxy_vaucouleurs.html.