The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926. It is often known colloquially 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 (originally on photographic plates). A fourth class contains galaxies with an irregular appearance. To this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy.
Classes of galaxies 
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 representing their degree of ellipticity on the sky. By convention, 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 and 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.
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
- Sd (SBd) - very loosely-wound, fragmentary arms; most of the luminosity is in the arms and not the bulge
Although strictly part of the de Vaucouleurs system of classification, the Sd class is often included in the Hubble sequence. 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 example56 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.
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 by Allan Sandage.
Lenticular and spiral galaxies, taken together, are often referred to as disk galaxies.
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 his extension to the Hubble sequence, de Vaucouleurs called the Irr I galaxies 'Magellanic irregulars', after the Magellanic Clouds - two satellites of the Milky Way which Hubble classified as Irr I. The discovery of a faint spiral structure in the Large Magellanic Cloud led de Vaucouleurs to further divide the irregular galaxies into those that, like the LMC, show some evidence for spiral structure (these are given the symbol Sm) and those that have no obvious structure, such as the Small Magellanic Cloud (denoted Im). In the extended Hubble sequence, the Magellanic irregulars are usually placed at the end of the spiral branch of the Hubble tuning fork.
Physical significance 
Elliptical and lenticular galaxies are commonly referred to together as “early-type” galaxies, while spirals and irregular galaxies are referred to as “late types”. This nomenclature is the source of the common, but erroneous, belief that the Hubble sequence was intended to reflect a supposed evolutionary sequence, from elliptical galaxies through lenticulars to either barred or regular spirals. In fact, Hubble was clear from the beginning that no such interpretation was implied:
The nomenclature, it is emphasized, refers to position in the sequence, and temporal connotations are made at one's peril. The entire classification is purely empirical and without prejudice to theories of evolution...
The evolutionary picture appears to be lent weight by the fact that the disks of spiral galaxies are observed to be home to many young stars and regions of active star formation, while elliptical galaxies are composed of predominantly old stellar populations. In fact, current evidence suggests the opposite: the early Universe appears to be dominated by spiral and irregular galaxies. In the currently favored picture of galaxy formation, present-day ellipticals formed as a result of mergers between these earlier building blocks. Lenticular galaxies may also be evolved spiral galaxies, whose gas has been stripped away leaving no fuel for continued star formation.
A common criticism of the Hubble scheme is that the criteria for assigning galaxies to classes are subjective, leading to different observers assigning galaxies to different classes (although experienced observers usually agree to within less than a single Hubble type). The different classification criteria can also be at odds with each other: for example, a more dominant bulge component does not always go hand-in-hand with more loosely-wound spiral arms. Another criticism of the Hubble classification scheme is that, being based on the appearance of a galaxy in a two-dimensional image, the classes are only indirectly related to the true physical properties of galaxies. In particular, problems arise because of orientation effects (the same galaxy looks very different when viewed edge-on, as opposed to face-on), because visual classifications are less reliable for faint or distant galaxies, and because the appearance of galaxies changes depending on the wavelength of light in which they are observed. Nevertheless, the Hubble sequence is still commonly used in the field of extragalactic astronomy and Hubble types are known to correlate with many physically relevant properties of galaxies, such as luminosities, colours, masses (of stars and gas) and star formation rates.
See also 
- Edwin Hubble
- Gérard de Vaucouleurs
- Galaxy color-magnitude diagram
- Galaxy morphological classification
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- Galaxies and the Universe - an introduction to galaxy classification
- Near-Infrared Galaxy Morphology Atlas, T.H. Jarrett
- The Spitzer Infrared Nearby Galaxies Survey (SINGS) Hubble Tuning-Fork, SINGS Spitzer Space Telescope Legacy Science Project
- Galaxy Zoo - Galaxy classification participation project.