In 1936 Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence.

Types of galaxies according to the Hubble classification scheme.

The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated.

In the Hubble classification scheme, these are designated by an SB, followed by a lower-case letter ( a, b or c ) that indicates the form of the spiral arms ( in the same manner as the categorization of normal spiral galaxies ).

( Barred lenticular galaxies receive Hubble classification SB0.

An Irr-I galaxy has some structure but does not align cleanly with the Hubble classification scheme.

Irr-II galaxies do not possess any structure that resembles a Hubble classification, and may have been disrupted.

The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926.

Although strictly part of the de Vaucouleurs system of classification, the Sd class is often included in the Hubble sequence.

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.

The star spectral classification and discovery of the main sequence, Hubble's law and the Hubble sequence were all made with spectrographs that used photographic paper.

The Hubble classification of elliptical galaxies contains an integer that describes how elongated the galaxy image is.

Hubble recognized that his shape classification depends both on the intrinsic shape of the galaxy, as well as the angle with which the galaxy is observed.

At the large end of the elliptical spectrum, there is further division, beyond Hubble classification.

Using the Hubble classification, the bulge of Sa galaxies is usually composed of population II stars, that are old, red stars with low metal content.

In the modified Hubble sequence galaxy morphological classification scheme of French astronomer Gérard de Vaucouleurs, Messier 87 is categorized as an E0p galaxy.

A 2012 paper that suggests a new classification system-first proposed by the canadian astronomer Sidney van den Bergh-for lenticular and dwarf spheroidal galaxies ( S0a-S0b-S0c-dSph ) that parallels the Hubble sequence for spirals and irregulars ( Sa-Sb-Sc-Im ) reinforces this idea showing how the spiral-irregular sequence is very similar to this new one for lenticulars and dwarf ellipticals.

* Hubble sequence, a classification of galaxy types

He also developed several galaxy morphological classification systems, including the first systems to use the physical, quantifiable properties of galaxies, as opposed to simple, qualitative, eye-ball estimates favoured by Edwin Hubble.

The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926.

Among his contributions are the observational proof that many nebulous objects are actually galaxies beyond our own Milky Way galaxy, the classification of galaxies according to the Hubble sequence, and the development of Hubble's Law relating a galaxy's observed red shift to its distance away.

Hubble ’ s scheme divides regular galaxies into 3 broad classes-ellipticals, lenticulars and spirals-based on their visual appearance ( originally on photographic plates ).

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 ).

Hubble ’ s scheme divides galaxies into 3 broad classes based on their visual appearance ( originally on photographic plates ):

As in Hubble ’ s original scheme, spiral galaxies are assigned to a class based primarily on the tightness of their spiral arms.

The de Vaucouleurs scheme extends the arms of Hubble ’ s tuning fork to include several additional spiral classes:

Most galaxies in these three classes were classified as Irr I in Hubble ’ s original scheme.

Hubble showed that the spiral nebulae were galaxies by determining their distances using measurements of the brightness of Cepheid variable stars.

Wilson, Hubble identified Cepheid variables ( a kind of star ; see also standard candle ) in several spiral nebulae, including the Andromeda Nebula and Triangulum.

Wilson telescope, Edwin Hubble was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way.

These are categorized as Hubble type S0, and they possess ill-defined spiral arms with an elliptical halo of stars.

While the LMC is often considered an irregular type galaxy ( the NASA Extragalactic Database lists the Hubble sequence type as Irr / SB ( s ) m ), the LMC contains a very prominent bar in its center, suggesting that it may have previously been a barred spiral galaxy.

The Pinwheel Galaxy ( Messier 101 / NGC 5457 ): a spiral galaxy classified as type Scd on the Hubble sequence

On the right of the Hubble sequence diagram are two parallel branches encompassing the spiral galaxies.

Hubble originally described three classes of spiral galaxy.

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.

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

* 1936 — Edwin Hubble introduces the spiral, barred spiral, elliptical, and irregular galaxy classifications,

Edwin Hubble used this method to prove that the so-called spiral nebulae are in fact distant galaxies.

As usual on spiral galaxies of the Virgo Cluster, in the rest of the disk both star formation and neutral hydrogen, of which M100 is deficient compared to isolated spirals of similar Hubble type, are truncated within the galaxy's disk, something caused by interactions with the intracluster medium of Virgo.

During the early part of the 20th century important articles published in Science included papers on fruit fly genetics by Thomas Hunt Morgan, gravitational lensing by Albert Einstein, and spiral nebulae by Edwin Hubble.

They are one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae, along with spiral and lenticular galaxies.

Originally, Edwin Hubble thought that elliptical galaxies may evolve into spiral galaxies, which later turned out to be false.

A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence.

The bright galaxy NGC 3810 demonstrates classical spiral structure in this very detailed image from Hubble.