Галактикалардың морфологиялық жіктелуі және Хаббл диаграммасы

Galaxy morphological classification scheme advocated by Edwin Hubble
The Hubble sequence is a morphological classification scheme for galaxies published by Edwin Hubble in 1926. It is often colloquially known as the Hubble tuning fork diagram because the shape in which it is traditionally represented resembles a tuning fork. It was invented by John Henry Reynolds and Sir James Jeans. The tuning fork scheme divided regular galaxies into three broad classes – ellipticals, lenticulars and spirals – based on their visual appearance (originally on photographic plates). A fourth class contains galaxies with an irregular appearance. The Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy.

On the left (in the sense that the sequence is usually drawn) lie the ellipticals. Elliptical galaxies have relatively 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 in 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 a the semi major axis length and b the semi minor axis length. 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 highly elliptical if viewed edge on). Observationally, the most flattened "elliptical" galaxies have ellipticities (denoted E7). However, from studying the light profiles and the ellipticity profiles, rather than just looking at the images, it was realised in the 1960s that the E5–E7 galaxies are probably misclassified lenticular galaxies with large scale disks seen at various inclinations to our line of sight. Observations of the kinematics of early type galaxies further confirmed this. Examples of elliptical galaxies: M49, M59, M60, M87, NGC 4125.

At the centre of the Hubble tuning fork, where the two spiral galaxy branches and the elliptical branch join, 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. When simply looking at a galaxy's image, lenticular galaxies with relatively face on disks are difficult to distinguish from ellipticals of type E0–E3, making the classification of many such galaxies uncertain. When viewed edge on, the disk becomes more apparent and prominent dust lanes are sometimes visible in absorption at optical wavelengths. 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. Missing from the Hubble sequence are the early type galaxies with intermediate scale disks, in between the E0 and S0 types, Martha Liller denoted them ES galaxies in 1966. Lenticular and spiral galaxies, taken together, are often referred to as disk galaxies. The bulge to disk flux ratio in lenticular galaxies can take on a range of values, just as it does for each of the spiral galaxy morphological types (Sa, Sb, etc.). Examples of lenticular galaxies: M85, M86, NGC 1316, NGC 2787, NGC 5866, Centaurus A.

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; while some lenticular galaxies may have formed this way, others may have accreted their disks around pre existing spheroids. Some lenticular galaxies may also be evolved spiral galaxies, whose gas has been stripped away leaving no fuel for continued star formation, although the galaxy LEDA 2108986 opens the debate on this.

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). Although not really a shortcoming, since the 1961 Hubble Atlas of Galaxies, as with the lenticular galaxies. 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 would look very different, if viewed edge on, as opposed to a face on or 'broadside' viewpoint. As such, the early type sequence is poorly represented: The ES galaxies are missing from the Hubble sequence, and the E5–E7 galaxies are actually S0 galaxies. Furthermore, the barred ES and barred S0 galaxies are also absent. Visual classifications are also less reliable for faint or distant galaxies, and the appearance of galaxies can change depending on the wavelength of light in which they are observed. Nonetheless, 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. In June 2019, citizen scientists in the Galaxy Zoo project argued that the usual Hubble classification, particularly concerning spiral galaxies, may not be supported by evidence. Consequently, the scheme may need revision.