Hubble sequence
[1][2][3][4] It is often colloquially known as the Hubble tuning-fork diagram because the shape in which it is traditionally represented resembles a tuning fork.
[5] The tuning fork scheme divided regular galaxies into three broad classes – ellipticals, lenticulars and spirals – based on their visual appearance (originally on photographic plates).
The Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy.
Elliptical galaxies have relatively smooth, featureless light distributions and appear as ellipses in photographic images.
When viewed edge-on, the disk becomes more apparent and prominent dust-lanes are sometimes visible in absorption at optical wavelengths.
Membership of one of these subdivisions is indicated by adding a lower-case letter to the morphological type, as follows: Hubble originally described three classes of spiral galaxy.
Our own Milky Way is generally classed as Sc or SBc,[15] making it a barred spiral with well-defined arms.
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...[3] 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.
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).
[21][22] Although not really a shortcoming, since the 1961 Hubble Atlas of Galaxies,[23] the primary criteria used to assign the morphological type (a, b, c, etc.)
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.
