Hubble classification

(redirected from Hubble sequence)
Also found in: Dictionary, Wikipedia.
Hubble classificationclick for a larger image
Hubble classification: Hubble's 'tuning fork' diagram

Hubble classification

(hub -'l) A classification scheme for galaxies introduced by Edwin Hubble in 1925. Although more complex and refined schemes have since been devised the simple but slightly revised Hubble scheme is still widely used. The scheme recognizes three main types of galaxy: elliptical, spiral, and barred spiral. Each is divided into subtypes according to gross observable characteristics, the principle one being shape. These subtypes are arranged into a morphological sequence (see illustration).

Elliptical galaxies are denoted by the letter E followed by a number from 0 to 7 that indicates the apparent degree of flattening; this number is the nearest integer to 10(1 – b /a ), where a and b are, respectively, the semimajor and semiminor axes of the observed profile. Thus E0 galaxies are almost circular in outline and E7, the most elliptical, have an a :b ratio of about 3:1.

The spirals (type S) and barred spirals (type SB) are separated into subtypes a, b, and c along a sequence that follows a progressive decrease in central bulge luminosity compared to the disk and an increase in the openness and prominence of the spiral arms. An additional subtype, d, has been introduced. Galaxies with intermediate characteristics can be classified as Sab, SBab, Sbc, SBbc, etc. The S0 galaxies, placed at the junction of the elliptical and spiral sequences, resemble the spirals in general shape but lack spiral arms. This type was predicted by Hubble in 1925 but identified only later.

The less commonly observed irregular galaxies were not included in Hubble's original diagram, but are now placed to the right of the S and SB sequences. They were originally divided into two types. Irr I galaxies lack both symmetry and a nucleus and are mainly comprised of blue population I stars and H II regions. Irr II galaxies were objects that could not otherwise be classified, possessing either a highly perturbed morphology or some prominent feature unusual for their class. The term irregular galaxy is now applied only to Hubble type Irr I, and Irr II systems are now usually reclassified as interacting or starburst galaxies as appropriate.

References in periodicals archive ?
She said that the Hubble Sequence underpins a lot of what we know about how galaxies form and evolve.
There was previous evidence that the Hubble Sequence holds true as far back as around 8 billion years ago, the authors point out, but their new observations push a further 2.
In the 1960s University of Texas astronomer Gerard de Vaucouleurs further subdivided the Hubble sequence on the basis of inner and outer rings that sometimes appear within the overall spiral pattern.
Today most astronomers use Sandage's revision of the Hubble sequence as the primary means of classifying galaxies.
Since these galaxies, in general, are diffuse blobs without much spiral structure, they fall well outside the Hubble sequence.
This in fact may be the most important aspect of the Hubble sequence.
In this view, the Hubble sequence is a sequence in the average age of each galaxy's stars.
Over the last 25 years astronomers have produced a number of papers with the catchy title "The Origin of the Hubble Sequence.
Given that the observed Hubble sequence is largely based on relative amounts of star formation, the issue is which of the above parameters determines a galaxy's star-formation history.
Bulge mass: There remains the relatively widespread belief that the Hubble sequence is a sequence in bulge mass or bulge-to-disk ratio.
However, data indicate that there is no difference in specific angular momentum (angular momentum per unit mass) along the Hubble sequence of spirals.
As noted before, galaxies can be grouped along the so-called Hubble sequence on the basis of their morphology and kinematics.