The de Vaucouleurs technique for classifying galaxies is a widely used extension to the Hubble sequence, first described by Gérard de Vaucouleurs in 1959.[2] De Vaucouleurs argued that Hubble's two-dimensional classification of spiral galaxiesâbased on the tightness of the spiral arms and the presence or absence of a bar did not adequately describe the full range of observed galaxy morphologies. In particular, he argued that rings and lenses were important structural parts of spiral galaxies.[3]
The de Vaucouleurs technique retains Hubbles basic division of galaxies in to elliptical s, lenticulars, spirals and irregulars. To complement Hubbleâs technique, de Vaucouleurs introduced a more elaborate classification technique for spiral galaxies, based on morphological characteristics:
* Bars. Galaxies are divided on the basis of the presence or absence of a nuclear bar. De Vaucouleurs introduced the notation SA to denote spiral galaxies without bars, complementing Hubbleâs use of SB for barred spirals. He also allowed for an intermediate class, denoted SAB, containing weakly barred spirals. Lenticular galaxies are also classified as unbarred (SA0) or barred (SB0), with the notation S0 reserved for those galaxies for which it is impossible to tell if a bar is present or not (usually because they are edge-on to the line-of-sight).
# Rings. Galaxies are divided in to those possessing ring-like structures (denoted â) & thse without rings (denoted â(s)â. So-called âtransitionâ galaxies are given the symbol (rs).
# Spiral arms. As in Hubbleâs original system, spiral galaxies are assigned to a class based primarily on the tightness of their spiral arms. The de Vaucouleurs system extends the arms of Hubbleâs tuning fork to include several additional spiral classes:
* Sd (SBd) - diffuse, broken arms made up of individual stellar clusters & nebulae; faint central bulge
* Sm (SBm) - irregular in appearance; no bulge part
* Im - highly irregular galaxy
Most galaxies in these classes were classified as Irr I in Hubbleâs original system. In addition, the Sd class contains some galaxies from Hubbleâs Sc class. Galaxies in the classes Sm & Im are termed the âMagellanicâ spirals & irregulars, respectively, after the Magellanic Clouds. The Giant Magellanic Cloud is of type SBm, while the Little Magellanic Cloud is an irregular
The de Vaucouleurs technique retains Hubbles basic division of galaxies in to elliptical s, lenticulars, spirals and irregulars. To complement Hubbleâs technique, de Vaucouleurs introduced a more elaborate classification technique for spiral galaxies, based on morphological characteristics:
* Bars. Galaxies are divided on the basis of the presence or absence of a nuclear bar. De Vaucouleurs introduced the notation SA to denote spiral galaxies without bars, complementing Hubbleâs use of SB for barred spirals. He also allowed for an intermediate class, denoted SAB, containing weakly barred spirals. Lenticular galaxies are also classified as unbarred (SA0) or barred (SB0), with the notation S0 reserved for those galaxies for which it is impossible to tell if a bar is present or not (usually because they are edge-on to the line-of-sight).
# Rings. Galaxies are divided in to those possessing ring-like structures (denoted â) & thse without rings (denoted â(s)â. So-called âtransitionâ galaxies are given the symbol (rs).
# Spiral arms. As in Hubbleâs original system, spiral galaxies are assigned to a class based primarily on the tightness of their spiral arms. The de Vaucouleurs system extends the arms of Hubbleâs tuning fork to include several additional spiral classes:
* Sd (SBd) - diffuse, broken arms made up of individual stellar clusters & nebulae; faint central bulge
* Sm (SBm) - irregular in appearance; no bulge part
* Im - highly irregular galaxy
Most galaxies in these classes were classified as Irr I in Hubbleâs original system. In addition, the Sd class contains some galaxies from Hubbleâs Sc class. Galaxies in the classes Sm & Im are termed the âMagellanicâ spirals & irregulars, respectively, after the Magellanic Clouds. The Giant Magellanic Cloud is of type SBm, while the Little Magellanic Cloud is an irregular