Yerkes classification system

The Yerkes classification system of galaxies was devised by W.W. Morgan, former director of the Yerkes Observatory of the University of Chicago, in 1958 (Publications of the Astronomical Society of the Pacific 70, 364). It was designed to classify galaxies purely according to morphology, and in turn to isolate the physical differences between different classes of galaxies. It classifies galaxies with three parameters:

• Concentration class -- how centrally concentrated is the galaxian light?
• Form-family -- what is the fundamental type of galaxy?
• Shape -- does the galaxy appear circular or elliptical on the plane of the sky?

Concentration class

This parameter uses the letters a, g, f, and k, with a indicating diffuse galaxies, and g, f, and k indicating progressively more centrally-concentrated light distributions.

Form-family

This parameter describes the overall shape of the galaxy. Although Morgan defined this parameter in his 1958 paper, it was revised in 1975 by Morgan, Kayser, and White (Astrophysical Journal 199, 545) as follows:

• S - Spiral galaxy
• B - Spirals with central bars
• E - Elliptical galaxy
• I - Irregular galaxy
• R - galaxies showing some form of rotational symmetry, not clearly spiral or elliptical
• N - Galaxies with bright nuclei against a faint host galaxy; further subdivided into N- (weak nuclei) and N+ (extremely bright nuclei)
• Q - unresolved, high-redshift galaxies
• C - small, barely resolved galaxies with large surface brightnesses
• D - elliptical galaxies with large, diffuse envelopes. Also, cD galaxies are defined as the supergiant D galaxies, commonly found at the centers of clusters of galaxies, and are the products of mergers of two or more large galaxies.
• db - dumbbell galaxies, another subset of D galaxies with two distinct nuclei

Shape

This parameter is simply a number between 1 and 7, designating the degree of ellipticity, with 1 being circular, and 7 being highly elliptical.

The Yerkes system is less commonly used than the Hubble and de Vaucouleurs classification systems, I suspect more out of habit than anything else. However, the Yerkes system is often used in studies of galaxy clusters. The designation cD is often made for the massive central galaxies in clusters, for example. N galaxies are also mentioned, though often the N galaxies are usually simply classified into types of active galaxies -- Seyfert galaxy, AGN, quasar, blazar, and so on.

Sources: Galactic Astronomy by D. Mihalas and J. Binney; Morgan, Kayser, and White, obtained from http://adsabs.harvard.edu/