galaxy classification

Astronomers classify galaxies based on their overall shape (elliptical, spiral or barred spiral) and further by the specific properties of the individual galaxy (for example degree of ellipse, number of spirals or definition of bar). The system of galaxy classification is called the Hubble "tuning fork" diagram, and is the Hubble sequence.

The "tuning fork" system

The Hubble "tuning fork" diagram starts from the left with elliptical galaxy as its base. Elliptical galaxies can be named from E0 to E7. E stands for elliptical while the number indicates how oval-shaped the ellipse is with 0 being ball shape to 7 being discus shape. After the elliptical galaxies the diagram splits into two branches. The upper branch covers spiral galaxy also called lenticular galaxies. It starts off with S0. The "S" means lenticular, the "0" means no arms, and the subscript number indicates how heavily a stripe is absorbed out of the image of the galaxy by dust in the galactic disc. On the same branch are the next 3 types which all have spiral arms. The "S" here also means lenticular, but the lower case letter after it tell how wound up the arms are. They range from "a" to "d" having the following meanings:

Sa - tightly-wound, smooth arms, and a bright central disc
Sb - better defined spiral arms than Sa
Sc - much more loosely wound spiral arms than Sb
Sd - very loose arms, most of the luminosity is in the arms and not the disc

The lower branch of the diagram covers spiral-barred galaxy given the symbol "SB". This branch starts with SBO galaxies which is followed by a subscript number that indicates how heavily defined the bar is. After that the branch continues with the SB galaxies which have lower case letters after them that indicates how heavily defined the bar is. They range from "a" to "c" having the following meanings:

SBa - a bright center and tight spirals
SBb - better defined arms than SBa galaxy and are more loosely wound
SBc - even looser arms, and a much dimmer central portion of the galaxy

The de Vaucouleurs system

There is an extension to the Hubble sequence that is in wide use, these are the de Vaucoleurs extensions.

Galaxy Classification

The "tuning fork" system

The de Vaucouleurs system

See also