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The Hubble galaxy classification scheme is subjective, but:
(1) A spiral galaxy with a small bulge is a lenticular galaxy.
(2) A galaxy shaped like a cigar is an elliptical galaxy.
(3) A galaxy that looks like the remnant of an explosion is an irregular galaxy.
(4) A galaxy with well-defined spiral arms is a spiral galaxy.
Note that a sprial galaxy might appear elliptical if seen edge on, and lenticular galaxies can also be mistaken for ellipticals. Subclassifications within the types help distinguish galaxy classes -- for instance the tighter the spiral arms are wound give different classifications like Sa,Sb,Sc,Sd etc...
The sub-classifications are due to De Vaucouleurs; they take into account the difficulty in placing many galaxies into perfect classes.
SA are spiral lenticular galaxies.
SAB are weakly barred spiral galaxies.
SB are barred galaxies.
SO is reserved for galaxies where it cannot be determined if there is a bar or not; the bar would be in the line of sight.
Sd are diffuse spiral galaxies while Sm are irregular spiral galaxies.
In addition, numbers from -6 to 10 can be assigned that take into account the age of the galaxy.
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