galaxy classification

The commonly-used galaxy classification indicates a galaxy's morphology (galaxy morphology), i.e., shape and overall structure, based upon its visual appearance. Edwin Hubble developed the basic classification still used, the Hubble types, known collectively as the Hubble sequence or Hubble classification (though current usage includes refinements subsequent to Hubble's work, e.g., by Allan Sandage and Gérard de Vaucouleurs):

• elliptical galaxy: E0 through E7, the smaller the number, the closer to spherical.
• spiral galaxy: S or SB (for barred) or SA (to specifically indicate unbarred) or SAB (for something between barred and unbarred, an intermediate spiral galaxy) followed by a, b, c, or d, from the tightest winding of arms to the loosest. Pairs ab, bc, and cd indicate between a and b, etc.
• lenticular galaxy: SO, SAO or SBO: bright central bulge in disk but no visible spiral, the A and B for unbarred and barred.
• irregular galaxy (or subsequently described as amorphous): Irr, Im, or sometimes just I.

(The term disk galaxy for a disk-like shape includes the above spiral and lenticular galaxies.) Hubble laid out the types in a Y-like diagram in the shape of a horizontal tuning fork and you do see references to the classification-structure as a tuning fork. Among the improvements over time have been suffixes and some prefixes, to specify more detail, often describing galaxies formerly classified as "irregular":

• m suffix to SA or SB - spiral but irregular, such as Large Magellanic Cloud (LMC), which is classified as "SBm", "m" for "Magellanic type spiral".
• (R) prefix - outer ring, i.e., a ring galaxy.
• r suffix - "inner ring":
• s suffix - does not have an inner ring, and rs indicates something between.
• t suffix - with tidal arm, a spiral arm presumed distorted by tidal forces.
• pec as a subsequent word - peculiar: odd in some other way.
• Sy as a subsequent word - Seyfert.
• subsequent Roman numeral, i.e., I, II, etc. - luminosity class analogous to stellar luminosity classes; note that V designates a "dwarf galaxy" luminosity. Inclusion of this is termed the van den Bergh galaxy classification or DDO classification, developed for the David Dunlap Observatory Catalog.

Parenthesis around suffix letters appear to be a means of distinguishing them, e.g., the parenthesis in "(s)m" makes clear that there isn't some specific meaning to the pair, "sm". Dwarf galaxies have their own terms: dE, dS, and dIrr for dwarf elliptical, spiral, and irregular, and dSph for dwarf spheroidal galaxy. Some example galaxies/types:

• LMC: SB(s)m
• Small Magellanic Cloud (SMC): SB(s)m pec
• Andromeda: SA(s)b
• Triangulum Galaxy: SA(s)cd
• Canis Major Dwarf Galaxy: Irr
• Circinus Galaxy: SA(s)b
• ESO 137-001: SBc
• M64: (R)SA(rs)ab
• M74: SA(s)c
• M82: I0
• M87: E0 pec
• NGC 1600: E3
• NGC 253: SAB(s)c
• NGC 2770: SBc
• NGC 3314 (a pair of galaxies): SBbc and SAab
• Sagittarius Dwarf Irregular Galaxy: IB(s)m V
• Sagittarius Dwarf Elliptical Galaxy: dSph(t)
• Ursa Major II Dwarf: dSph

Another classification system is the Morgan classification, and one of its classifications, cD, is still commonly seen for central elliptical galaxies. There also exists some classification specific to radio galaxies.