Galaxy Morphology

There are lots of galaxies out there, and they come in many different flavors.

Hubble proposed that galaxies be classified according to their appearance.

Spiral Galaxies

Spiral galaxies are disk shaped galaxies with spiral structure. Hubble classified them as Sa, Sb, Sc:

Sa: Large central bulge, smooth, tightly wrapped spiral structure

M104, the Sombrero Galaxy

Sb: Less noticable bulge, looser spiral structure

M31, the Andromeda Galaxy

Sc: Weak or no bulge, open spiral structure, very knotty appearance

M33, the Pinwheel Galaxy

There is a parallel sequence for spiral galaxies that have central bars: SBa, SBb, SBc

The Milky Way is probably an Sbc or SBbc, something like this galaxy, M83.

Elliptical Galaxies

Elliptical galaxies are much smoother, spheroidal galaxies. Hubble classified them as E0 - E7, where the number refers to their axis ratio 1-(b/a). [E0=round, E7=very squashed]. This is M87, an E0 galaxy in the center of the Virgo Cluster.

And here is NGC 4881, another E0 which sits in the Coma Cluster. Notice the other galaxies...

Note that the observed shape may have nothing to do with the true shape, due to projection effects:

From morphology alone we can't tell if elliptical galaxies are

spherical (a=b=c)
prolate (a>b=c), a cigar
oblate (a=b>c), a hamburger bun
triaxial (a>b>c)

S0 or Lenticular Galaxies

S0's (or SB0's) are "transitional" (morphologically!) between spirals and ellipticals. They are disky, but very smooth like ellipticals, and have no spiral arms.

Irregular Galaxies

Galaxies which don't "fit the mold"

Like the Large and Small Magellenic Clouds, nearby companions to our own Milky Way

Peculiar Galaxies

Galaxies which REALLY don't fit the mold!

Hubble put these all together into his famous "tuning fork" diagram of galaxy classification:

Hubble believed this was an evolutionary diagram. It's not!

Here is a discussion of the pitfalls associated with using Hubble typing to discuss galaxy evolution (courtesy Greg Bothun, University of Oregon)