Hubble proposed that galaxies be classified according
to their appearance.
M31, the Andromeda Galaxy
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 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
|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.|
Like the Large and Small Magellenic
Clouds, nearby companions to our own Milky Way
|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)