Galaxy Taxonomy

Astronomers have classified galaxies into different groups, based largely on their appearance in photographs. There's quite a variety, as this section of the Virgo Cluster shows:

Let's look at a few galaxies:

The basic groups may be distinguished in several ways:

                  Elliptical             Spiral              Irregular
------------------------------------------------------------------------

looks like        sphere                spiral             train wreck
                  squashed sphere       pinwheel
                                        barred wheel

stars colors      reddish               blue in arms       mostly blue
                                        red in bulge
                                        red in halo

star ages         old                   young in arms      mostly young
                                        old in bulge
                                        old in halo

gas/dust?         not much              in arms            lots, all over

The "Hubble Tuning Fork" classification scheme is shown below; it separates

elliptical galaxies into classes E0 - E7, depending on their roundness
spiral galaxies in regular (S) and barred (SB) classes,
- subdivided into categories Sa - Sb - Sc and SBa - SBb - SBc depending on the size of the bulge and smoothness of the arms

Click on the diagram to see a larger version, in which the text in captions is legible.

For examples of each type of galaxy, look at this montage of galaxies .

Good sites for pictures of galaxies:

Hubble Heritage Gallery of Galaxy images
Spiral Galaxies from the Astronomy Picture of the Day
Elliptical Galaxies from the Astronomy Picture of the Day
SEDS gallery of Messier galaxies
SEDS gallery of non-Messier objects