GALAXIES: Previous generations of observers sometimes referred to galaxies as "star cities". This is an interesting moniker, but somehow falls a bit short of describing what we now know to be distant islands containing billions of stars, which are gravetationally bound and frequently accompanied by massive clouds or gas and dust (nebulae).
Apart from stars themselves, these are the most abundant type of celestial object visible to observers and imagers.These massive islands of stars are all quite distant as even our nearest galactic neighbor is 2.2 million light years away (12.83 trillion miles).
There are primarily six morphological types (Class) of galaxies; Elliptical, Lenticular, Spiral, Barred Spiral, Irregular and Peculiar. By far the most common are the Spirals, Barred spirals and Ellipticals.
These galaxies are further broken down into families, varieties and stages, which are assigned designations to indicate the dominant and secondary structures.
You may have noticed designations such as SB(s) bII in the galaxy image descriptions on this site. The SB represents the Class and Family; Spiral bar. The (s) describes the inner structure - "s" shaped, and the bII describes the development stage of the galaxy.
Because they are so distant, galaxies appear small and faint, but are neither. The brightest galaxies are supergiant ellipticals, which reach integrated absolute magnitudes of -24. This is roughly the equivalent of 1 trillion of our Suns and can span diameters or several thousand light years.
On the other end of the spectrum are the dwarf galaxies, which have absolute magnitudes of -9 or so. These are about the same size as many of the globular clusters.
Our own Milky Way compares pretty favorably with an absolute magnitude of -20.5 with a diameter of 150 million light years.
Galaxy congregate in larger formations called "galaxy clusters" and it is believed that there are correlations of these clusters into "super clusters".
Galaxies have 3 componants that are generally visible in larger telescopes and to a much greater degree in CCD images. Most galaxies have a star-like concentrated nucleus. It is generally accepted that a black hole exists at the center of most larger galaxies. These dense "cores" produce more concentrated light. The halo is generally a fainter less distinct region with various shapes as described above. Some of the brighter galaxies also exhibit dust lanes.
In CCD images star clusters and star forming nebulae can somtimes be resolved as well.