Spiral Galaxies like our galaxy are the most eye catching and the type that led Wright, Kant and others to speculate on "other universes". A glance at M31 or M33 or any of the many photographs available tells how they got their name. Like adding cream to stirred coffee.

The spirals divide into two major types, the regular spirals (S) and the barred spirals(SB). About a fifth of the spirals are SB. Depending on how tightly the arms are wound the classes are further subdivided "a" for tightest wrapping through "c" for most open. So "SBab" would indicate a barred spiral with tight arms, intermediate between the tightest "a" and tight "b" class. Both kinds are similar to our Milky Way having spiral arms in a disk surrounding a central bulge all in a halo of old stars and globular clusters.

The spiral galaxies all show rotation curves (21cm) that remain "flat" almost as far as the eye (I mean dish) can see suggesting a huge dark corona containing at least twice as much mass than is visible and up to ten times as much according to observations of tidal disruptions of companion galaxies (like our LMC and SMC) and the virial of clusters of galaxies shows the same result that dark matter has ten times more mass than luminous (including radio) matter. There is a relatively narrow range of sizes for spirals, none much larger or smaller than our Milky Way and there is some suggestion that the smaller spirals contain more dark matter than the larger making the total mass range even narrower.

Elliptical galaxies are the simplest class, a simple cluster of stars not unlike our Milky Way with the disk removed. Usually rather featureless and not unlike squashed globular clusters, they are subclassified in terms of a flattening parameter 10(a-b)/a where a is the major and b the minor axis. This is not the eccentricity, it is simpler and I apologize to any mathematicians. So if a=b the class is E0 and if a is twice b the galaxy is E5. The most elliptical galaxies are E7 (a=3.3b) since more flattened systems show evidence of a disk but no arms. Such galaxies are labeled S0 or SB0.

The size range of the ellipticals is enormous. They range from giant ellipticals such as M87 in Virgo (giant ellipticals are invariably found at the centers of large clusters) down to ... single stars in intergalactic space? Many of our companions are dwarf ellipticals such as the E3d Sculptor galaxy.

The ellipticals are by far the most abundant in the cosmos but of the catalogued galaxies 77% are spirals, 20% ellipticals and 3% irregular.

Irregular galaxies (Irr I or II) are just what the name implies. Both the LMC and SMC are considered Irr I though some consider the LMC to be a small barred spiral. The Irr mostly exhibit gas and dust. If there is much gas and OB associations the galaxy is labeled Irr I while galaxies with lesser amounts of gas and young stars but conspicuous dust lanes are labeled Irr II such as M82.

Hubble devised a "tuning fork" classification of galaxies like:

                       |--S0----Sa----Sb----Sc
            E0---E4---E7
                       |--SB0---SBa---SBb---SBc

Of course in addition to the Hubble and Irr classes there are a number of unusual galaxies that just don't fit in such as the supergiant E0 or E1 galaxies frequently found at the centers of large clusters of galaxies labeled cD galaxies. cD galaxies are often strong radio sources and are sometimes called "cannibal galaxies". Compact galaxies are small but bright galaxies, usually elliptical or Irr, often called Starburst Galaxies. N galaxies are usually spiral galaxies with a very bright stellar nucleus belonging to a class of galaxies with active galactic nuclei AGN. More extreme AGN's are the Seyfert Galaxies first described by Seyfert. Seyfert's are spirals with a bright nucleus whose spectrum shows broad emission lines indicating hot gas. Like N galaxies they may be strong radio emitters.

Radio Galaxies are galaxies which may emit more radiation at radio than at optical wavelengths. Some radio galaxies appear normal but many are Seyfert or AGN galaxies. The giant elliptical M87 looks normal but the core, studied in detail when found to be a radio galaxy, exhibits AGN features such as an energetic jet blasting material out of the nucleus. "Cygnus A", the first extragalactic object identified as a radio source in 1951, is a complex source of radio waves identified with the Cygnus cluster at a distance of a billion light years. Unless identified with a known object these sources are known by their Cambridge catalogue number such as 3C271.

The structure of the sources can be seen by very long baseline interferometry (VLBI) or aperture synthesis techniques. Many strong sources are point like but the majority have most of the radiation coming from extended regions or jets on either side of the galaxy. 3C236 for example has radio jets that are 2Mpc either side of the galaxy.

Interacting or Colliding Galaxies are not uncommon. Big galaxies, not like the LMC interacting with the milky way which are very common. A good example is The Antennae NGC4038 and NGC4039, two interacting galaxies throwing off "tails" of material. Alar and Juri Toomre have made computer simulations of interacting galaxies that mimic the strange appearance of many interacting pairs of galaxies.

A glance at the Palomar Sky Survey shows that the galaxies are not distributed at random. Even when interstellar dust is taken into consideration (the zone of avoidance) we see the universe has structure. But first we have to see how far away the galaxies are.