In the early years of the 20th Century, the prevailing view appears to have been that there was only one galaxy - our Milky Way. Although M31 and other galaxies had appeared on Messier's List of nebulas ever since the 18th Century, they appear not to have been widely recognized as galaxies as such in their own right.
In 1923, the 'Great Debate' was staged to try and solve the problem as to whether other 'Island Universes' did actually exist, as some astronomers were coming to believe. The outcome of this debate appears to have been inconclusive. It was the work of Hubble that finally brought the matter to a definite conclusion, in 1924 - he made use of Cepheid variables to deduce that M31 was external to our galaxy.
Today, in contrast, the Universe is assumed to contain around 100 billion galaxies, each having typically 100 billion stars.
The above diagram is from Hubble's own book, The Realm of the Nebulae.
Broadly speaking, Hubble classified all galaxies into Spirals, Ellipticals and Irregular Galaxies. Although originally there appeared to be a tendency to view spirals and ellipticals as being different stages of a galaxy's evolution, spirals and ellipticals now tend to be viewed as totally separate types of galaxies (there are however theories that purport to make a connection between the two types, although these are coming from a 'different direction' from before).
Ellipticals are classified according to their degree of ellipticity
Spirals are first divided into Barred and Normal (plus a class called S0). Each of these sub-classes is then split into three types, as shown above.
The S0 is considered a 'spiral' with no arms.
Spirals contain both young and old stars, and a fair amount of dust and gas.
The adjacent image shows the Milky Way but its basic features are typical of all spiral galaxies
The spiral arms actually delineate regions of bright objects and stars rather than areas of increased number of stars. The density of stars is the same throughout the entire disk
The nature of the arms is still a bit of a mystery. They cannot be rotating as independent entities because they would soon 'wind-up'. Present-day theories commonly attempt to describe a pressure wave maintaining the spiral arm pattern as it appears.
The bulge is populated primarily by older Population II stars.
The halo is a low density spherical region.
The younger population I stars are to be found predominantly in the disk
The first spiral to be actually recognized as a spiral was M51 (the Whirlpool Galaxy) in 1845, although it was not known to be a galaxy at that time. It was seen from the 1.83m reflector at Birr Castle in Ireland (run by the 'Earl of Rosse')
M31 (NGC 224) is similar to the Milky Way. It is 2.2 million light years away and is the most distant object we can see with the unaided eye.
Barred spiral are much less numerous than 'conventional' spirals.
Elliptical Galaxies contain mostly Population I stars. They have little dust and gas, as expected.
The range in size is large - from 100 thousand solar masses to 10 million solar masses. Their dimensions range from about a tenth of the diameter of that of the Milky Way to ten times the diameter of the Milky Way.
This covers anything that doesn't fit into the two main groups.
The best known examples are the Magellanic Clouds at 46 kpc and 63 kpc , the well-known satellite galaxies of our own galaxy.
The ISM would be classed as a vacuum if such conditions could be replicated on Earth. It fact, it is a better 'vacuum' than those that are produced on Earth.
It is mostly Hydrogen and can be classified in the following manner to a first approximation
ISM is the raw material for new stars.
It is seeded by supernovae with heavier elements
It also contains organic molecules
Clusters of Galaxies
There are two main classes of cluster
- Regular Clusters relatively compact, with a concentration of
density towards the center. Contains mostly ellipticals and S0s.
Many emit radio waves from active galaxies, as well as intergalactic gas. Some intergalactic
is at temperature of 100 million degrees, emitting in X-rays.
- Irregular Clusters Little central concentration. Mostly composed of spirals and irregulars. Fewer radio waves and X-rays, and less hot gas.
Clusters of galaxies are grouped together into superclusters.
These superclusters appear to form into 'sheets' of galaxies surrounding voids, giving a 'holy' effect.
The Great Wall is the largest supercluster known.