What are the three major categories of galaxies?

On 1930 the astronomer Edwin Hubble classified the galaxies into elliptical, spiral and irregular, being the first two classes the most frequent.  

However, it should be noted that this classification is based only on the visual appearance of the galaxy, and does not take into account other aspects, such as the rate of star formation or the activity of the galactic nucleus.  

The classification is as follows:  

Their main characteristic is that the concentration of stars decreases from the nucleus, which is small and very bright, towards its edges. In addition, they contain a large population of old stars, usually little gas and dust, and some newly formed stars.  

They are symbolized by the letter E and subdivided into eight classes, from E0 with zero eccentricity (spherical) to E7 (called husiform).  

They have the shape of flattened disks containing some old stars and also a large population of young stars, enough gas and dust, and molecular clouds that are the birthplace of the stars.  

They are symbolized with the letter S and depending on the minor or major development that each arm possesses, it is assigned a letter: a, b or c (for example: Sa, Sb, Sc, SBa, SBb, SBc).  

These galaxies, are also divided into two types:  

-Lenticular galaxies  

-Barred spiral galaxies

They are symbolized by the letter I (or IR), although they are usually dwarf or rare and do not have well-defined structure and symmetry.  

They are classified in:  

-Irregular type 1 (Magellanic), which contain large numbers of young stars and interstellar matter.  

-Regular type 2, less frequent and whose content is difficult to identify.  

This type of irregular galaxies are generally located close to larger galaxies, and usually contain large amounts of young stars, gas and cosmic dust.