Types of Galaxy
Galaxies can be classified into several different main types:
Spiral galaxies are what many people think of as as galaxies, though actually less than 30% of galaxies are spiral. The masses of spiral galaxies range from about a billion to a few trillion solar masses and similarly for luminosity.
Elliptical galaxies are the most common type of galaxy – about 60%. The masses of elliptical galaxies range from about 100,000 to over 10 trillion solar masses and similarly for luminosity. Elliptical galaxies are typically made up of very old stars.
Irregular galaxies are the least common of these three types – about 15%. The masses of irregular galaxies range from 10 million to 10 billion solar masses and similarly for luminosity. Irregular galaxies tend to have a lot of gas and dust and a high proportion of young stars.
The above list is not exhaustive. There are a group which do not fall into any group – maybe because they are two galaxies merging together, or evolving from one of the groups into another for example.