Galaxies can be classified into several different main types:
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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. |
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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. |
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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. |
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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.