Galaxies
The universe is full of galaxies. Early in the twentieth century, Edwin Hubble sought to create a system to classify the galaxies. He came up with essentially three different categories; Spirals, such as the Milky Way, Ellipticals, both giant and dwarf, and Irregulars. Every galaxy we see can fit into one of those categories.
When one looks at the Hubble Classification Scheme, or its updated version, it becomes apparent that there are only a few different ways to make a galaxy. Here is a summary of some of the properties of galaxy types. Here is a remarkable trend in the universe. When one examines larger and larger structures in the universe, they tend to become simpler and simpler. Let's look at some famous galaxies and see if you can identify their type.
Galaxies tend to appear in groups. Our part of the unverse has a group of galaxies called the Local Group. Our home galaxy, the Milky Way, is one member of this Local Group. The Local group is a collection of two dozen or so galaxies which are gravitationally bound to each other. Most are small, either dwarf ellipticals or irregulars. Only a few of the members are spiral galaxies, the most notable being the Great Galaxy in Andromeda. This near-twin of the Milky way is more two million light years distant. Another spiral in the group is the relatively small galaxy called M33 in Triangulum.
Many of the galaxies visible in small telescopes are included in Charles Messier's list of non-stellar objects. How many galaxies have we actually imaged? The Hubble Deep Field project can give you some idea.
Galaxies do occasionally collide. Here is some fun with galaxies!
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Updated May 4, 2011