Our Group of Galaxies
Edwin Hubble first used the term “Local Group" to describe the isolated group of about a dozen nebulae, or island universes that could be observed around our galaxy. These galaxies, as we know them today, showed a blueshift in their spectra as compared to the other galaxies that were further away and whose spectra exhibited a redshift.
Our group is defined by its two largest members, the Milky Way and Andromeda (M31) galaxies. The center of mass for the group lies between these two galaxies.
Each galaxy has a collection of satellite galaxies that accompany it. The more famous ones for the Milky Way are the Large and Small Magellanic Clouds, which are easily seen in the Southern Hemisphere, and Barnard’s Galaxy, NGC 6822, a barred irregular galaxy.
Andromeda has M32, M110, and the spiral galaxy Triangulum (M33) also known as the Pinwheel galaxy, which is the third largest galaxy in the group and about half the size of the Milky Way. M33 is about 3 million light-years from our galaxy.
The galactic cluster is loosely bound within a volume that extends over a 10 million light year diameter. Contained in this volume of space are:
- 3 spiral galaxies
- 2 elliptical galaxies
- 12 irregular galaxies
- 4 dwarf elliptical galaxies
- 22 dwarf spheroidal galaxies
- 5 galaxies of unidentified class.
In time, Andromeda and the Milky Way will close the 2 million light-year gap that separates them and eventually collide. It will take about 3 billion years before this will happen, so it’s not something we have to worry about today. For an idea of what we are going to miss, take a look at this short video at the Hayden Planetarium’s web site.
For a review of the some of the galaxies in the our group, check out the article "Survey of Our Nearest Galaxies."