The Local Group and Other Galaxy Clusters

Armed with an ability to measure very great distances, we can begin looking at the relationships among galaxies.

Within 1 million parsecs (3 million light-years) of the Milky Way lie about 20 galaxies, the most prominent of which is Andromeda (M31). This galactic grouping, called the Local Group, is bound together by gravitational forces.

The generic name for our Local Group is a galaxy cluster, of which thousands have been identified. Some clusters contain fewer than the 20 or so galaxies of the Local Group, while some contain many more. The Virgo Cluster, an example of a rich cluster, is about 15 million parsecs from the Milky Way and contains thousands of galaxies, all bound by their mutual gravitational attraction. Giant elliptical galaxies are often found at the centers of rich clusters where there are very few spirals.

From the velocities and positions of galaxies in clusters, one thing is very clear. We cannot directly observe at least 90 (perhaps as much as 99) percent of the mass that must be there. Galaxy clusters, like the outer reaches of spiral galaxies, must be teeming with dark matter.

The Local Group is a galaxy cluster, a gravitationally bound group of galaxies, which includes the Milky Way, Andromeda, and other galaxies.

Star Words

The Local Group is a galaxy cluster, a gravitationally bound group of galaxies, which includes the Milky Way, Andromeda, and other galaxies.

The galaxy cluster called Abell 2218 is about 2 billion light years from the earth. When these photons left the cluster, life on earth was very simple indeed. The effect of the "unseen " mass in this cluster can be seen as the arcs of light that are "lensed" light from background sources.

(Image from NASA, A. Fruchter, and the ERO team)

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