Galaxy Groups and Clusters: Practice Problem Solutions

1. A group of about 80 galaxies about 10 million light-years across, including the Milky Way, Andromeda and Triangulum, plus many dwarf galaxies.

2. They will pass close and probably merge into a single large elliptical galaxy in several billion years; the stars themselves will almost never hit each other.

3. A group of hundreds or thousands of galaxies held together by gravity, such as the Virgo Cluster or the Coma Cluster.

4. The galaxies were moving much faster than the visible mass could hold, so there must be far more mass than we can see: dark matter.

5. A massive object (like a galaxy cluster) whose gravity bends and magnifies the light from more distant galaxies behind it. Astronomers use it to see faint, far galaxies and to map dark matter.

6. The large-scale pattern of galaxies lying along filaments and sheets around huge empty voids.

7. Star cluster, galaxy, galaxy cluster, supercluster.