Summary: ASTR 1120 Section 1 (3 credit hours): Spring 2006
SUMMARY OF KEY CONCEPTS: GALAXIES AND COSMOLOGY
Lecture #17 textbook Chapter 20
We discussed galaxies a galaxy is an island of stars held together as a single unit by gravity. A
large galaxy, such as the Milky Way galaxy that the Sun is part of, contains around 100 billion
stars. There are about 100 billion galaxies in the observable Universe.
Galaxies are observed to often live in groups or clusters of galaxies. There is no clear
demarcation between these, but a group normally refers to a system with a handful of large
galaxies while a cluster is a much richer structure sometimes containing hundreds of large
galaxies. The Milky Way belongs to the Local Group which also includes Andromeda, the
nearest large galactic neighbor to the Milky Way.
An important distinction between stars and galaxies is that galaxies are separated by distances
that are typically around 10 times the size of a galaxy. This means that galaxies are packed
together relatively tightly, and as they orbit around under the action of gravity they often
collide with each other. Those collisions lead to mergers, and allow large galaxies to grow by
swallowing smaller ones. This is unlike the situation for stars stars are extremely small
compared to the typical separation between stars and as a result stars almost never collide.
Galaxies are classified based primarily on their appearance. The main distinction is between
elliptical galaxies and spiral galaxies (the Milky Way is a spiral), with the latter being further
divided into barred and unbarred spirals. Elliptical galaxies are smooth, `round', and rather