The following glossary includes terms that are used in the student texts for this activity. You will probably want to study the terms and their definitions before you read these student texts.

Compression
Compressions are regions in which sound waves are squeezed together. The effect of compression is to increase the frequency of sound waves, making peaks and troughs of sound waves pass a given point in space very rapidly.

Galactic Clusters
Galactic Clusters are cosmic structures in which many galaxies are found in close proximity to each other.

Galaxy
A Galaxy is a large aggregation of usually over a million stars, bound together by gravity. There are three major classifications of galaxies—spiral, elliptical, and irregular.

Gravity
In Newtonian physics, gravity is the universal, mutual attraction that all massive objects have for one another. Its force is directly proportional to the mass of each object, and decreases by the square of the distance separating the objects. This force is also called gravitational attractive force.

Rarefactions
Rarefactions are regions in which sound waves are spread out. The effect of rarefaction is to decrease the frequency of sound waves because peaks or troughs are separated by greater distances.