Photo of a Shell Galaxy
Astronomers don’t know exactly what a Shell Galaxy is, or what causes it, but they do know where to find them and how to photograph them.
This photo shows the structure of galaxy NGC 474. The “Shell Galaxy” description is due to stars creating the well-defined structures through some as-yet-not-understood process. Some speculate that the structure is a result of galaxies colliding with one another over hundreds of millions of years in interactions called tidal stripping, as galaxies rip each other apart.
The full-size image can be seen here: [Link].