Galaxy Zoo: What the Shell?

Galaxies grow over time through collisions with other galaxies. When a small galaxy hits a large galaxy in a head-on collision, its stars are released into the bigger galaxy. As these stars turn around in their orbits, like a bungee jumper reaching the end of their rope, they slow down and make large rings. We call these structures "shells".

Shells are special because the strength of gravity is equal all along each shell. This means that the shell's shape can tell us about the shape of the galaxy's gravitational field. Gravity comes from both stars and dark matter. Since we can see the stars, the shells can help us work out how much dark matter is in the galaxy, and where it is.

In this project, you'll be shown images of galaxies. It's your job to determine whether they contain one or more shells. If they do, you'll be asked to draw over the edge of each shell you see.

This project is led by researchers of the Minnesota Institute for Astrophysics, at the University of Minnesota Twin Cities. Image credit for home page: DESI Legacy Imaging Surveys & NSF’s NOIRLab