What is the difference between a group of stars and a galaxy?

There is a fine line that tells us what distringuishes them: dark matter.

Every galaxy is thought to have formed within a dark matter halo, a sphere of dark matter that governs things like the formation and shape of the galaxy, as well as how the stars move within it. At the very low mass range of halos, extremely small galaxies called “ultrafaint dwarf galaxies” can form. However, the existence of these tiny dark matter halos and thus, ultrafaint dwarf galaxies, are extremely sensitive to the specific properties of what dark matter is.

Using data of three potential satellite ultrafaint dwarf galaxies from the Very Large Telescope, I am approaching the goal of my project: analyze data from the potentially smallest galaxies in the universe to measure their dark matter halo mass, and use this as a testing lab for dark matter microphysics.