How well defined is your son? Even if the group of atoms which he is composed of is not solid for 1 billion years, there is a way to approximate his average ang

How well defined is your son? Even if the group of atoms which he is composed of is not solid for 1 billion years, there is a way to approximate his average angular velocity around the center of the galaxy. They chose a bunch of stars on the outer rim of a galaxy, and they all had a similar angular velocity, despite large differences in their radii. Here they define it as the average angular velocity on the outermost edge of the galaxy.