Around the center of the galaxy: I guess it's made of stuff that was circling around the center of the galaxy fast enough to not fall into the supermassive black hole at the center of the galaxy.

Around the sun: I guess it's also made of stuff that was circling around the sun fast enough to not fall into the sun.

Around itself with its current direction and speed: I have no idea, this part might be outside what my brain can understand without learning more math or something.