They have a center,and arms of matter that seem to circle the center like a pinwheel.

VOA: special.2009.04.29

When a particle moves in a circle, it has an acceleration towards the center of this size, v^2 over R.

当一个质点在圆周上运动时,它有一个指向圆心的加速度,v^2 / R

耶鲁公开课 - 基础物理课程节选