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

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That means the acceleration is pointing towards the center of the circle and it has a magnitude a.

也就是说加速度,始终指向圆心,其模长为 a

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