It is not speed, but variations in speed that cause density waves.

并不是速度高低引起了拥堵波，而是速度的不一致性。

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Last go-round, M82's gravity likely raised density waves rippling around M81, resulting in the richness of M81's spiral arms.

最近一个周期中，M82的重力增强了M81周围密度波的脉动，导致M81的旋臂更浓密。

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Gravity, however, causes density waves to move out through the galaxy which in turn triggers the formation of hot, bright young stars, producing the ring that we see in this picture.

引力会使密度波穿透星系，继而形成炽热、耀眼的年轻恒星，产生我们在图片中看到的光环。

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If the gas sheet had the same density throughout, radio waves would pass through it without distortion, like light through a window pane.

ECONOMIST: Pulsars: In a wink | The

Introduce variations in density into the gas, however, and the waves will be bent and scattered, just as they would be by irregularities in a pane of glass.

ECONOMIST: Pulsars: In a wink | The