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。

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

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本文从密度波理论出发，讨论三维盘状星系的旋臂结构。

In this paper we discuss the spiral structure of dish galaxies in three-dimension according to the density wave theory.

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