Description: 北极星是距离北天极最近的亮星（1度以内）。它的正式名称是小熊座α星，通常被称为北极星，在中国也叫勾陈一、北辰、宸极。北天球的所有恒星都似乎围绕着它旋转，因此它是进行导航和天体测量的绝佳固定点。它的地平高度可以提供观测地点的大致纬度。不过，由于地球自转轴的进动，北极星在天球上的位置正在慢慢发生变化，因此再过几个世纪，北极星就不再能指示北天极的位置了。北极星是一个由三合星系统。主星是一颗黄超巨星，被命名为北极星Aa；伴星是一颗较小的主序星，被命名为北极星Ab。它们在非常近的轨道上相互绕转运行。这对恒星还有一颗较大的伴星——北极星B，它是一颗主序星，在2400个天文单位的距离上运行，轨道周期约为29.3年。北极星B可以用普通望远镜分辨出来。哈勃太空望远镜能够分辨出北极星三合星系统的所有三个成员。北极星的视星等会有波动，这是因为北极星Aa是一个仙王座变星。北极星系统距离地球约447光年。

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