描述: 彗星是太阳系中的小天体，由不同类型的冰、岩石和尘埃物质混合组成的核——一个“脏雪球”。彗星的核大小可以从几百米到几十公里不等。大多数彗星的轨道是高度椭圆化的。当彗星接近太阳时，部分表面冰层会被蒸发，并被太阳风吹向后方，形成独特的彗发和彗尾。我们之所以能够看到彗星，是因为阳光反射到彗发、彗尾，或者是（对于远离太阳的彗星）彗核上。彗星根据其轨道周期分类，如果其轨道周期已经被多次观测到或已知小于200年，则被分类为“周期彗星”或“短周期彗星”；否则，则被归类为“非周期彗星”。
相关词条
彗发
彗核
彗尾
太阳系小天体
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