Description: 當彗星靠近太陽時，太陽輻射會加熱彗星表面。表面的冰變成氣體（"升華"），同時帶走岩石和塵埃物質。由此產生的混合物在彗核周圍形成一團雲，這就是彗發。一般來說，彗星會有兩條尾巴：噴射出的塵埃粒子形成彗尾，彗尾呈典型的彎曲狀。它由從表面釋放出來的塵埃粒子組成，這些塵埃粒子沿著彗星的軌道繞太陽運行。塵埃尾的長度可達數百萬公里或更長。它們反射太陽光，如果條件合適，肉眼能觀測到的彗星大部分都是由它們的白色漫射形狀組成的。

相當一部分氣體被太陽風--太陽發出的帶電粒子--吹走並電離。這些離子形成了彗星的離子尾，通常呈藍色。離子尾總是直指太陽。如果彗星本身正在遠離太陽，離子尾就會在彗星之前。

Related Terms:

• 彗發
• 彗星
• 彗核

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