彗发（Coma）：彗发是彗核的蒸发物，其形状和大小与距离太阳的远近密切相关。一般来说，离太阳越近，彗发越亮越大，直径可达数十万千米，有时还可与太阳相当，甚或超过太阳直径。
彗星气态彗发里的分子被阳光激发而发光。
The molecules of the comet's gaseous coma fluoresce in sunlight.
很多彗星接近太阳时，我们可以用肉眼看见，因为它们的彗发和彗尾可以反射阳光，甚至因为受阳光激发而自发光。
We can see a number of comets with the naked eye when they pass close to the sun because their comas and tails reflect sunlight or even glow because of energy they absorb from the sun.
但一些彗星的彗发宽度可达将近100万英里(160万公里)，一些彗尾长度则可达1亿英里(1.6亿公里)。
Some comets have comas that can reach nearly 1 million miles (1.6 million kilometers) wide, and some have tails reaching 100 million miles (160 million kilometers) long.
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