彗尾和彗发是彗星在内太阳系受到太阳照射，从地球可以看见的结构，是由直接反射阳光的灰尘和从发射出光辉的离子化气体两种形成来源结合成的。多数的彗星都很暗淡，必须用望远镜才能看见，但是每十年左右，都会有几颗亮到可以用裸眼直接看见的彗星。

每颗彗星的气体和尘埃喷流形成的彗尾都是独特的，指向的方向也都略有不同。尘埃尾会被拖曳在彗星轨道的后方，他经常会因为曲线的形状而形成反尾。同时，由气体构成的离子尾永远都指向背向太阳的方向，因为这些气体受到太阳风的影响远比尘埃来得强烈，跟随的是磁力线，而不是轨道的路径。从地球观测的视差有时会使彗尾看似指向相反的方向。

A comet tail—and coma—are features visible in comets when they are illuminated by the Sun and may become visible from Earth when a comet passes through the inner Solar System. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. Separate tails are formed of dust and gases, becoming visible through different phenomena; the dust reflects sunlight directly and the gases glow from ionisation. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye.