The Tails

There are two main types of cometary tails: the ion and dust tail. Both the ion and the dust tails can reach great lengths, up to about one Astronomical Unit (AU) in the most spectacular cases.

Comet Hale Bopp showing its two tails Courtesy of NASA

The ion tail
The ion tail is formed because the neutral gas, initially present in the cometary comae, is ionized by the solar radiation, giving birth to ions. These ions are susceptible to magnetic solar forces and are therefore swept out of the coma into a long, characteristic ion tail.
Because the most common ion (CO+) scatters blue light better than red (click here to know more about the spectrum of light), the ion tail often appears to the human eye as blue. Also, the magnetic force is very strong and produces ropes, knots and streamers that distinguish the ion tail from the dust tail. Another important characteristic of the ion tail is its orientation. In fact, the solar wind that sweeps past the comet has a very high speed (about 500 km/s) and causes the ion tail to be always orientated exactly in the anti-solar direction.

The dust tail
The dust tail consists of dust particles that have been pushed out of the coma by the radiation from the Sun. Compared to the ion tail, the dust tail is much more diffuse, and appears white or slightly pink (since dust grains reflect sunlight slightly better at longer wavelengths than at shorter wavelengths).

Furthermore, the dust tail normally has an orientation that differs from the ion tail's one. In fact, once they have been expulsed from the coma, the dust particles in the tail will be individually in orbit around the Sun, causing the dust tail to be curved as the comet follows its trajectory. In the animation on the right, the orientation of both the dust tail and the ion tail are visible. The variations of the curvature of the dust tail during the orbit is also evident .