Editing Comet Hale–Bopp (section)

=== Sodium tail ===
[[File:Hale-Bopp sodium tail.gif|thumb|right|Comet Hale–Bopp's neutral sodium tail (the straight tail extending up to the left from the nucleus){{r|Cremonese_1997a}}]]
One of the most remarkable discoveries was that the comet had a third type of tail. In addition to the well-known gas and dust tails, Hale–Bopp also exhibited a faint [[sodium]] tail, only visible with powerful instruments with dedicated filters. Sodium emission had been previously observed in other comets, but had not been shown to come from a tail. Hale–Bopp's sodium tail consisted of [[electric charge|neutral atoms]] (not [[ion]]s), and extended to some {{convert|50|e6km|e6mi|abbr=unit}} in length.{{r|Cremonese_1997a}}

The source of the sodium appeared to be the inner [[coma (cometary)|coma]], although not necessarily the [[comet nucleus|nucleus]]. There are several possible mechanisms for generating a source of sodium atoms, including collisions between dust grains surrounding the nucleus, and "sputtering" of sodium from dust grains by [[ultraviolet]] light. It is not yet established which mechanism is primarily responsible for creating Hale–Bopp's sodium tail, and the narrow{{r|Cremonese_1997a}} and diffuse{{r|Wilson_1998}} components of the tail may have different origins.{{r|Cremonese_1997b}}

While the comet's dust tail roughly followed the path of the comet's orbit and the gas tail pointed almost directly away from the Sun, the sodium tail appeared to lie between the two. This implies that the sodium atoms are driven away from the comet's head by [[radiation pressure]].{{r|Cremonese_1997a}}