Halogens in space - Fluorine

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From the perspective of cosmology, fluorine is relatively rare with 400 ppb in the universe. Within stars, any fluorine that is created is rapidly eliminated through nuclear fusion: either with hydrogen to form oxygen and helium, or with helium to make neon and hydrogen. The presence of fluorine at all—outside of temporary existence in stars—is somewhat of a mystery because of the need to escape these fluorine-destroying reactions.

Three theoretical solutions to the mystery exist. In type II supernovae, atoms of neon are hit by neutrinos during the explosion and converted to fluorine. In Wolf-Rayet stars (blue stars over 40 times heavier than the Sun), a strong solar wind blows the fluorine out of the star before hydrogen or helium can destroy it. In asymptotic giant branch (a type of red giant) stars, fusion reactions occur in pulses and convection lifts fluorine out of the inner star. Only the red giant hypothesis has supporting evidence from observations.

In space, fluorine commonly combines with hydrogen to form hydrogen fluoride. This compound has been suggested as a proxy to enable tracking reservoirs of hydrogen in the universe. HF emits radiation that is more easily detected than that of hydrogen. (Currently carbon monoxide is used as the proxy for hydrogen.)

Even though elemental fluorine, due to its chemical activity, does not exist in its elementary state on Earth, it can be found in the interstellar medium. Fluorine cations have been seen in planetary nebulae and in stars, including our Sun.
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