The univalent five-atom radical C4H is found both in interstellar molecular clouds and in the envelopes of evolved carbon stars (evolved stars whose atmospheres contain more carbon than oxygen, the excess carbon presumably produced by helium fusion in the latter stages of the star’s life). It is an intermediary in the rich carbon chemistry of these stars and of cold, dark interstellar clouds; both contain polyacetylenes, cyanopolyynes, and related carbon-chain species. Both deuterated and 13C variants of C4H have been detected in astronomical sources (Cernicharo et al. 2000).
The astronomical identification of C4H in the envelope of the carbon star IRC + 10216 (Guélin et al. 1978) prior to laboratory measurements of the rotational spectrum provides another example of the ability of high-frequency radio astronomy to contribute to fundamental molecular physics (cf. the discussion for C3N). Subsequently, this radical was also detected in interstellar...