  The origin of lithium (Li) and its production process have long been an
unsettled question in cosmology and astrophysics. Candidates environments of Li
production events or sites suggested by previous studies include big bang
nucleosynthesis, interactions of energetic cosmic rays with interstellar
matter, evolved low mass stars, novae, and supernova explosions. Chemical
evolution models and observed stellar Li abundances suggest that at least half
of the present Li abundance may have been produced in red giants, asymptotic
giant branch (AGB) stars, and novae. However, no direct evidence for the supply
of Li from stellar objects to the Galactic medium has yet been found. Here we
report on the detection of highly blue-shifted resonance lines of the singly
ionized radioactive isotope of beryllium, $^{7}$Be, in the near ultraviolet
(UV) spectra of the classical nova V339 Del (Nova Delphini 2013). Spectra were
obtained 38 to 48 days after the explosion. $^{7}$Be decays to form $^{7}$Li
within a short time (half-life 53.22 days). The spectroscopic detection of this
fragile isotope implies that it has been created during the nova explosion via
the reaction $^{3}\mbox{He}(\alpha,\gamma)^{7}\mbox{Be}$, and supports the
theoretical prediction that a significant amount of $^{7}$Li could be produced
in classical nova explosions. This finding opens a new way to explore $^{7}$Li
production in classical novae and provides a clue to the mystery of the
Galactic evolution of lithium.
