  Individual highly magnified stars have been recently discovered at lookback
times of more than half the age of the Universe, in lensed galaxies that
straddle the critical curves of massive galaxy clusters. Having confirmed their
detectability, it is now important to carry out systematic searches for them in
order to establish their frequency, and in turn learn about the statistical
properties of high-redshift stars and of the granularity of matter in the
foreground deflector. Here we report the discovery of a highly magnified star
at redshift $z = 0.94$ in a strongly lensed arc behind a Hubble Frontier Field
galaxy cluster, MACS J0416.1-2403, discovered as part of a systematic archival
search. The bright transient (dubbed "Warhol") was discovered in Hubble Space
Telescope data taken on 2014 September 15 and 16. This single image faded over
a period of two weeks, and observations taken on 2014 September 1 show that the
duration of the microlensing event was at most four weeks in total. The light
curve may also exhibit slow changes over a period of years consistent with the
level of microlensing expected from stars responsible for the intracluster
light (ICL) of the cluster. Optical and infrared observations taken near peak
brightness can be fit by a stellar spectrum with moderate host-galaxy
extinction. A blue supergiant matches the measured spectral energy distribution
near peak, implying a temporary magnification of at least several thousand.
While the spectrum of an O-type star would also fit the transient's spectral
energy distribution, extremely luminous O-type stars are much less common than
blue supergiants. The short timescale of the event and the estimated effective
temperature indicate that the lensed source is an extremely magnified star.
