  Star formation (SF) takes place in unusual places such as way out in the
intergalactic medium out of material expelled from parent galaxies. We wish to
answer whether SF proceeds in this specific environment in a similar way than
in galactic disks. We have carried out a multiwavelength analysis of the
interacting system NGC 5291, which is remarkable for its extended HI ring
hosting numerous intergalactic HII regions. We combined new ultraviolet (GALEX)
observations with archival Halpha, 8 mu m (Spitzer Space Telescope) and HI (VLA
B-array) images of the system. We have found that the morphology of the star
forming regions, as traced by the ultraviolet, Halpha, and 8 mu m emission is
similar. There is a clear excess of ultraviolet emission compared to individual
HII regions in spirals, i.e. the [8.0]/[NUV] and [Halpha]/[NUV] SFR ratios are
on average low although there are some large variations from one region to
another, which cannot be explained by variations of the metallicity or the dust
extinction along the HI structure. Comparing the observed SFR with a model of
the evolution of [Halpha]/[NUV] with time favours young, quasi-instantaneous
though already fading starbursts. The total star formation rate measured in the
intergalactic medium (which accounts for 80% of the total) surrounding NGC 5291
is up to 1.3 Msun/yr, a value typical for spirals, assuming the standard SFR
calibrations are valid. The SFR drops by a factor of 2 to 4 in case the star
formation is indeed quasi-instantaneous. (abridged)
