ISO-SWS spectroscopy of the WR galaxy NGC 5253 is presented , and analysed to provide estimates of its hot young star population .
Our approach differs from previous investigations in that we are able to distinguish between the regions in which different infrared fine-structure lines form , using complementary ground-based observations .
The high excitation nebular [ S iv ] emission is formed in a very compact region , which we attribute to the central super-star-nucleus , and lower excitation [ Ne ii ] nebular emission originates in the galactic core .
We use photo-ionization modelling coupled with the latest theoretical O-star flux distributions to derive effective stellar temperatures and ionization parameters of T _ { eff } \geq 38 kK , log Q \sim 8.25 for the compact nucleus , with T _ { eff } \sim 35kK , \log~ { } Q \leq 8 for the larger core .
Results are supported by more sophisticated calculations using evolutionary synthesis models .
We assess the contribution that Wolf-Rayet stars may make to highly ionized nebular lines ( e.g .
[ O iv ] ) .

From our Br \alpha flux , the 2 ^ { \prime \prime } nucleus contains the equivalent of approximately 1 000 O7 V star equivalents and the starburst there is 2–3 Myr old ; the 20 ^ { \prime \prime } core contains about 2 500 O7 V star equivalents , with a representative age of \sim 5 Myr .
The Lyman ionizing flux of the nucleus is equivalent to the 30 Doradus region .
These quantities are in good agreement with the observed mid-IR dust luminosity of 7.8 \times 10 ^ { 8 } L _ { \odot } .
Since this structure of hot clusters embedded in cooler emission may be common in dwarf starbursts , observing a galaxy solely with a large aperture may result in confusion .
Neglecting the spatial distribution of nebular emission in NGC 5253 , implies ‘ global ’ stellar temperatures ( or ages ) of 36kK ( 4.8 Myr ) and 39kK ( 2.9 or 4.4 Myr ) from the observed [ Ne iii/ii ] and [ S iv/iii ] line ratios , assuming \log~ { } Q =8 .