Clusters of massive stars are born in `Starbursts' when galaxies collide, as seen below in the Antennae galaxies. Our group is involved in studies of WR stars in starburst regions, both individual stars in nearby giant HII regions, plus examples in more distant starburst galaxies, where only integrated properties are feasible.

Giant HII regions are gigantic nebulae photo-ionized by tremendous amounts of Lyman continuum radiation from member stars, and are the signature of recent bursts of star formation in galaxies. Within one such region of our own Galaxy, NGC3603, three WN and six massive O stars inhabit a volume of less than a cubic light year! Our analysis ( Crowther & Dessart MNRAS 1998 296 622 see here ) of stars in NGC3603 and R136 , the core of 30 Doradus in the LMC, confirm that WR stars dominate the energetics of starburst regions for several million years. 30 Doradus can be seen to the left of the centre of a HII image of the LMC (courtesy of Mike Bessell, Mt Stromlo - for more see HERE).

Our studies have been greatly enhanced by the launch of the ESA ( Infrared Space Observatory (ISO) . We have used the Short Wavelength Spectrometer to obtain mid-IR (2.6-30 micron) spectroscopy of several WR galaxies. These represent a subset of blue emission-line galaxies which contain large numbers of WR stars as evidenced from their spectroscopic signature. Work recently completed ( Crowther et al. 1999 MNRAS 304 654 see here) for one such galaxy, NGC5253 (shown left) one of the closest known, has allowed us to provide estimates of its young, hot star population. Our approach differs from previous investigations in that we are able to distinguish between the formation regions of different infrared fine-structure lines, using complementary ground-based observations. We use photoionization modeling coupled with the latest theoretical O-star flux distributions to identify the region of high excitation nebular [SIV] emission to a hot, compact region, which we attribute to the central super-star-nucleus, containing stars with T(eff) in excess of 38,000K and high ionization parameter. In contrast, lower excitation [NeII] nebular features originate in a much lower density region containing cooler stars, with T(eff) around 36,000K, which we attribute to the galactic core. Our results are compared with calculations obtained with evolutionary synthesis models, and the possible contribution of Wolf-Rayet stars towards high excitation nebular emission lines is discussed.

Our Br alpha flux indicates approximately 1000 equivalent O7V stars in the central nucleus, with an age of 3Myr, plus 2500 equivalent O7V stars in the larger core, at a somewhat later age of greater than 5 Myr. The nucleus Lyman ionizing flux is equivalent to 30 Doradus. These quantities are consistent with the observed mid-IR dust luminosity. Since a structure of hot clusters embedded in cooler emission may be common in dwarf starbursts, confusion may result for cases where the galaxy is observed only with a large aperture.