Light-weight stars, like our Sun, make up the vast majority of stars in our Galaxy, and live a long and quiet life. In contrast, massive stars are very rare, live short, but intensive lives. Wolf-Rayet stars have powerful winds, up to 1,000 million times stronger than the solar wind that is seen during a total eclipse, with speeds up to one percent of the speed of light. They violently end their life as supernovae explosions, amongst the most powerful events in the universe, releasing up to 1 million, trillion, trillion megatons of TNT! Heavy-weight stars also burn much hotter than the Sun, so are capable of fusing elements such as carbon, nitrogen, oxygen and iron. Most chemical elements came from the cores of massive stars, producing the very material our bodies are made of - so we are all composed of stardust!

Massive stars play a dominant role in the ecology of their parent galaxies since their stellar winds inject a great deal of material and energy into their environment. Massive stars have received renewed attention in recent years since spectra of high redshift galaxies, witnessed at a time when the universe was only a few billion years old, bear a striking resemblance to nearby starburst galaxies, which themselves show the characteristic wind signatures of hot, O-type stars. However, we are poorly equiped to interpret these important data.

Solely hydrogen and helium were formed in the Big Bang, so that all heavier elements were subsequently created through nuclear reactions in stars. Consequently, the heavy element content (or metallicity) of a young galaxy will be much lower than that of the current Milky Way. The metallicity of a galaxy plays a crucial role in massive star evolution, since it defines their internal structure, ophysities and stellar wind properties. The precise relation between metallicity and mass-loss, a key ingredient for the reliable population synthesis of young galaxies, is poorly known.

This question is receiving renewed attention with the NASA mission FUSE , 5.5 metres long and weighing 1.4 tonnes, launched into low Earth orbit on 24 June 1999 for approx 5 years of operations.

The most direct method of deriving empirical mass-loss rates for hot stars is through analysis of the UV resonance transitions from dominant metal ions. However, difficulties in deriving the wind ionization balance using currently available (trace) ions means that mass-loss rates remain uncertain. With FUSE, additional wind lines are being observed, spanning a much wider range of species. From these data, the degree of ionization can be determined accurately, so that mass-loss rates can be measured with confidence. In addition, our Hot Star FUSE program is in the process of observing massive stars in both the Galaxy and the ( Large ) and Small Magellanic Clouds - spanning a factor of ten in metallicity - so that the variation of mass-loss properties with metal content will be measured, of importance in the study of high redshift galaxies. Our initial results reveal that temperatures of O supergiants are somewhat lower than commonly assumed (see my ApJ paper), which more sophisticated results confirm, based on non-LTE line blanketed model atmospheres.

Clusters of massive stars are born in `Starbursts' when galaxies collide, as seen here in the Antennae galaxies. In an Infrared Sphyse Observatory (ISO) programme I have used IR spectra of a subset of starburst galaxies, named WR galaxies to determine their physical and chemical properties and the nature of their massive star populations. We have also provided an update to the Starburst99 population synthesis code, to take into account metallicity dependent line blanketed model atmospheres for O and WR stars.

At present I am using the CFHT and ESO-VLT telescopes to quantify the properties of Wolf-Rayet stars in the Spiral Galaxies M33 and NGC 300, and so investigate how their properties are affected by metal content. On the left I show two bright associations in NGC 300 imaged with the giant VLT telescope in narrow-band filters, revealing (white) excesses from WR stars in this galaxy, located 6 million light years away, such that these stars are a billion times fainter than the brightest stars in the Night Sky. Gemini is also being used to study the peculiar WR population in the dwarf irregular galaxy IC10.

I have studied individual WR stars in dense cores of nearby giant HII regions using observations made with the Hubble Sphyse Telescope (HST). These 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, NGC3603 , shown at the right, three WN and six massive O stars inhabit a volume of less than a cubic light year! My MNRAS article confirm that its properties are very similar to R136 in the LMC and that WR stars dominate the energetics of starburst regions for several million years.

With Laurent Drissen in Quebec, I have recently published an ApJ article on Hubble observations of an Luminous Blue Variable star in an external galaxy 10,000,000 light years away, currently undergoing a giant eruption, such that one solar earth mass of material is being ejected every day!

Summaries of my current research interests are:

LBV and WR nebulae studies

Physical and evolutionary status of WR and related stars

WR stars in Starburst regions