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 core-collapse supernovae, and most likely Gamma Ray Bursts, amongst the most powerful events in the universe. Heavy-weight stars also burn much hotter than the Sun, so are capable of fusing elements producing carbon, 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!
We have reanalysed VLT/SINFONI spectroscopy of the brightest members of the NGC 3603 and R136 star clusters, together with high spatial resolution near-IR imaging, revealing high stellar temperatures and luminosities. These have been combined with contemporary evolutionary models for rotating main-sequence to suggest initial masses as high as 320 solar masses (R136a1), and are supported by dynamical mass determinations for the NGC3603 A1 eclipsing binary system. These findings have been reported by ESO (Stars just got bigger). The artists impression to the right compares the relative sizes of young stars, from red dwarfs (0.1 solar masses), yellow dwarfs (1 solar mass), blue dwarfs (8 solar masses) and R136a1 - see FAQ.
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. They 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 equipped 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, opacities and stellar wind properties. The precise relation between metallicity and mass-loss, a key ingredient for the reliable population synthesis of young galaxies, remains imprecisely known.
Our surveys of Wolf-Rayet stars within nearby star-forming galaxies have been compared to deep X-ray images to reveal candidates of rare Wolf-Rayet plus black hole or neutron star systems. Prior to our studies only one Milky Way system was known (Cyg X-3), while we now have confirmed three instances of Wolf-Rayet plus black hole systems (IC10 X-1, NGC300 X-1, M101 ULX-1), whose black hole components are the highest known to date amongst stellar-mass systems. These findings have been reported by NASA for IC10 X-1 (Massive Black Hole Smashes Record), ESO for NGC 300 X-1 (Black Hole Hunters Set New Distance Record) and Gemini for M101 ULX-1 (Fast, Furious, Refined: Smaller Black Holes Can Eat Plenty). The image to the left shows an artists impression of these high-mass X-ray binary systems.
- NGC 300 X-1 is a Wolf-Rayet/Black-Hole binary Crowther et al. 2010
- The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 Msun stellar mass limit Crowther et al. 2010
- Evolution and fate of very massive stars Yusof et al. 2013 MNRAS 433 1114
- Puzzling accretion onto a black hole in the ultraluminous X-ray source M 101 ULX-1 Liu et al. 2013 Nat 503, 500