Overabundance of massive stars gives new insights into the universe

Tarantula NebulaAn international team of astronomers has revealed an astonishing overabundance of massive stars in a neighbouring galaxy.

The discovery, made in the gigantic star-forming region 30 Doradus in the Large Magellanic Cloud galaxy, has far-reaching consequences for our understanding of how stars transformed the pristine universe into the one we live in today.

The results, published in the journal Science, were produced by a team including Professor Paul Crowther and Dr Joachim Bestenlehner from the Department of Physics and Astronomy at the University of Sheffield.

Professor Crowther said: "We've known that some monster stars lurk in the core of this giant star forming complex for some time, but our analysis establishes that there are more monsters throughout the entire stellar nursery than we had thought."

As part of the VLT-FLAMES Tarantula Survey (VFTS), the team used European Southern Observatory's Very Large Telescope to observe nearly 1,000 massive stars in 30 Doradus, a gigantic stellar nursery also known as the Tarantula nebula. 

The team used detailed analyses of about 250 stars with masses that are between 15 and 200 times greater than the mass of our Sun to determine the distribution of massive stars born in 30 Doradus – the so-called initial mass function (IMF).

Dr Bestenlehner led the study of the most extreme stars in 30 Doradus, which was integral to lead author Dr Fabian Schneider's analysis of the entire VFTS sample.

Dr Schneider, a Hintze Research Fellow in the University of Oxford's Department of Physics, said: "We were astonished when we realised that 30 Doradus has formed many more massive stars than expected."

Massive stars are particularly important for astronomers because of their enormous feedback. They can explode in spectacular supernovae at the end of their lives, forming some of the most exotic objects in the Universe – neutron stars and black holes.

Co-author Professor Hugues Sana from the University of Leuven in Belgium said: "We have not only been surprised by the sheer number of massive stars, but also that their IMF is densely sampled up to 200 solar masses."

Until recently, the existence of stars up to 200 solar masses was highly disputed, but the Science paper, 'An excess of massive stars in the local 30 Doradus starburst', shows that a maximum birth mass of stars of 200-300 solar masses appears likely.

In most parts of the universe studied by astronomers to date, stars become rarer the more massive they are. The IMF predicts that most stellar mass is in low-mass stars and that less than 1% of all stars are born with masses in excess of ten times that of the Sun. Measuring the proportion of massive stars is extremely difficult – primarily because of their scarcity – and there are only a handful of places in the local universe where this can be done.

The team turned to 30 Doradus, the biggest local star-forming region, which hosts some of the most massive stars ever found, and determined the masses of massive stars with unique observational, theoretical and statistical tools.

This large sample allowed the scientists to derive the most accurate high-mass segment of the IMF to date, and to show that massive stars are much more abundant than previously thought. Dr Chris Evans, the principal investigator of VFTS and a co-author of the study, said: "In fact, our results suggest that most of the stellar mass is actually no longer in low-mass stars, but a significant fraction is in high-mass stars."

The team’s research leaves many open questions, which they intend to investigate in the future: how universal are the findings, and have other starburst regions produced a similar overabundance of massive stars? What are the exact consequences of this for the evolution of our cosmos and also the occurrence of supernovae and gravitational wave events?