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17 March 2016
Astronomers using the unique ultraviolet capabilities of the NASA/ESA Hubble Space Telescope have identified nine monster stars with masses over 100 times the mass of the Sun in the star cluster R136. This makes it the largest sample of very massive stars identified to date. The results, which will be published in the Monthly Notices of the Royal Astronomical Society, raise many new questions about the formation of massive stars.
R136 observed with WFC3
An international team of scientists using the NASA/ESA Hubble Space Telescope has combined images taken with the Wide Field Camera 3 (WFC3) with the unprecedented ultraviolet spatial resolution of the Space Telescope Imaging Spectrograph (STIS) to successfully dissect the young star cluster R136 in the ultraviolet for the first time .
R136 is only a few light-years across and is located in the Tarantula Nebula within the Large Magellanic Cloud, about 170 000 light-years away. The young cluster hosts many extremely massive, hot and luminous stars whose energy is mostly radiated in the ultraviolet . This is why the scientists probed the ultraviolet emission of the cluster.
As well as finding dozens of stars exceeding 50 solar masses, this new study was able to reveal a total number of nine very massive stars in the cluster, all more than 100 times more massive as the Sun. However, the current record holder R136a1 does keep its place as the most massive star known in the Universe, at over 250 solar masses. The detected stars are not only extremely massive, but also extremely bright. Together these nine stars outshine the Sun by a factor of 30 million.
The scientists were also able to investigate outflows from these behemoths, which are most readily studied in the ultraviolet. They eject up to an Earth mass of material per month at a speed approaching one percent of the speed of light, resulting in extreme weight loss throughout their brief lives.
“The ability to distinguish ultraviolet light from such an exceptionally crowded region into its component parts, resolving the signatures of individual stars, was only made possible with the instruments aboard Hubble,” explains Paul Crowther from the University of Sheffield, UK, and lead author of the study. “Together with my colleagues, I would like to acknowledge the invaluable work done by astronauts during Hubble’s last servicing mission: they restored STIS and put their own lives at risk for the sake of future science!” 
Pseudo image of R136
In 2010 Crowther and his collaborators showed the existence of four stars within R136, each with over 150 times the mass of the Sun. At that time the extreme properties of these stars came as a surprise as they exceeded the upper-mass limit for stars that was generally accepted at that time. Now, this new census has shown that there are five more stars with more than 100 solar masses in R136. The results gathered from R136 and from other clusters also raise many new questions about the formation of massive stars as the origin of these behemoths remains unclear .
Saida Caballero-Nieves, a co-author of the study, explains: “There have been suggestions that these monsters result from the merger of less extreme stars in close binary systems. From what we know about the frequency of massive mergers, this scenario can’t account for all the really massive stars that we see in R136, so it would appear that such stars can originate from the star formation process.”
In order to find answers about the origin of these stars the team will continue to analyse the gathered datasets. An analysis of new optical STIS observations will also allow them to search for close binary systems in R136, which could produce massive black hole binaries which would ultimately merge, producing gravitational waves.
Hubble and the sunrise over Earth
“Once again, our work demonstrates that, despite being in orbit for over 25 years, there are some areas of science for which Hubble is still uniquely capable,” concludes Crowther.
1] R136 was originally listed in a catalogue of the brightest stars in the Magellanic Clouds compiled at the Radcliffe Observatory in South Africa. It was separated into three components a, b, c at the European Southern Observatory, with R136a subsequently resolved into a group of eight stars (a1-a8) at ESO, and confirmed as a dense star cluster with the NASA/ESA Hubble Space Telescope after the first servicing mission in 1993.
 Very massive stars are exclusive to the youngest star clusters because their lifetimes are only 2-3 million years. Only a handful of such stars are known in the entire Milky Way galaxy.
 STIS’s capabilities were restored in 2009 by astronauts who successfully completed Serving Mission 4 (SM4), one of the Hubble’s most challenging and intense servicing missions, involving five spacewalks.
 The ultraviolet signatures of even more very massive stars have also been revealed in other clusters — examples include star clusters in the dwarf galaxies NGC 3125 and NGC 5253. However, these clusters are too distant for individual stars to be distinguished even with Hubble.
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
The results were published in the paper “The R136 star cluster dissected with Hubble Space Telescope/STIS. I. Far-ultraviolet spectroscopic census and the origin of Heii λ1640 in young star clusters” in the Monthly Notices of the Royal Astronomical Society.
The international team of astronomers in this study consists of Paul A. Crowther (Department of Physics and Astronomy, University of Sheffield, Sheffield, UK), S.M. Caballero-Nieves(Department of Physics and Astronomy, University of Sheffield, Sheffield, UK), K.A. Bostroem (Space Telescope Science Institute, Baltimore MD, USA; Department of Physics, University of California, Davis CA, USA), J. Maíz Apellániz (Centro de Astrobiología, CSIC/INTA, Madrid, Spain), F.R.N. Schneider (Department of Physics, University of Oxford, Oxford, UK; Argelanger-Institut fur Astronomie der Universität Bonn, Bonn, Germany), N.R. Walborn(Space Telescope Science Institute, Baltimore MD, USA), C.R. Angus (Department of Physics and Astronomy, University of Sheffield, Sheffield, UK; Department of Physics, University of Warwick, Coventry, UK), I. Brott (Institute for Astrophysics, Vienna, Austria), A. Bonanos (Institute of Astronomy & Astrophysics, National Observatory of Athens, P. Penteli, Greece), A. de Koter (Astronomical Institute Anton Pannekoek, University of Amsterdam, Amsterdam, Netherlands; Institute of Astronomy, Leuven, Belgium), S.E. de Mink (Astronomical Institute Anton Pannekoek, University of Amsterdam, Amsterdam, Netherlands), C.J. Evans (UK Astronomy Technology Centre, Royal Observatory Edinburgh, Edinburgh, UK), G. Gräfener (Armagh Observatory, Armagh, UK), A. Herrero (Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain; Departamento de Astrofísica, Universidad de La Laguna, La Laguna, Tenerife, Spain), I.D. Howarth (Department of Physics & Astronomy, University College London, London, UK), N. Langer (Argelanger-Institut fur Astronomie der Universität Bonn, Bonn, Germany), D.J. Lennon (European Space Astronomy Centre, ESA, Villanueva de la Cañada, Madrid, Spain), J. Puls (Universitäts-Sternwarte, Munchen, Germany), H. Sana (Space Telescope Science Institute, Baltimore MD, USA; Institute of Astronomy, Leuven, Belgium), J.S. Vink (Armagh Observatory, Armagh, UK).
Wide Field Camera 3 (WFC3): http://www.spacetelescope.org/about/general/instruments/wfc3/
Space Telescope Imaging Spectrograph (STIS): http://www.spacetelescope.org/about/general/instruments/stis/
European Southern Observatory (ESO): http://www.eso.org/
Images of Hubble: http://www.spacetelescope.org/images/archive/category/spacecraft/
Link to science paper: http://www.spacetelescope.org/static/archives/releases/science_papers/heic1605a.pdf
Image of R136 taken in 2009: http://www.spacetelescope.org/images/opo0932a/
ESO release on the discovery of R136a1 in 2010: http://www.eso.org/public/news/eso1030/
Images, Video, Text, Credits: ESO/NASA/ESA/P Crowther (University of Sheffield)/Hubble/K.A. Bostroem (STScI/UC Davis).