British astronomers have discovered a cluster of nine "monster" stars 30 million times brighter than the Sun which could change our understanding of the way stars are formed.
Using the Hubble Space Telescope, an international team led by astronomers from Sheffield University also found dozens of stars more than 50 times the mass of the Sun.
The star cluster, named R136, is 170,000 light years from Earth and is the largest group of so-called very massive stars identified to date.
Until now, astronomers had theorised that huge stars were formed when two smaller stars merged, but the sheer number of massive stars in the cluster suggests another process must be at work.
Saida Caballero-Nieves, from Sheffield University's Department of Physics and Astronomy, said: "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 such stars can originate from the star formation process."
The star cluster is in the Tarantula Nebula within the Large Magellanic Cloud - a satellite galaxy of our own Milky Way.
The team said the young cluster hosts many extremely massive, hot and luminous stars whose energy is mostly radiated in the ultra violet, which is why the scientists used Hubble to probe the ultraviolet emission of the cluster.
It includes four stars which are more than 150 times the mass of the Sun, five which are more than 100 times as massive and dozens of stars exceeding 50 solar masses.
None of the stars identified have unseated R126a1, also in the Tarantula Nebula, as the most massive star in the known universe at more than 250 solar masses.
The team combined images taken with the Wide Field Camera 3 on Hubble with the unrivalled ultraviolet spatial resolution of its Space Telescope Imaging Spectrograph (STIS).
Professor Paul Crowther, lead author of the study, published in the monthly notices of the Royal Astronomical Society, said: "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."
The professor said identifying individual stars in this crowded region of space was only possible because of Hubble and he praised astronauts who risked their lives in 2009 to repair the STIS.
The team is continuing to analyse the Hubble data, allowing them to search for close binary systems - in which two stars circle around each other - in R136 which could produce massive black hole binaries.
The professor said this could contribute to the study of gravitational waves - the ripples in space time which were predicted by Albert Einstein and recently detected.
The star cluster
The largest group of monster stars identified to date.
170,000 light years from Earth in the Tarantula Nebula within the Large Magellanic Cloud.