British astronomers have used the Hubble Space Telescope to find a group of nine massive stars 30 million times brighter than the Sun.
An international team, led by astronomers from Sheffield University, said the cluster, 170,000 light years from Earth, is the largest group of very massive stars identified to date.
The discovery raises many new questions about the formation of massive stars and also highlights the continuing capabilities of the now ageing Hubble Space Telescope.
The cluster - named R136 - is only a few light years across and 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 nine monster stars which are more than 100 times the mass of the Sun 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.
Professor Paul Crowther, from Sheffield University's Department of Physics and Astronomy, showed with his international team in 2010 the existence of four stars within R136, each more than 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 the time.
Now, the team's new research has shown there are five more stars with more than 100 solar masses in R136, raising many new questions about the formation of massive stars as the origin of these huge creations remains unclear.
Saida Caballero-Nieves, also from the university, 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 that such stars can originate from the star formation process."
Prof 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 team combined images taken with the Wide Field Camera 3 on Hubble with the unprecedented ultraviolet spatial resolution of its Space Telescope Imaging Spectrograph (STIS).
The professor said identifying individual stars in this crowded region of space was only possible because of Hubble and he praised the work done by astronauts who risked their lives in 2009 to repair the STIS.
He said the team is continuing to analyse the Hubble data, allowing them to search for close binary systems 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 became worldwide news last month when another team announced they had been detected.