The Hubble Space Telescope has found 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 is the largest group of very massive stars identified to date. The cluster is 170,000 light years from Earth.
The discovery has highlighted the continuing capabilities of the now-ageing telescope, while raising a number of questions about the formation of massive stars.
The cluster is called R136 and is only a few light years across in the Tarantula Nebula within the Large Magellanic Cloud, a satellite galaxy of the 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 ultra-violet 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.
Saida Caballero-Nieves, from the University of Sheffield, 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 Paul Crowther, who led the study which made the discovery, 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."
Meanwhile, scientists watching out for Earth-threatening asteroids have spotted the ExoMars spacecraft streaking towards Mars at more than 20,000mph.
Images captured by ground-based telescopes in Australia, New Zealand and Brazil (above) just after the vehicle broke free from Earth orbit show a rapidly moving bright mass.
The first stage of the joint European and Russian ExoMars mission to search for life on Mars was successfully launched on Monday.A Russian Proton heavy-lift rocket blasted two unmanned probes, Trace Gas Orbiter (TGO) and the lander Schiaparelli, into space from the Baikonur Cosmodrome in Kazakhstan.
Flying together as a combined stack, the craft are now coasting on a trajectory that will cause them to rendezvous with Mars in October after a journey of 300 million miles.