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An international team of astronomers, including Selma de Mink and Alex de Koter from the University of Amsterdam, has identified a set of nine monster stars with the assistance of the Hubble Space Telescope. It constitutes the largest set of massive stars discovered to date. Each star is more than one hundred times the mass of our Sun. The study will be published in the journal 'Monthly Notices of the Royal Astronomical Society' and raises new questions as to the formation and ultimate fate of massive stars.

NASA, ESA, P. Crowther
The centre of the Tarantula Nebula. In the righthand corner is the young, densely packed R136 cluster. Credits: NASA, ESA, P. Crowther (University of Sheffield)

The stars are located in the cluster R136 in the centre of the Tarantula Nebula. This nebula is found in the Large Magellanic Cloud, which is visible from the southern hemisphere in the Dorado constellation. The stars are 170,000 light years away from the Earth. R136 is just a couple of light years in diameter, but contains numerous extreme, massive, hot and luminous stars.

The mass record is held by one of the stars in this cluster, namely R136a1. At approximately 250 times the mass of the Sun, it has the highest mass of any star observed up to now. ‘For a long time, we did not believe that stars could be heavier than around 100 times the mass of the Sun,’ states co-author Selma de Mink. ‘This is what you find written in the majority of text books. It is high time to rewrite the books.’

Hubble Space Telescope

The astronomers were not able to observe the stars from Earth due to the fact that massive stars primarily emit ultraviolet light, which is obstructed by our atmosphere. This is why the astronomers used the Hubble Space Telescope, which is in orbit around the Earth and is not affected by the planet’s atmosphere.

The nine supermassive stars are not only extremely massive, but also very luminous. Combined, they emit thirty million times as much light as our Sun. Luminous stars of this kind burn up quickly. They reach the end of their life cycles after around two to three million years. The scientists calculated that each month, every one of the monster stars emits material weighing more than the Earth’s mass in the form of a stellar wind. Despite this enormous loss of mass, these stars are expected to ultimately produce black holes.

Formation of monster stars

It is not clear how monster stars form. When four of the nine stars were discovered in 2010, the researchers viewed them as oddities as, according to prevailing theories at the time, stars larger than 100 times the mass of the Sun could not exist. Now that a further five monster stars have joined the cluster, it seems as though stars of this magnitude are more normal than was previously thought.

Some monster stars may form through two or more stars merging together. However, this does not account for the existence of all monster stars. The astronauts therefore believe that the stars may form via normal star formation processes as well.

For the time being, the researchers will study their observations in more detail to gain a better understanding of the properties of the stars. In addition to the nine monster stars, it appears as though the cluster also contains several massive double stars. One of the main questions is which of these stars may ultimately end up forming double black holes. In turn, these black holes might merge again and create gravitational waves.

‘It is great that we have now been able to find more of these supermassive stars,’ explains co-author Alex de Koter. ‘Especially because we believe that the first stars to appear after the Big Bang must have been as massive as these ones. By studying the stars in the Tarantula Nebula, we might be able to learn more about the role of the first stars in the early universe.’