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In 2010, astronomers discovered four stars, all of which are at least 300 times the mass of the Sun. Prior to their detection, stars with this solar mass were thought to be impossible to exist; not one star that has been accounted for and studied has a mass that exceeds the 150 solar mass limit, which is a universal limit. These four colossal stellar bodies have been the only ones detected in the Universe. Their origin stumped astronomers.
Recently, however, one team of astronomers – Samabaran Banerjee, Raval Kroupa, Seungkyung Oh – from the University of Bonn in Germany determined the cause of the “monster” stars’ existence by creating and using a computer model: Because the stars in the tiny R136 cluster are so close to one another, the binary systems are unusually tight; hence, the intense gravitational tug the stars impose on each in each system caused the stars to smash together and fuse to become their present hyper-massive and luminous selves.
“They start appearing very early in the life of the cluster,” Dr. Banerjee states in Royal Astronomical Society press release. “With so many massive stars in tight binary pairs, themselves packed closely together, there are frequent random encounters, some of which result in collisions where two stars coalesce into heavier objects. The resulting stars can then quite easily end up being as ultramassive as those seen in R136.”
These four stars are located in the Large Magellanic Cloud (LMC), which is one of the closest galaxies to the Milky Way and a hotbed for star formation, harboring approximately ten billion stars. Specifically, their home lies in the R136 star cluster, which is a mere 35 light-years across, in the well-known Tarantula Nebula, the LMC’s most active star formation region.
A star cluster is a group of stars tightly held together by gravity. The number of stars range from a few hundred to several hundreds of thousands. Roughly, there are more than 1000 star clusters in the LMC alone.
For accuracy, the model Banerjee, Kroupa, and Oh produced resembled the R136 region. To calculate the shape of the star cluster, the team utilized the NBODY6 – or “N-body” – integration code developed by Sverre Aaseth, a research scientist of the Institute of Astronomy at the University of Cambridge. The model contained 170,000, which were normal in mass and luminosity (that is, they were stars from the Main Sequence of the Hertzsprung-Russell diagram) and were distributed as the stars were in R136.
For Banerjee, Kroupa and Oh to monitor and analyze how the stars interacted with one another and changed over time, the computer had to solve 510,000 calculations multiple times while taking into account stellar winds, nuclear reactions caused by stellar collisions, gravity, and the result of each collision – all of which happened in the supposed densely packed environment. The N-body code the team used helped speed up these calculations.
Once the calculations were completed, the team concluded that the leviathan stars inhabiting R136 used to be ordinary stars that merged with one another, and that they are not anomalies which had formed outside our knowing of how star’s normally form.
“Not only the upper mass limit but the whole mass ingredient of any newborn assembly of stars appears identical irrespective of the stellar birthplace: the star birth process seems to [still] be universal,” Dr. Kroupa says. “This helps us relax because the collisions mean that the ultramassive stars are a lot easier to explain. The universality of star formation prevails after all.”
The team published their paper in Monthly Notices of the Royal Astronomical Society.