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August 11, 2012
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Study said to crack mystery of “monster” stars
Aug. 11, 2012
Courtesy of the Royal Astronomical Society
and World
Science staff
Four “monster” stars whose
colossal size has defied explanation turn out to have been formed thanks to stellar mergers, scientists report. An artist's illustration of the
Wolf-Rayet star R136a1, the most massive star known. (Credit: Wikipedia) Send us a comment
on this story, or send
it to a friend Homepage image: R136a1 and
its stellar neighborhood (courtesy European Southern Observatory)
In 2010 scientists discovered the stars, the biggest of which weighs the equivalent of more than 300 Suns,
in a nearby galaxy known as the Large Magellanic Cloud. Despite their incredible brightness, similar objects have turned up nowhere else.
Now a group of astronomers at the University of Bonn in Germany present their findings on how the objects arose, in a paper in the journal
Monthly Notices of the Royal Astronomical Society. Their conclusion: the “ultramassive” stars were created from the merger of stars in tight binary systems, which consist of a pair of stars orbiting each other.
The Large Magellanic Cloud is the third nearest satellite of the Milky Way galaxy we live in and it contains around 10 billion stars. It has many actively star-forming regions, with by far the most active being one that’s 1,000 light-years wide (a light-year is the distance light travels in a year).
This area, called the Tarantula Nebula, is where the four supermassive stars lie. It’s a cloud of gas and dust that serves as a highly fertile breeding ground of stars, also known as the “30 Doradus” (30 Dor) complex. Near the center of 30 Dor is a giant star cluster called R136, by far the brightest stellar nursery not just in
that galaxy but in our entire “Local Group” of more than 50 galaxies.
Until the monster stars appeared, it was thought that stars heavier than about 150 Suns were impossibilities. “The star birth process seems to be universal,” and simply doesn’t allow for that, explained
Pavel Kroupa of the University of Bonn, a co-author of the findings. But the monster stars presented a stark challenge to that assumption. What gives?
The Bonn group, also including lead investigator Sambaran Banerjee, modelled the interactions between stars in a R136-like cluster. Their computer simulation assembled the model cluster star by star, so as to resemble the real cluster as closely as possible, creating a
group of more than 170,000 stars packed closely together.
To compute how this system changes over time, the model had to solve 510,000 equations many times over. The simulation is complicated by the effect of the nuclear reactions and hence energy released by each star and what happens when two stars happen to collide, a frequent event in such a crowded environment.
These highly intensive, star by star calculations are known as “direct N-body simulations” and are considered the most reliable and accurate way to model clusters of stars. The Bonn researchers used a computer code called “NBODY6,” developed primarily by Sverre Aarseth of the Institute of Astronomy in Cambridge, and took advantage of the unprecedented hardware acceleration of video-gaming cards installed in otherwise ordinary workstations to fast forward their calculations.
With all this, the simulation involved the most difficult, intensive N-body calculations ever made, said
the astronomers. “Once these calculations were done, it quickly became clear that the ultramassive stars are no mystery,” added
Banerjee. “They start appearing very early in the life of the cluster. 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.
“Imagine two bulky stars closely circling each other but where the duo gets pulled apart by the gravitational attraction from their neighbouring stars. If their initially circular orbit is stretched enough, then the stars crash into each other as they pass and make a single ultramassive
star,” Banerjee explained.
“This helps us relax,” added Kroupa. “The universality of star formation prevails after all.”
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Four “monster” stars whose extreme immensity has defied explanation turn out to have been formed thanks to stellar mergers, scientists report. In 2010 scientists discovered the stars, the biggest of which weighs the equivalent of more than 300 Suns, in a giant star cluster called R136 in a nearby galaxy known as the Large Magellanic Cloud. Despite their incredibly brightness, similar objects have turned up nowhere else. Now a group of astronomers at the University of Bonn in Germany present their findings on how the objects arose, in a paper in the journal Monthly Notices of the Royal Astronomical Society. Their conclusion: the “ultramassive” stars were created from the merger of stars in tight binary systems, which consist of a pair of stars orbiting each other. The Large Magellanic Cloud is the third nearest satellite of the Milky Way galaxy we live in and it contains around 10 billion stars. It has many actively star-forming regions, with by far the most active being one that’s 1,000 light-years wide (a light-year is the distance light travels in a year). This area, called the Tarantula Nebula, is where the four supermassive stars lie. It’s a cloud of gas and dust that serves as a highly fertile breeding ground of stars, also known as the “30 Doradus” (30 Dor) complex. Near the centre of 30 Dor is R136, by far the brightest stellar nursery not just in the LMC but in our entire “Local Group” of more than 50 galaxies. Until the monster stars appeared, it was thought that stars heavier than about 150 Suns were impossibilities. “The star birth process seems to be universal,” and simply doesn’t allow for that, explained Pavel Kroupa of the University of Bonn, a co-author of the findings. But the monster stars presented a stark challenge to that assumption. What gives? The Bonn group, also including lead investigator Sambaran Banerjee, modelled the interactions between stars in a R136-like cluster. Their computer simulation assembled the model cluster star by star, so as to resemble the real cluster as closely as possible, creating a cluster of more than 170,000 stars packed closely together. To compute how even this relatively basic system changes over time, the model had to solve 510,000 equations many times over. The simulation is complicated by the effect of the nuclear reactions and hence energy released by each star and what happens when two stars happen to collide, a frequent event in such a crowded environment. These highly intensive, star by star calculations are known as ‘direct N-body simulations’ and are considered the most reliable and accurate way to model clusters of stars. The Bonn researchers used a computer code called “NBODY6,” developed primarily by Sverre Aarseth of the Institute of Astronomy in Cambridge, and took advantage of the unprecedented hardware acceleration of video-gaming cards installed in otherwise ordinary workstations to fast forward their calculations. With all this, the simulation involved the most difficult, intensive N-body calculations ever made, said Pavel and Seungkyung. “Once these calculations were done, it quickly became clear that the ultramassive stars are no mystery,” added Banerjee. “They start appearing very early in the life of the cluster. 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. “Imagine two bulky stars closely circling each other but where the duo gets pulled apart by the gravitational attraction from their neighbouring stars. If their initially circular orbit is stretched enough, then the stars crash into each other as they pass and make a single ultramassive star”, Banerjee explained. “This helps us relax,” added Kroupa. “The universality of star formation prevails after all.”