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.

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A team of astronomers from the University of Leicester in the United Kingdom and Monash University in Australia have collaborated to put together a new theory that explains how black holes grow to be massive so quickly. With this theory, astronomers and astrophysicists are closer to understanding the nature of black holes.

These outer space oddities are born as the result of a star’s death: when a star collapses upon itself and continues to do so until it becomes a tiny point in space-time. A black hole’s mass is highly compressed, so its gravity is large enough to distort time.

Black holes eat anything and everything: stars, nebulas, planets, space debris, and even light, all of which spiral into the tiny point known as the event horizon. Black holes are hundreds to billions times more massive than the sun. Many galaxies contain supermassive black holes at their centers, including our very own Milky Way.

No one can observe black holes because they cannot be seen (they absorb all light and do not reflect any), but evidence for their presence can be located in distortions in space and light. Smaller black holes are usually found in binary systems, in which the black hole slowly eats away at its companion star.

Most black holes have been feeding since the early years of the Universe. While eating, a disk of gas and other materials (called the accretion disk) forms around them, slowing down their munching and growing time. Astronomers and astrophysicists have determined that certain black holes can grow considerably by eating or crashing into each another to create one massive black hole, and supermassive black holes are usually the result of galaxies colliding. These collisions, though, are quite rare.

Still, the nature of other large black holes cannot be explained. How do other black holes – those that do not collide with anything and that are not in any binary systems – grow to be so big in such a little amount of time? Usually, when black holes feed, a disk (called the accretion disk) of gas and other materials forms around them. This accretion disk is what slows down their munching and growing time.

The astronomers that are part of the research team have developed a theory to account for this mystery. They created computer simulations of a black hole that have two accretion disks orbiting it at different angles. As the simulations continue over time, the disks eventually spread, then collapse. This collapse allows the black hole to swallow heaps of produced gas and enables it to grow 1,000 times faster, according to the University of Leicester press release.

“If two guys ride motorbikes on a Wall of Death, and they collide, they lose the centrifugal force holding them to the walls and fall,” Andrew King clarified. King, one of the astronomers that is part of the team, is from the Department of Physics and Astronomy at the University of Leicester.

The team will publish their research in the Monthly Notices of the Royal Astronomical Society. Their simulations can be found here.

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Shogo Masaki of the Department of Physics at Nagoya University and Masataka Fukugita and Naoki Yoshida of the University of Tokyo’s Institute of Physics and Mathematics of the Universe (IMPU) collaborated in an experiment to create a computer simulation that would hopefully figure out the location of dark matter. In late January, their experiment was successful.

The term intergalactic refers to the physical space between galaxies where matter is hardly distributed. Scientists previously thought that intergalactic space comprised of nothing, being only empty, and that galaxies, in contrast, have the highest concentration of matter. Masaki, Fukugita, and Yoshida, however, have discovered that these intergalactic zones are packed with clumps of dark matter.

In addition, they also learned that galaxies do not have clear, defined edges; instead, they “have long outskirts of dark matter that extend to their nearby galaxies” according to IMPU’s press release. These “outskirts” contain much of the matter – and dark matter – in the universe.

The existence of dark matter was proposed by Swiss astronomer Fritz Zwicky in the 1930s. Since then, there have been numerous experiments around the globe involving dark matter. Dark matter’s nature is still enigmatic: it is an invisible, dense substance, and it cannot even be detected by instruments. Scientists do know that dark matter takes up about 23% of the Universe, with dark energy taking up 72% and the rest (planets and stars, for example) only 4%.

Furthermore, contrary to popular belief, dark matter is not random – it is uniform and organized. Masaki and his colleagues gathered recent observational data of 24 million galaxies from the Sloan Digital Sky Survey (SDSS) and created a large simulation of matter distribution. With their knowledge of the large density of dark matter, they used gravitational lensing to find the substance’s location.

Because dark matter is so dense, it causes space and light from stars, galaxies, and other light-emitting objects to bend, making these celestial objects appear bigger and brighter. With gravitational lensing, Masaki and his colleagues measured how the galaxies’ light was bent, allowing them to locate dark matter.

Dark matter remains as elusive as ever: although we have found exactly where dark matter is, we still do not know what it is, but scientists are closer than ever to understanding the mysterious substance’s nature. Masaki, Fukugita, and Yoshida have published a paper describing details of their experiment in The Astrophysical Journal. A PDF of the preprint version is found here.

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