Near the centre of the galaxy, chaos rules. The stars that orbit close to the Milky Way鈥檚 central supermassive black hole, called S-stars, are so chaotic that our best simulations of their motions fall apart in just 462 years, making it impossible to predict their locations after that.
Traditionally, it has been difficult to calculate the motions of many objects orbiting near one another because of ever-present rounding errors; long decimals involved in the calculations must be cut off somewhere, which throws off the results by tiny amounts. To make matters worse, orbiting objects may experience chaotic motion, where even minuscule changes in their behaviour can have enormous impacts on their final positions. This聽means it is crucial to keep the numbers in the calculations as precise as possible.
To solve this issue, at Leiden University in the Netherlands and his colleagues developed an algorithm called Brutus. It runs simulations of many objects, called N-body simulations, without significant rounding errors, leaving the decimals intact up to hundreds of digits. That makes its calculations extremely precise, but also slow to perform, leading Portegies Zwart to call it 鈥渢he slowest N-body code on the planet鈥�.
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The code allowed the researchers to place limits on the predictability of the motions of 27 S-stars for the first time 鈥� and those limits are much tighter than expected. 鈥淭he galactic centre looks kind of like a bigger solar system, so I thought it would be very similar, but the time scale of the chaos turned out to be less than 500 years rather than millions of years like the solar system,鈥� says Portegies Zwart. 鈥淭hat was a surprise 鈥� from the chaotic perspective, it鈥檚 a totally different beast.鈥�
This is because of a phenomenon that the researchers have dubbed 鈥減unctuated chaos鈥�. Things seem calm for a long time, and then suddenly two or three of the S-stars get a little bit too close together, shifting their orbits slightly. That shift pulls on the black hole, which in turn pulls on the other stars, changing their trajectories in ways that are impossible to predict.
Over time, their expected positions could change by up to 40 times the distance between Earth and the sun. But those shifts are expected to take a long time to build up. 鈥淲e鈥檙e hardly going to see the change 鈥� it would be great to see it, and it would be amazing to see one of these S-stars fall into the black hole, but that鈥檚 not going to happen,鈥� says Portegies Zwart. 鈥淭he orbits of these S-stars are not going to make drastic changes鈥� that would require a really close encounter or a collision.鈥�
Monthly Notices of the Royal Astronomical Society
