A star twirling in a tight orbit around the centre of our galaxy has finally pinned down the existence of a monstrous black hole.
The size of the orbit and estimated mass of the star S2 rules out all but the most exotic possibilities for the relatively small but massive object lurking at the heart of the Milky Way.
Previous observations of X-ray radiation, as well as the movement of other central stars, have strongly suggested that the compact radio source known as Sagittarius A* is a supermassive black hole. But other explanations, such as a cluster of smaller black holes, neutron stars or a ball of heavy neutrinos, could not be ruled out until now.
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鈥淢ost astronomers were pretty much convinced, but with this orbit we can exclude that it is a neutrino ball or some very dense cluster of normal-sized black holes or neutron stars,鈥� says Rainer Sch枚del, at the Max Planck Institute for Extraterrestrial Physics in Germany, who led the research. 鈥淚t really settles the issue.鈥�
The only remaining alternative is an star made of elementary particles called bosons, but even this would eventually collapse into a black hole says Sch枚del鈥檚 colleague Reinhard Genzel.
3.7 million Suns
The researchers used data on S2 gathered at various observatories over 10 years and also captured new images of the star using Adaptive Optics NAOS-CONICA instrument at Paranal Observatory in Chile. This provided images of unprecedented quality by correcting for the distortion caused by the Earth鈥檚 atmosphere.
S2 orbits Sagittarius A* every 15.6 years at a distance of between 17 light hours and five light days. The orbit and estimated mass of the star allow researchers to calculate that the Milky Way鈥檚 black hole has a mass 3.7 million times that of our Sun. The black hole鈥檚 Schwarzschild radius, equivalent to the 鈥渟ize鈥� of its event horizon, was found to be 11 times the radius of the Sun at 7.7 million kilometres.
Astronomer Paul O鈥橞rien of the UK鈥檚 Leicester University says this approach is much better than previous methods, such as observing X-rays. 鈥淚t鈥檚 a technique that no one will argue with,鈥� he says.
James Binney, a theoretical astrophysicist at Oxford University, UK, adds that confirming the black hole鈥檚 existence will mean detailed analysis of it can now begin.
He told 快猫短视频: 鈥淲e have now entered the era of precision measurements when the issue is determination of its spin and exactly what is going on in its environment.鈥� He says the discovery will also shed new light on the general relationship between black holes and galaxies.
Journal reference: Nature (vol 419, p 694)