London Times 
Wednesday, Dec 10, 2008
A swarm of stars orbiting a vast black hole at the centre of the Milky Way has been mapped with remarkable precision, providing astronomers with their most detailed look yet at the heart of our galaxy.
Observations by the European Southern Observatory (ESO) in Chile has found the strongest evidence yet for a supermassive black hole at the galaxy’s core, as well as charting the immense gravitational effects this has on the surrounding stars.
Over 16 years, the orbits of 28 stars in the Milky Way’s central region have been meticulously tracked by astronomers, allowing them to study the hidden black hole that influences their movements.
The black hole, known as Sagittarius A* (pronounced “Sagittarius A-star”), cannot be seen directly, but its nature can be inferred from the pattern of motion of the stars that surround it. Details of the research are published in the Astrophysical Journal.
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Reinhard Genzel, of the Max Planck Institute for Extraterrestrial Physics in Germany, who led the international study team, said: “Undoubtedly the most spectacular aspect of our long term study is that it has delivered what is now considered to be the best empirical evidence that supermassive black holes do really exist. The stellar orbits in the galactic centre show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt.
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“The centre of the galaxy is a unique laboratory where we can study the fundamental processes of strong gravity, stellar dynamics and star formation that are of great relevance to all other galactic nuclei, with a level of detail that will never be possible beyond our galaxy.”