Black hole at the heart of the Milky Way

http://www.timesonline.co.uk/tol/news/uk/science/article5316001.ece

December 9, 2008
Astronomers confirm black hole at the heart of the Milky Way
The nature of the black hole can be inferred from the pattern of motion of the stars that surround it
Mark Henderson, Science Editor

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.

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.

“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.”

His colleague Stefan Gillessen said: “The galactic centre harbours the closest supermassive black hole known. Hence, it is the best place to study black holes in detail.”

The observations have also allowed astronomers to pinpoint the Earth’s distance from the centre of the galaxy with greater precision, measuring it at 27,000 light years. Scientists have also been able to identify common properties among the stellar orbits at the galactic centre.

“The stars in the innermost region are in random orbits, like a swarm of bees,” Dr Gillessen said. “However, further out, six of the 28 stars orbit the black hole in a disc. In this respect the new study has also confirmed explicitly earlier work in which the disc had been found, but only in a statistical sense.

“Ordered motion outside the central light-month, randomly oriented orbits inside - that’s how the dynamics of the young stars in the galactic centre are best described.”