Astronomers confirm black hole at the heart of the Milky Way

Mark Henderson
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.

(Gillessen et al/PA)
The nature of the black hole can be inferred from the pattern of motion of the stars that surround it

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.”