Posted: 10 December, 2008
By plotting the motions of stars around the Milky Way’s central region over a period of 16 years, astronomers have probed the nexus of forces thought to be controlled by the Galaxy’s supermassive black hole.
Astronomers tracked the motions of 28 stars, one of which even completed a full orbit in the 16 year long study, marking the first time that the orbits of so many central stars have been calculated precisely. The research reveals information about both the enigmatic formation of these stars and the properties of the black hole – known as Sagittarius A* – to which they are bound. Sagittarius A* is the closest supermassive black hole known, making it the best place to become more acquainted with such galactic monsters.
The central parts of our Milky Way Galaxy, as observed in the near-infrared with the NACO instrument on ESO's VLT. By following the motions of the most central stars over more than 16 years, astronomers were able to determine the mass of the supermassive black hole that lurks there. Image: ESO/S. Gillessen et al.
"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," says Reinhard Genzel, leader of the team from the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.
By studying the motions of all 28 stars, the astronomers found that at least 95 percent of the mass sensed by the stars had to be in the black hole, equating to a mass of just over four million times the mass of the Sun. "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,” says Genzel. “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 new work also improved the accuracy by which astronomers can measure the positions of the stars by a factor of six compared to previous studies, to 300 microarcseconds, equivalent to seeing a one euro coin from a distance of roughly 10,000 kilometres. The observations also allowed the team to pinpoint our distance to the centre of the Galaxy with great precision, which is now measured as 27,000 light years.
The orbits of the central stars led to the deduction that the black hole has a mass of around four million times the mass of the Sun, and is located 27,000 light years away from the Earth. Credit: ESO.
These groundbreaking results stemmed from observations dating back to 1992 with the SHARP camera attached to ESO's 3.5 metre New Technology Telescope located at the La Silla observatory in Chile. More observations have subsequently been made using two instruments mounted on ESO's 8.2 metre Very Large Telescope (VLT). In total, roughly 50 nights of observing time with ESO telescopes have been used to compile this incredible set of observations, resulting in a catalogue of stellar motions that, for the first time, allows scientists to look for common properties among the stars.
"The stars in the innermost region are in random orbits, like a swarm of bees," says Gillessen. "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 [the distance light travels in one full month], randomly oriented orbits inside – that's how the dynamics of the young stars in the Galactic Centre are best described."
One central star, S2, completed an orbit in just 15 years, enabling the astronomers to study its orbit and relation to the black hole in unprecedented detail. Credit: ESO.
One particular star, known as S2, completed one full revolution around the centre within 15 years, approaching the black hole to within just one light day, about five times the distance between the planet Neptune and the Sun. Yet the mystery still remains as to how these young stars came to be in the orbits they are observed to be in today. They are much too young to have migrated far, but it seems even more improbable that they formed in their current orbits where the tidal forces of the black hole act. We’ll have to wait for future observations to find the answers. "ESO still has much to look forward to," says Genzel. "For future studies in the immediate vicinity of the black hole, we need higher angular resolution than is presently possible."
According to Frank Eisenhauer, principal investigator of the next generation instrument GRAVITY, ESO will soon be able to obtain that much needed resolution. "The next major advance will be to combine the light from the four 8.2-metre VLT unit telescopes – a technique known as interferometry. This will improve the accuracy of the observations by a factor 10 to 100 over what is currently possible. This combination has the potential to directly test Einstein's general relativity in the presently unexplored region close to a black hole."
There are exciting times ahead for the laws of physics. The paper describing the 16 year study is currently in press for the Astrophysical Journal.