The Milky Way, like other galaxies, is thought to harbour a black hole at its centre. The remarkable observation of a star in close orbit around the Galactic Centre is the first firm evidence that this is so.
Reporting on page 694 of this issue, Schödel et al.1 describe the first observation of a nearly complete orbit of a star around the black hole at the centre of our Galaxy, the Milky Way. We have long been able to trace the movement of planets in our own Solar System, and astronomers are now beginning to observe planetary orbits in other stellar systems2, but this is the first time that an orbit has been detected on the galactic scale. The sheer size of galaxies normally makes the detection of such movement impossible within a human lifetime. For example, our own Sun takes 230 million years to circle the Milky Way. The star that Schödel et al. report on will complete its orbit around the central black hole of our Galaxy in a mere 15 years — lightning speed on the grand, slow scale of the Universe.
That we are now able to watch the orbit of a star across our Galaxy is only one of several important implications of Schödel and colleagues' findings. For many years now, astronomers have been reporting that supermassive black holes — more than a million times the mass of the Sun — exist in nearly every galaxy3; scientists now even have data that suggest that black holes also occupy the centres of smaller stellar systems called globular clusters4,5. But despite decades of research and discovery, no one had found conclusive evidence that supermassive black holes exist.
The problem was that astronomers had never been able to observe the centres of galaxies closely enough to rule out other possibilities, such as a collection of neutron stars masquerading as a central black hole6. But these new data probe the Galactic Centre more closely than ever before. The matter density that Schödel et al.1 infer from the details of the star's orbit is inconsistent with the presence of neutron stars, or other more exotic objects. The only compelling explanation is that there is a supermassive black hole lurking there. These results are the best evidence yet that supermassive black holes are not just theory, but fact.
The technique used by Schödel et al. to measure the stellar orbit is also impressive. Their success shows the true power of a relatively new observing tool, adaptive optics imaging7. Starlight reaching ground-based telescopes becomes blurred as it travels through the Earth's atmosphere. In an adaptive optics system, the distortions in the incoming beam are measured, and electronic signals sent to a deformable mirror. In response, the mirror can rapidly change its shape to correct for those distortions as it reflects the starlight. The blurring effects of the atmosphere are removed almost completely from the data and thus ground-based observations can be as sharp and informative as those from space-based telescopes.
Using this new technology, we are now able to see images that are up to 20 times sharper than they once appeared, making it possible to differentiate individual stars in the crowded stellar regions at the centre of the Milky Way (Fig. 1). Faint stars that were practically invisible can now be isolated, and more stars in orbit at the very centre of our Galaxy could be found. Measuring the orbits of these stars should provide even stronger evidence in favour of a black hole, and it should eventually be possible to test the predictions8 of general relativity using stars that pass even closer to the black hole. The power of adaptive optics may also enable us to determine how material is funnelled into a supermassive black hole or, in the case of the black hole in the Milky Way, why so little matter is actually consumed by it9.
Although the main point of interest for the layperson may be the proof that black holes are real, scientific research is more concerned with gathering the best possible data and determining the most accurate results. The black hole in our Galaxy does not have the best-determined mass, but with continued observations, following those of Schödel et al. and others10, it will soon be the best constrained. The exact value of its mass has important implications for understanding how our black hole compares with those at the centres of other galaxies11. We still have a way to go, but for this black hole the future is bright.
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