At the end of the 1960s and early 1970s, Donald Lynden-Bell and Martin Rees proposed that the Milky Way and perhaps most other galactic nuclei might contain a central massive black hole. But the evidence for such an object was lacking at the time because the centre of the Milky Way is obscured by interstellar dust, and was detected only as a relatively faint radio source.

In the late 1970s and early 1980s, Charles Townes and his collaborators including Reinhard Genzel, developed instruments capable of observing the centre of the Milky Way at infrared wavelengths, which can pass through the interstellar dust clouds with relatively little obscuration. By analyzing the spectrum of such radiation, they inferred that gas is swirling around a central concentration containing a few million solar masses. These authors suggested that the central object might be a supermassive black hole, but the observations did not have sufficient angular resolution to prove that conjecture.

Starting in 1990 Genzel and his collaborators continued to develop new instruments to observe the centre of the Milky Way at near-infrared wavelengths and with unprecedented angular resolution. In 1996, they reported two independent observations of the motions of stars clustered very close to the centre of the Milky Way, both of which provided compelling evidence that the central object was indeed a supermassive black hole. In the first technique they measured the velocities of the stars through the Doppler shifts of their spectra, while in the second technique they measured the transverse velocities of their positions. Subsequently, Genzel’s group and, independently, a group led by Andrea Ghez, continued to track the orbits of these stars around the black hole and refined the estimate of its mass, which we now know is about a million Suns. The Galactic Centre thus constitutes now the best evidence that the black holes predicted by General Relativity are realized in nature.

In their ongoing campaign to observe the centre of the Milky Way, Genzel and his group have continued to make major discoveries. By measuring the spectra and motions of the luminous stars near the centre of the Milky Way, they have found evidence that many of these stars were probably formed relatively recently (some six million years ago) from a disk of gas orbiting the supermassive black hole. They have also discovered simultaneous flares of infrared and X-ray emission from the central black hole, evidently due to sporadic accretion of gas.

The proof by Genzel’s group that the concentration of bright stars at the centre of the Milky Way are orbiting a supermassive black hole leaves little doubt that similar concentrations of stars seen at the centres of other nearby galaxies also manifest the presence of supermassive black holes. The fact that many galaxies contain supermassive black holes is one of the outstanding clues to the mystery of how galaxies form.

In addition to his work on the centre of the Milky Way, Reinhard Genzel and his group have made many contributions to the study of the formation of stars in other galaxies in the nearby and distant universe through his development of pioneering instrumentation to observe these systems at infrared wavelengths. For his great achievements in observational astronomy, he is a most worthy recipient of the Shaw Prize in Astronomy for 2008.

9 September 2008, Hong Kong