Reinhard Genzel

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Lore & Background

Reinhard Genzel led an international team at the Max Planck Institute for Extraterrestrial Physics that, on October 16, 2002, reported the observation of the motion of the star S2 near Sagittarius A* throughout a period of ten years. According to the team's analysis, the data ruled out the possibility that Sgr A* contains a cluster of dark stellar objects or a mass of degenerate fermions, strengthening the evidence for a massive black hole. The observations used near-infrared interferometry in the Ks-band (2.1 μm) because of reduced interstellar extinction in that band. SiO masers were used to align NIR images with radio observations. From examining the Keplerian orbit of S2, they determined the mass of Sagittarius A* to be 4.1±0.6 million solar masses, confined in a volume with a radius no more than 17 light-hours (120 AU). Later observations of the star S14 showed the mass to be about 4.1 million solar masses within a volume with radius no larger than 6.25 light-hours (45 AU). Genzel's team also determined the distance from Earth to the Galactic Center as 8,000 ± 600 parsecs (30,000 ± 2,000 light-years). In 1994, infrared and sub-millimetre spectroscopy studies by a Berkeley team involving Nobel Laureate Charles H. Townes and future Nobel Prize Winner Reinhard Genzel showed that the mass of Sgr A* was tightly concentrated and on the order of 3 million Suns. After monitoring stellar orbits around Sagittarius A* for 16 years, Gillessen et al. (with Genzel as team leader) estimated the object's mass at 4.31±0.38 million solar masses, announced in 2008 and published in 2009. Genzel said the study delivered 'what is now considered to be the best empirical evidence that supermassive black holes do really exist. The stellar orbits in the Galactic Center show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt.'

Reader's Guide

Reinhard Genzel's key contribution is the observational proof that Sagittarius A* is a supermassive black hole at the centre of the Milky Way. Using infrared and submillimetre spectroscopy, his team tracked the orbits of stars like S2 and S14 around Sgr A* over many years. The Keplerian orbit of S2 gave a mass of 4.1±0.6 million solar masses within a radius of 17 light-hours, and later S14 refined that to 4.1 million solar masses within 6.25 light-hours. These measurements ruled out alternative explanations such as a cluster of dark stellar objects or degenerate fermions. Genzel's work, alongside Andrea Ghez's, earned them each a quarter-share of the 2020 Nobel Prize in Physics, with Sir Roger Penrose receiving the other half. The discovery confirmed that the only explanation for the compact object at the Galactic Center is a black hole, fundamentally changing our understanding of the Milky Way's structure and the role of supermassive black holes in galaxies.

Did You Know?

• Reinhard Genzel was awarded a quarter-share of the 2020 Nobel Prize in Physics for his discovery that Sagittarius A* is a supermassive compact object, for which a black hole was the only explanation.
• In 1994, Genzel was part of a Berkeley team (including Nobel Laureate Charles H. Townes) that showed the mass of Sgr A* was tightly concentrated and on the order of 3 million Suns.
• On October 16, 2002, Genzel's team reported that observations of star S2 ruled out the possibility that Sgr A* contains a cluster of dark stellar objects or a mass of degenerate fermions.
• Genzel's team determined the distance from Earth to the Galactic Center as 8,000 ± 600 parsecs (30,000 ± 2,000 light-years).
• After 16 years of monitoring stellar orbits, Genzel's team estimated the mass of Sgr A* at 4.31±0.38 million solar masses, announced in 2008.