Reinhard Genzel

Major contributions

• Led a decades-long infrared monitoring programme of stars in the central parsec of the Milky Way, measuring their motions and orbits and showing that they move under the gravity of a compact, dark mass of about four million solar masses (Milky Way)
• With the star S2 (orbital period about 16 years), helped demonstrate that the central mass is confined to a region so small that, on current evidence, a supermassive black hole is by far the most plausible explanation
• Co-led the GRAVITY collaboration at the European Southern Observatory's Very Large Telescope Interferometer, which measured the gravitational redshift of S2 (2018) and the Schwarzschild precession of its orbit (2020), both consistent with general relativity
• Contributed widely to infrared and submillimetre astronomy, including studies of star formation, active galactic nuclei (AGN), and galaxies in the early universe

Named concepts

Concept	Statement	Codex page
Sagittarius A* evidence	Stellar orbits show a ~4 million solar-mass compact object at the Galactic Centre	View entry
S2 gravitational redshift (GRAVITY, 2018)	First detection of gravitational redshift in a star orbiting a massive black hole	View entry

Career

Genzel studied physics at the University of Freiburg and the University of Bonn, and earned his doctorate in 1978 from the University of Bonn, working at the Max Planck Institute for Radio Astronomy. He then held positions at the Harvard–Smithsonian Center for Astrophysics and the University of California, Berkeley, where he worked with Charles Townes. In 1986 he became a director at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, and he also held a professorship at Berkeley for many years.

Beginning in the early 1990s, Genzel's group used near-infrared cameras and spectrographs, first at the New Technology Telescope and later with adaptive optics at the Very Large Telescope in Chile, to follow individual stars through the dust that hides the Galactic Centre at visible wavelengths. Their measured stellar proper motions and velocities (Eckart and Genzel, 1996) pointed to a very concentrated central mass; with years of additional data, including full orbits for S2, the evidence for a compact object of about 4 million solar masses became very strong. The independent programme led by Andrea Ghez at the Keck Observatory reached consistent results, and their friendly rivalry is often credited with accelerating progress.

Key publications

• Eckart, A., Genzel, R. (1996). “Observations of stellar proper motions near the Galactic Centre.” Nature, 383, 415–417.
• Genzel, R., Eisenhauer, F., Gillessen, S. (2010). “The Galactic Center massive black hole and nuclear star cluster.” Reviews of Modern Physics, 82, 3121–3195.
• GRAVITY Collaboration (2018). “Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole.” Astronomy & Astrophysics, 615, L15.

Nobel Prize

Genzel shared the 2020 Nobel Prize in Physics. One half went to Roger Penrose “for the discovery that black hole formation is a robust prediction of the general theory of relativity”, and the other half was shared equally by Reinhard Genzel and Andrea Ghez “for the discovery of a supermassive compact object at the centre of our galaxy”.

Historical context

Black holes were for decades regarded as mathematical consequences of general relativity (see Schwarzschild metric) rather than observed objects. The Galactic Centre work turned a theoretical prediction into measured orbital dynamics of individual stars, complementing the direct detection of gravitational waves from merging black holes (2015) and the later Event Horizon Telescope images. The Nobel citation says “compact object” because orbital data constrain mass and size but do not by themselves prove a horizon; the black-hole interpretation is, however, the only one consistent with all current observations. Two independent groups reaching the same result is itself a strong form of verification.