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The History of the Discovery of Sgr A*
At the center of the Milky Way hides a four-million solar mass black hole. The confirmation of its existence, which took place thanks to the study of star orbits conditioned by its immense gravity, earned the Nobel Prize 2020 for physics to Andrea Ghez and Reinhard Genzel. But the story of the observations that led to the discovery of this supermassive black hole begins in the 1950s and deserves to be told
A powerful radio source towards the Galaxy’s darkened center
In 1968, Eric E. Becklin and Gerald Neugebauer, two Caltech astronomers, managed to scan the central parsecs of the Milky Way in four different infrared wavelengths, obtaining the best results at 2.2 µm. Overcoming 25 magnitudes of obscuration due to the dust in the interposed spiral arms, they discovered swarms of stars huddled together with an unlikely density, compared to the enormous distances that, in the galactic periphery, separate the Sun from its neighbors. An article published in Scientific American in April 1974 (R.H. Sanders and G.T. Wrixon, “The Center of the Galaxy”) evocatively summarized what Becklin and Neugebauer had observed.
The two authors wrote that infrared observations showed that the galactic center contains about one million stars per cubic parsec, a stellar density about one million times that of the Sun’s surroundings. This implies that a living being on a planet orbiting a star at the Milky Way’s center would see a million bright stars like Sirius, the brightest star in our sky. The integrated luminosity of all the stars in the night sky of such a planet would be equal to about 200 full moons. Under these conditions, optical astronomy should limit itself to studying only the brightest nearby objects. Even the light from the closest galaxies would be dimmed. However, they also pointed out that it is quite unlikely that any form of life could exist on planets in the galactic nucleus, given that at such high stellar densities, the stars would graze each other so frequently that they would tear the planets from…
