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The heavyweight of the Milky Way

Images taken between March and July 2021 show the trajectory of stars orbiting Sagittarius A*

ESO / GRAVITY collaboration

Highly accurate measurements of the movement of stars closest to the black hole at the center of the Milky Way have allowed scientists to accurately calculate the distribution of mass at the heart of the galaxy. Using telescopes based in the planet’s northern and southern hemispheres, an international group of astronomers and astrophysicists found that 99.9% of the matter at the center of the galaxy is held by Sagittarius A*, the black hole whose mass is 4.3 million times that of the Sun. The remaining 0.1% is due to stars, smaller black holes, clouds of interstellar gas and dust, and dark matter (Astronomy & Astrophysics, in press). Despite decades of close observation, it had previously been difficult to prove that nearly all of the region’s mass was concentrated in this black hole. According to Albert Einstein’s theory of general relativity, stars orbiting high-mass objects have a rosette-shaped trajectory. The group of researchers, which included astrophysicist Reinhard Genzel, director of the Max Planck Institute for Extraterrestrial Physics and winner of the Nobel Prize in Physics 2020, inferred the distribution of mass in the region by following the movement of four stars (S2, S29, S38, and S55) in the immediate vicinity of Sagittarius A*.