Why the Milky Way's central black hole earned an asterisk
When Robert Brown named the compact radio source at the heart of our galaxy in 1982, he tacked on an asterisk because he found it exciting, borrowing the physicists' symbol for an excited atom. Sagittarius A* turned out to be a black hole weighing about 4.3 million Suns.
The trail began in April 1933, when Karl Jansky picked up radio noise from the direction of Sagittarius. Australian observers later pinned down a bright source there as the probable galactic centre. In February 1974 Bruce Balick and Brown, using a National Radio Astronomy Observatory interferometer, found a tiny, intense component buried inside the larger Sagittarius A complex. Thick dust and gas dim it by 25 magnitudes, so it has never been seen in visible light.
Proof that it was a black hole came from stars whipping around it. In 1994 a Berkeley team including Charles Townes and Reinhard Genzel showed that roughly 3 million solar masses were packed into the centre. In 2002 Genzel's group at the Max Planck Institute reported ten years of tracking the star S2, whose elliptical orbit centred exactly on the radio source. That orbit implied about 4.1 million solar masses squeezed within 17 light-hours, excluding alternatives such as a swarm of dark stars. Genzel and Andrea Ghez shared half of the 2020 Nobel Prize in Physics for the finding, with Roger Penrose taking the other half for theoretical work.
On May 12, 2022, the Event Horizon Telescope Collaboration released the first picture of the glowing material around its event horizon, only the second image of any black hole after the one in M87 in 2019. Eight radio observatories at six sites had gathered the data in 2017; turning them into an image took five years, partly because the source flickers within minutes. The ring measures about 51.8 million kilometres across, larger than Mercury's closest approach to the Sun, and the results matched Einstein's relativity.
The black hole is surprisingly quiet. Airborne observations in 2019 showed magnetic fields steering the surrounding gas and dust into orbit rather than letting it pour straight in, keeping its emissions low.
Source: Sagittarius A*
