Sagittarius A*

Sagittarius A* is a compact radio source at the Milky Way’s center, associated with a black hole of about four million solar masses.

Event horizon: The boundary around a black hole beyond which nothing can escape to distant observers. It marks the boundary of the black hole associated with this radio source.
Karl Jansky: An American radio engineer whose 1930s observations identified radio emission from the direction of the Milky Way’s center. His discovery established the central radio source’s observational history.
Milky Way: The barred spiral galaxy containing the Sun and the Solar System. Sagittarius A* lies at the center of this galaxy.
Event Horizon Telescope: A global network of radio observatories that acts as an Earth-sized telescope at millimeter wavelengths. Its observations resolved emission around Sagittarius A* at horizon-scale resolution.
M87*: The supermassive black hole at the center of the galaxy Messier 87, imaged by the Event Horizon Telescope in 2019. It is a much more massive black hole whose image appeared before Sagittarius A*’s.
Accretion disk: A rotating flow of gas and dust that spirals toward a massive central object. Infalling gas near the black hole can heat and emit radiation.
Bruce Balick: An American astronomer who, with Robert Brown, identified Sagittarius A* as a compact radio source in 1974. The name Sagittarius A* dates to their identification of the compact component.
Galactic Center: The central region of a galaxy, where stars, gas, and often a massive black hole are concentrated. The radio source is embedded in the Milky Way’s central region.
Sagittarius A* black hole image: The Event Horizon Telescope’s 2022 image of a bright ring of emission surrounding a central dark region at the Milky Way’s center. It provided the first horizon-scale image of this source.
Stellar-mass black hole: A black hole formed from the collapse of a massive star, typically containing a few to dozens of solar masses. Its mass is far smaller than Sagittarius A*’s supermassive black hole.

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