Sagittarius A*

The existence of a compact radio source at the Galactic center was first detected in 1974 by Bruce Balick and Robert Brown using the National Radio Astronomy Observatory's interferometer.¹ The source, named Sagittarius A*, was later identified as a supermassive black hole through the monitoring of stellar orbits by Reinhard Genzel's group at the Max Planck Institute for Extraterrestrial Physics and Andrea Ghez's group at UCLA.² These observations, which tracked stars like S2, provided the most compelling evidence for a black hole, leading to the 2020 Nobel Prize in Physics shared by Genzel and Ghez.

Sagittarius A* has a mass of approximately 4.3 million solar masses, as determined from the orbits of stars around it.³ Its event horizon has a diameter of about 12 million kilometers, which is roughly the size of Mercury's orbit. The black hole is relatively quiet, emitting low levels of X-ray and radio radiation, but it occasionally flares, likely due to accretion of gas clouds. In 2019, the Event Horizon Telescope (EHT) captured the first image of Sagittarius A*, revealing a shadow consistent with a black hole.⁴

The region around Sagittarius A* is dense with stars, gas, and dust. The black hole is surrounded by a cluster of young, massive stars, known as the S-stars, which orbit at high speeds. The central region also contains a circumnuclear disk of molecular gas and a number of X-ray binaries. The black hole's gravitational influence extends over a large area, shaping the dynamics of the entire Galactic center.⁵

Early observations of Sagittarius A* were limited to radio and infrared wavelengths due to dust obscuration. The development of adaptive optics and infrared interferometry allowed astronomers to resolve the orbits of individual stars. In 2002, the star S2 was observed to pass close to the black hole, providing a precise measurement of its mass. More recently, the GRAVITY instrument on the Very Large Telescope Interferometer has detected flares from the black hole, offering insights into its accretion flow.

One lesser-known fact is that Sagittarius A* is not exactly at the Galactic center; it is offset by a few light-years due to the gravitational influence of nearby stars. Another is that the black hole's spin has been estimated from the shape of its shadow, suggesting it is rotating. Additionally, the name “Sagittarius A*” was coined by Robert Brown in 1982, with the asterisk denoting a “star” in the sense of an excited state, as in atomic physics. The black hole is also a source of high-energy gamma rays, possibly from dark matter annihilation, though this remains speculative.⁶

Supermassive black hole

A black hole with a mass millions to billions of times that of the Sun, typically found at the centers of galaxies.

Event horizon

The boundary around a black hole beyond which nothing, not even light, can escape.

Solar mass

A unit of mass equal to the mass of the Sun, used to express the masses of stars and black holes.

Accretion

The process by which matter falls into a gravitational well, such as a black hole, often forming a disk.