Sagittarius A*

Sagittarius A*
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Name	Sagittarius A*
Type	Supermassive black hole
Location	Sagittarius (constellation), Milky Way
Mass	~4.1 million M☉
Distance	~26,000 light-years
Constellation	Sagittarius (constellation)
Epoch	J2000
Discovery date	1974
Discoverers	Bruce Balick, Robert L. Brown

Sagittarius A* is the supermassive black hole residing at the dynamical center of the Milky Way galaxy. Located in the constellation Sagittarius (constellation), it is the closest such object to Earth, providing a unique laboratory for studying the physics of extreme gravity. Its existence and properties have been inferred through decades of observations of the motions of stars and gas in its immediate vicinity, culminating in the first direct image released by the Event Horizon Telescope collaboration.

Discovery and observation

The discovery of Sagittarius A* emerged from radio astronomy studies of the Galactic Center. In February 1974, astronomers Bruce Balick and Robert L. Brown used the National Radio Astronomy Observatory's Green Bank Telescope to identify a compact, bright radio source at the Milky Way's core, which Brown later designated Sagittarius A*. Subsequent observations with instruments like the Very Large Array and the Keck Observatory revealed high-velocity stars, such as S2, orbiting an unseen massive object. The Max Planck Institute for Extraterrestrial Physics played a leading role in these infrared monitoring campaigns, which tracked stellar orbits to precisely determine the central mass. A major breakthrough came in 2022 when the Event Horizon Telescope, a global network of radio observatories including the Atacama Large Millimeter Array and the South Pole Telescope, produced the first resolved image of its shadow.

Physical characteristics

Sagittarius A* has a mass equivalent to approximately 4.1 million times that of the Sun, confined within a region smaller than the orbit of Mercury (planet). Its Schwarzschild radius, the point of no return, is about 12 million kilometers. The black hole's spin and exact magnetic field structure remain active areas of study, with data from the Chandra X-ray Observatory and the Neil Gehrels Swift Observatory providing clues. Its relative quiescence and low accretion rate, compared to active galactic nuclei like Messier 87's central black hole, make it an intriguing subject. The environment is characterized by intense gravitational lensing effects and powerful flares observed in X-ray and infrared wavelengths by telescopes like NuSTAR.

Role in the Milky Way

As the central gravitational anchor of the Milky Way, Sagittarius A* profoundly influences the dynamics and evolution of our galaxy. It governs the orbits of millions of stars in the surrounding Nuclear Star Cluster and the innermost region of the Galactic Bulge. The black hole's gravity also shapes the structure of the Central Molecular Zone, a dense region of gas and dust. While currently quiet, past episodes of higher activity, possibly recorded in Fermi Bubbles observed by the Fermi Gamma-ray Space Telescope, may have regulated star formation. Its presence is fundamental to models of galaxy formation, linking the Milky Way to other spirals like the Andromeda Galaxy.

Event horizon and accretion

The immediate vicinity of Sagittarius A* is a region of extreme spacetime curvature dominated by its event horizon. Infalling material from the surrounding accretion disk becomes heated to millions of degrees, emitting radiation before crossing the horizon. The accretion flow is notably tenuous and radiatively inefficient, described by models like the Advection-Dominated Accretion Flow. Occasional bright flares, likely caused by magnetic reconnection or discrete accretion events, are detected by instruments on the Hubble Space Telescope and the Spitzer Space Telescope. The first image from the Event Horizon Telescope revealed a bright ring of photons orbiting in the strong gravity, consistent with predictions from Albert Einstein's general relativity.

Comparison with other black holes

Sagittarius A* serves as a crucial benchmark for understanding black holes across the mass spectrum. It is significantly less massive and less active than the supermassive black holes powering quasars, such as those in Messier 87 or Centaurus A. Compared to stellar-mass black holes like Cygnus X-1, it is millions of times more massive but has a much lower accretion luminosity relative to its mass. Its observed properties bridge the gap between the well-studied stellar remnants in the Milky Way and the enormous black holes in distant galaxies observed by the Sloan Digital Sky Survey. Studies of its stellar orbits provide the strongest empirical evidence for the existence of supermassive black holes, a cornerstone of modern astrophysics.

Category:Supermassive black holes Category:Milky Way Category:Astronomical radio sources