Sagittarius A*

Sagittarius A*
EHT Collaboration · CC BY 4.0 · source
Name	Sagittarius A*
Type	Supermassive black hole
Constellation	Sagittarius
Epoch	J2000
Distance	26,000 ly
Mass	~4.3×10^6 M☉
Radius event horizon	~12.7 million km

Sagittarius A* Sagittarius A* is the compact radio source at the center of the Milky Way associated with a supermassive black hole. It lies in the constellation Sagittarius near the radio source Sagittarius A and has been studied across radio, infrared, submillimeter, X-ray, and gamma-ray bands by observatories and missions worldwide. Observations by teams from institutions such as the Event Horizon Telescope, European Southern Observatory, National Radio Astronomy Observatory, Harvard–Smithsonian Center for Astrophysics, Max Planck Institute, and NASA missions like Chandra and NuSTAR have established its mass, dynamics, and accretion behavior.

Discovery and observations

Early radio surveys by the National Radio Astronomy Observatory and the Jodrell Bank Observatory identified strong emission from the Galactic Center region near the radio complex Sagittarius A. Infrared detections using the European Southern Observatory's facilities and the W. M. Keck Observatory resolved stellar motions reported by groups at the Max Planck Institute for Extraterrestrial Physics and the University of California, Los Angeles (UCLA), while X-ray variability was revealed by the Chandra X-ray Observatory and XMM-Newton teams. Very long baseline interferometry efforts by collaborations involving the Very Long Baseline Array, Atacama Large Millimeter/submillimeter Array, and the Event Horizon Telescope consortium provided high-resolution constraints. Groundbreaking proper-motion studies of stars by researchers affiliated with Harvard University, the California Institute of Technology, and the Space Telescope Science Institute used adaptive optics on instruments such as NIRC2 and instruments at the Keck Observatory and Very Large Telescope to track orbits of stars like S2 and S0-2. Historical radio mapping tied to surveys from the Green Bank Telescope and the Parkes Observatory contextualized the source within the complex interstellar environment cataloged by the Two Micron All-Sky Survey and the Infrared Astronomical Satellite.

Physical properties

Mass estimates stem from orbital analyses of stars by groups from the Max Planck Institute for Extraterrestrial Physics, Harvard–Smithsonian Center for Astrophysics, and teams led by researchers at UCLA and Caltech, yielding a mass of several million solar masses. Its low luminosity relative to supermassive black holes in active galactic nuclei was characterized by multiwavelength campaigns involving Hubble Space Telescope observations of the nuclear cluster and the Spitzer Space Telescope mid-infrared studies. Spectroscopic work using instruments at the European Southern Observatory and the National Aeronautics and Space Administration enabled measurement of radial velocities and stellar dynamics. The accretion flow models developed by theorists at Princeton University, Columbia University, and the University of Arizona consider radiatively inefficient accretion flows, magnetohydrodynamic simulations by groups at the Princeton Plasma Physics Laboratory and the Max Planck Institute. Comparisons to extragalactic nuclei such as M87, NGC 4258, and Centaurus A illustrate differences in jet activity and radiative efficiency.

Event horizon and structure

Constraints on the event horizon scale derive from very long baseline interferometry by the Event Horizon Telescope collaboration, the Global mm-VLBI Array, and follow-up analyses by teams at the MIT Haystack Observatory and the Max Planck Institute for Radio Astronomy. Relativistic models informed by general relativity tests performed using orbits measured by groups at Harvard University and UCLA evaluate the Schwarzschild radius and potential spin parameters, employing ray-tracing codes developed at Rutgers University, Perimeter Institute, and Princeton University. Comparisons to theoretical predictions by researchers at Cambridge University, Yale University, and Stanford University probe the shadow, photon ring, and lensing signatures expected from strong-field gravity. Numerical relativity simulations from collaborations at NASA Jet Propulsion Laboratory and the Kavli Institute for Theoretical Physics contribute to imaging expectations and polarization structure.

Variability and flares

X-ray and near-infrared flares recorded by the Chandra X-ray Observatory, XMM-Newton, and the Keck Observatory reveal transient increases in flux; coordinated campaigns with the Submillimeter Array and ALMA tracked correlated variability. Theoretical interpretations by groups at Cambridge University, Max Planck Institute for Astrophysics, and Columbia University invoke magnetic reconnection, hot-spot orbital motion, and turbulent episodic accretion, with models tested against timing studies from the European Space Agency missions and ground arrays such as the Very Large Array. High-energy detections and upper limits from the Fermi Gamma-ray Space Telescope and INTEGRAL informed particle acceleration scenarios developed at Princeton University, University College London, and the University of Chicago.

Surrounding environment and stellar dynamics

The nuclear star cluster and the S-star cluster near Sagittarius A* were characterized by adaptive optics teams at the Keck Observatory, the Very Large Telescope, and institutions including the Max Planck Institute for Extraterrestrial Physics and UCLA. Long-term monitoring of stellar orbits like S2 and S0-102 provided strong evidence for a compact massive object and enabled relativistic tests pursued by scientists at Harvard University, Caltech, and MPI. Gas features such as the circumnuclear disk, mini-spiral, and molecular clouds were mapped by the Atacama Pathfinder Experiment, ALMA, and the James Clerk Maxwell Telescope; interactions with sources like the G2 cloud were monitored by teams at UC Berkeley and ETH Zurich. Tidal disruption event studies and hypervelocity star research connecting to the Galactic Center involved researchers from University of Cambridge, Ohio State University, and University of Toronto.

Imaging and interferometry studies

The Event Horizon Telescope collaboration, with contributions from institutions including the Max Planck Institute for Radio Astronomy, MIT Haystack Observatory, Harvard-Smithsonian Center for Astrophysics, and the National Radio Astronomy Observatory, produced the first horizon-scale images at millimeter wavelengths, analyzed with algorithms developed at Perimeter Institute, MIT, and Caltech. VLBI arrays such as the Very Long Baseline Array, the Global mm-VLBI Array, and facilities at the IRAM observatory contributed to baseline coverage. Polarimetric and closure-phase analyses by groups at University of Oxford, University of Arizona, and University of Copenhagen constrained magnetic field geometry and emission models. Comparative imaging work connecting to long-baseline studies of M87 leveraged software and statistical methods from research teams at Princeton University, Cambridge University, and Yale University to validate image fidelity and interpret the observed crescent morphology.

Category:Supermassive black holes Category:Galactic Center objects