Sgr A*

Sgr A*
EHT Collaboration · CC BY 4.0 · source
Name	Sgr A*
Type	Supermassive black hole (radio source)
Constellation	Sagittarius
Epoch	J2000
Ra	17h 45m 40.0409s
Dec	−29° 00′ 28.118″
Distance ly	25,640
Mass	≈4.1×10^6 M☉
Radius Schwarzschild	≈0.08 AU
Discovered	1974
Discoverers	Bruce Balick; Robert L. Brown

Sgr A* is the compact radio source at the dynamical center of the Milky Way identified with a supermassive black hole. Located in the direction of the constellation Sagittarius, it anchors the Galactic nucleus and influences the orbits of nearby stars and gas clouds. Observations across radio, infrared, X-ray, and submillimeter bands have established its mass, compactness, and variable emission, making it a focal point for studies connecting general relativity, accretion physics, and galactic nuclei.

Introduction

The radio source was first cataloged during surveys with the National Radio Astronomy Observatory and later studied with the Very Large Array, linking a compact nonthermal source to the dynamical center of the Milky Way. Subsequent work employed instruments and facilities including the Keck Observatory, Very Long Baseline Array, Atacama Large Millimeter/submillimeter Array, Event Horizon Telescope, and X-ray satellites such as Chandra X-ray Observatory. Key researchers and teams involved include Bruce Balick, Robert L. Brown, Reinhard Genzel, Andrea Ghez, and the collaborations behind the GRAVITY instrument and the EHT consortium.

Physical Properties

Mass estimates derive from stellar dynamical measurements of stars like S2 using adaptive optics on W. M. Keck Observatory and the Very Large Telescope, yielding ≈4 million solar masses comparable to the central black holes in Andromeda Galaxy and in galaxies studied by the Sloan Digital Sky Survey. The inferred Schwarzschild radius is on the order of tens of microarcseconds as probed by very long baseline interferometry such as the Event Horizon Telescope. The source exhibits low bolometric luminosity compared with quasars studied by the Sloan Digital Sky Survey and Hubble Space Telescope surveys, placing it in the class of low-luminosity active galactic nuclei explored by the Chandra X-ray Observatory and XMM-Newton. Magnetic fields in the vicinity have been probed through polarization measurements with facilities like the Submillimeter Array and ALMA.

Observational History

Radio discovery in 1974 followed early mapping campaigns by the Green Bank Telescope and the Effelsberg 100-m Radio Telescope. Infrared astrometry advanced via speckle imaging on the Keck Observatory and the Very Large Telescope instrumentation including NAOS-CONICA and later the GRAVITY instrument on the Very Large Telescope Interferometer. X-ray flares were first resolved by Chandra X-ray Observatory and further characterized by XMM-Newton and NuSTAR. The source became a prime target for millimeter VLBI with the Event Horizon Telescope, a global collaboration integrating sites like the Submillimeter Array, IRAM 30m Telescope, Large Millimeter Telescope Alfonso Serrano, and the South Pole Telescope.

Accretion and Emission Processes

Accretion flows near the compact object are modeled by radiatively inefficient accretion flow frameworks developed in studies associated with RIAF models, magnetohydrodynamic simulations performed with codes from groups at institutions like Harvard-Smithsonian Center for Astrophysics, Max Planck Institute for Extraterrestrial Physics, and Princeton University. Synchrotron emission dominates the radio and submillimeter regime as described by theory from researchers at Stanford University and University of California, Berkeley. X-ray flares are interpreted using magnetic reconnection scenarios tested by teams at NASA Goddard Space Flight Center and by authors affiliated with Columbia University. Particle acceleration, turbulence, and jet launching mechanisms are compared to benchmarks from studies of M87 and blazar work involving the Fermi Gamma-ray Space Telescope.

Surrounding Environment and Stellar Dynamics

The nuclear star cluster hosts young massive stars including the IRS 16 group and the S-star cluster with objects like S2 whose orbital parameters were crucial to mass determination, studied by groups led by Reinhard Genzel and Andrea Ghez. Gas structures such as the circumnuclear disk and features like G2 (a gas cloud or stellar object) were monitored by researchers at European Southern Observatory and University of California, Los Angeles. Dynamics link to tidal disruption event theory developed at University of Leicester and to stellar evolution models from Institute for Advanced Study collaborations. Interactions with molecular clouds in the Central Molecular Zone and with nonthermal filaments mapped by the Very Large Array affect star formation traced by the Herschel Space Observatory and Spitzer Space Telescope.

Imaging and Interferometry Studies

High-angular-resolution imaging used interferometers including the Event Horizon Telescope, Very Long Baseline Array, and the Very Large Telescope Interferometer with the GRAVITY instrument; the EHT imaging algorithms were developed by teams across MIT Haystack Observatory, Haystack Observatory, Harvard University, and Max Planck Institute for Radio Astronomy. Closure phase, visibility amplitude, and polarimetric VLBI constrain horizon-scale structure consistent with predictions from general relativity tests and ray-tracing codes used by groups at Perimeter Institute and Kavli Institute for Particle Astrophysics and Cosmology. Comparative analyses reference imaging of M87* and methods from radio astronomy standards set by the International Astronomical Union.

Theoretical Models and Open Questions

Competing theoretical frameworks include RIAF, jet-dominated, and magnetically arrested disk models developed at Princeton University, Caltech, and Cambridge University; numerical relativity and GRMHD simulations come from teams at Max Planck Institute for Astrophysics and NASA Ames Research Center. Open questions address horizon-scale tests of general relativity, black hole spin measurement efforts paralleling studies at LIGO Scientific Collaboration and Virgo Collaboration, and the origin of flares relative to magnetic flux accumulation analyzed by groups at University of Oxford and Columbia University. Future constraints will come from next-generation facilities including the expanded Event Horizon Telescope, the Thirty Meter Telescope, the European Extremely Large Telescope, and space missions proposed to build on legacy from Hubble Space Telescope and James Webb Space Telescope.

Category:Milky Way Category:Black holes Category:Radio sources