Sagittarius A*

Sagittarius A* (Sgr A*) is the compact radio source associated with the Milky Way’s central massive object. Stellar orbits establish a roughly four-million-solar-mass concentration at the Galactic Center; millimeter very-long-baseline interferometry resolves emission on scales comparable to its gravitational radius. These mutually consistent results make the massive-black-hole interpretation exceptionally strong. They do not, by themselves, directly detect a material horizon or measure all Kerr multipoles.

Sagittarius A* in the Galactic Center

Property	Working statement
Object class	Named observed system
Designation	Sagittarius A*; Sgr A*
Location	Dynamical center of the Milky Way
Physical status	High-confidence massive-black-hole identification
Direct measurements	Stellar angles and radial velocities; radio-to-infrared fluxes and variability; calibrated interferometric visibilities and closure quantities
Model-dependent posteriors	Central mass, Galactocentric distance, orbital elements, angular gravitational scale, accretion geometry, and spacetime parameters
Evidence cutoff	20 August 2026

The radio source and the gravitational focus are tied through a common astrometric reference frame and the small proper motion of Sgr A*. The system is unusually valuable because individual stellar trajectories can be followed through periapse, but source crowding, extinction, reference-frame construction, and unresolved companions remain part of the measurement model.

Measured parameters

These representative numbers retain the inference layer and uncertainty convention reported by the source. In particular, an emission-ring diameter is an image-domain summary of calibrated interferometric data, whereas an angular gravitational radius additionally uses a spacetime and plasma-model calibration.

Quantity and dataset	Reported value	Measurement or inference status
Central mass from the GRAVITY S2 fit through 2019	$M_\bullet=(4.261\pm0.012)\times10^6\,M_\odot$	Parameter of a 14-parameter relativistic orbit fit; the quoted one-sigma fit and MCMC widths agree, but they are not a complete cross-analysis systematic budget (GRAVITY Collaboration 2020, Appendix E, Table E.1)
Galactocentric distance from the same fit	$R_0=8246.7\pm9.3\ \mathrm{pc}$	Joint orbit-fit parameter, strongly covariant with the central mass and astrometric reference frame; it is not statistically independent of the mass (GRAVITY Collaboration 2020, Appendix E, Table E.1)
Schwarzschild-precession scale	$f_{\rm SP}=1.10\pm0.19$	Posterior-fit result, where zero is Newtonian and one is the general-relativistic monopole prediction; the paper also reports an MCMC width of 0.21 (GRAVITY Collaboration 2020, Abstract and Appendix E, Table E.1)
2017 EHT emission-ring diameter	$d=51.8\pm2.3\ \mu\mathrm{as}$	Consensus image and geometric-model result; 68% credible interval, including method dispersion but conditional on the adopted variability treatment (Event Horizon Telescope Collaboration 2022, Table 1 and §5)
EHT angular gravitational radius	$\theta_g=4.8^{+1.4}_{-0.7}\ \mu\mathrm{as}$	68% credible interval after calibrating the emission-diameter-to-gravitational-scale ratio with Kerr GRMHD synthetic data; this is not a direct ring-size measurement (Event Horizon Telescope Collaboration 2022, Table 1 and §5.4)

The orbit posterior includes priors and nuisance parameters for the reference frame, orbital elements, and relativistic term. The EHT posterior instead marginalizes interferometric calibration and image/model parameters, then adds a Kerr-GRMHD calibration. Their uncertainties therefore must not be combined as if every row were an independent Gaussian datum.

Stellar-orbit inference

The data are angular positions on the sky and spectroscopic line-of-sight velocities at dated epochs. The long-baseline orbit programs of Ghez et al. 2008, §§2–4, especially §4.1 and Tables 4–5 and Gillessen et al. 2009, §§2–6, especially §6.2 and Tables 4–5 show how a relativistic orbital model predicts these observables from the black-hole mass $M$, distance $R_0$, orbital elements, extended mass, and reference-frame parameters. At Newtonian order, astrometry constrains approximately $M/R_0^3$, whereas velocities introduce a physical scale and help break the covariance. The fitted $M$ and $R_0$ are therefore correlated posterior parameters, not independent direct measurements.

The short-period star S2 supplies gravitational-redshift and transverse-Doppler information around periapse (GRAVITY Collaboration 2018, §§2–3 and Appendix A, article pp. 2–8) and, over a complete orbit, Schwarzschild apsidal precession. The later GRAVITY analysis found the precession consistent with the general-relativistic monopole prediction under its orbital and extended-mass model (GRAVITY Collaboration 2020, §§3.1–3.2, Eq. (1), and Appendices C and E). This probes gravity well outside the horizon. Frame dragging and a quadrupole moment are much smaller effects and require careful separation from distributed mass and perturbations by other stars.

Layer	Quantity	Dominant uncertainty type
Observable	Pixel or interferometric position, wavelength-derived velocity, time stamp	Measurement noise, source confusion, wavelength calibration
Calibration	Infrared-to-radio reference frame and spectroscopic zero point	Correlated systematic uncertainty
Orbit posterior	$M$, $R_0$, elements, relativistic terms, extended mass	Statistical covariance plus model uncertainty
Interpretation	Compact central object consistent with a massive black hole	Alternative mass distributions, formation and lifetime constraints

The detailed dynamical machinery belongs with Relativistic Stellar Orbits as Probes and the hypothesis-testing framework with Relativistic Orbits Around Massive Black Holes.

From millimeter data to a ring scale

For independently inferred mass and distance, the angular gravitational scale is

$$\theta_g=\frac{GM}{c^2R_0}.$$

Using only the central values from the 2020 GRAVITY fit gives the transparent scale check

$$\theta_g =\frac{G(4.261\times10^6M_\odot)}{c^2(8246.7\ \mathrm{pc})} =5.10\ \mu\mathrm{as}.$$

This central-value conversion lies inside the much broader EHT image-based interval. It is not a new uncertainty estimate: $M_\bullet$ and $R_0$ share a correlated orbit posterior, so a defensible error bar must be computed from their joint samples rather than by adding the two tabulated widths in quadrature.

The Event Horizon Telescope records correlated millimeter signals on Earth-sized baselines. After station calibration and scattering mitigation, the principal data products are visibility amplitudes and closure phases or amplitudes. Images are regularized reconstructions of sparse Fourier data. For Sgr A*, variability on observing timescales makes a static source approximation especially delicate.

The 2017 data support a ring-like brightness structure with a scale consistent with the stellar-orbit prior under the collaboration’s geometric and general-relativistic magnetohydrodynamic model families (Event Horizon Telescope Collaboration 2022, Paper I, §§2–4 and 6–8). The direct observable is not a shadow diameter. Translating a fitted emission ring into a critical-curve scale depends on plasma emissivity, scattering, temporal averaging, calibration, imaging regularization, and the simulation library.

Polarization and infrared or X-ray flares constrain magnetic-field geometry and compact emitting regions, but neither a flare period nor a polarization pattern yields spin uniquely. Orbiting-hotspot, turbulence, reconnection, and lensing models can produce overlapping signatures.

Compactness, Kerr tests, and systematics

Stellar dynamics confines the mass far more tightly than any long-lived cluster of ordinary remnants can plausibly survive. Low luminosity and surface-emission arguments add evidence for an absorbing compact object, while horizon-scale imaging tests the expected angular scale. The combined case for a massive black hole is stronger than any one channel.

The conclusion ceiling is nevertheless precise. Present data do not constitute a local horizon detection, do not exclude every engineered horizonless model independently of its stability and emission physics, and do not measure the Kerr quadrupole. A robust imaging test must compare candidate spacetimes in visibility space, propagate the stellar-orbit $M/R_0$ posterior, and marginalize plasma and calibration uncertainty. That full assessment belongs with Horizon-Scale Imaging Tests of Kerr.

The validity domain of the simplest cross-check is restricted to the measured orbit and interferometric epochs: propagate the dynamical $M/R_0$ posterior into $\theta_g$, then ask whether independently calibrated visibility models recover a compatible angular scale.

Major systematics include reference-frame drift, source confusion near periapse, spectroscopic calibration, the extended stellar-remnant distribution, unmodeled stellar perturbations, interstellar scattering, millimeter variability, sparse baseline coverage, station gain calibration, and simulation-library misspecification. Statistical intervals from one orbit or image model do not include all of these effects.

Limitations

Sgr A* is one low-luminosity nucleus and is not a demographic sample. Its exceptionally accurate orbit scale does not make every accretion-flow parameter equally well known. Mass and distance are dynamical posteriors; inclination and spin depend much more strongly on which variable emission and plasma model is admitted. Any new claim should identify the released epochs or interferometric dataset, reproduce its calibration choices, and show how the result changes under credible dynamical and plasma alternatives.

References

• Event Horizon Telescope Collaboration. “First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way.” Astrophysical Journal Letters 930 (2022): L12. DOI.
• Ghez, Andrea M., et al. “Measuring Distance and Properties of the Milky Way’s Central Supermassive Black Hole with Stellar Orbits.” Astrophysical Journal 689 (2008): 1044–1062. DOI.
• Gillessen, Stefan, et al. “Monitoring Stellar Orbits Around the Massive Black Hole in the Galactic Center.” Astrophysical Journal 692 (2009): 1075–1109. DOI.
• GRAVITY Collaboration. “Detection of the Gravitational Redshift in the Orbit of the Star S2 Near the Galactic Centre Massive Black Hole.” Astronomy & Astrophysics 615 (2018): L15. DOI.
• GRAVITY Collaboration. “Detection of the Schwarzschild Precession in the Orbit of the Star S2 Near the Galactic Centre Massive Black Hole.” Astronomy & Astrophysics 636 (2020): L5. DOI.