Sagittarius A* (Sgr A*)
Coordinates, GRAVITY S2 stellar orbit distance (8,277 pc) & mass (4.297 × 10^6 M☉), EHT 51.8 μas shadow, and Galactic Center guide for Sgr A*.
Provenance-Backed Scientific Measurements
Every measurement below preserves its unit, uncertainty interval, observational method, reference epoch, quality status, and primary catalog source. See Uncertainty & Provenance for how competing measurements are handled.
| Parameter | Value | Uncertainty | Method & Epoch | Quality Status | Source Archive |
|---|---|---|---|---|---|
| Black hole mass (VLTI GRAVITY S-star orbits) | 4297000 M☉ | ± 12000 M☉ | Relativistic Keplerian orbit of star S2 + S29/S38/S55 (GRAVITY Collaboration 2022) | definitive | Event Horizon Telescope Collaboration Horizon-Scale Results |
| Galactic Center distance (R0) | 8277 pc | ± 9 pc | GRAVITY interferometric astrometry + radial velocity of S2 (26,996 ± 29 ly) | definitive | Event Horizon Telescope Collaboration Horizon-Scale Results |
| EHT emission ring diameter | 8.63e-7 arcmin | ± 3.8e-8 arcmin | Event Horizon Telescope 1.3-mm VLBI image (51.8 ± 2.3 microarcseconds) | definitive | Event Horizon Telescope Collaboration Horizon-Scale Results |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Sagittarius A* (Galactic Center Black Hole) at RA 17h 45m 40.04s, Dec -29° 00′ 28.1″ within Sagittarius, alongside peak meridian transit altitudes for 40°N and 30°S observers. Use the RA/Dec Coordinate Converter to compute exact culmination altitude for your latitude.
Astrophysical Context & Structure
Sagittarius A* (pronounced 'Sagittarius A-star', abbreviated Sgr A*) is the compact radio, infrared, and X-ray source marking the supermassive black hole at the dynamical center of our Milky Way galaxy, located 8,277 ± 9 parsecs (26,996 light-years) away on the Sagittarius–Scorpius border. First resolved as a compact sub-arcsecond radio point source inside the broader Sagittarius A complex by Bruce Balick and Robert Brown in February 1974, Sgr A* became the decisive proving ground for general relativity in the strong-field regime.
Over three decades of near-infrared adaptive-optics and VLTI/GRAVITY interferometry tracking the 'S-stars'—led by Andrea Ghez at Keck and Reinhard Genzel at ESO, who shared the 2020 Nobel Prize in Physics—mapped the 16.05-year orbit of the B-type star S2, which plunges to within 120 AU (four times Neptune's distance from the Sun) of Sgr A* at 7,650 km/s (2.55% of the speed of light). Measuring both gravitational redshift and Schwarzschild orbital precession pinned down the black hole's mass to 4.297 ± 0.012 million solar masses.
On May 12, 2022, the Event Horizon Telescope Collaboration released the direct 1.3-millimeter VLBI horizon-scale image of Sgr A*, revealing a glowing 51.8 ± 2.3 microarcsecond photon ring matching the exact shadow diameter predicted by Einstein's general relativity for a 4.3-million-solar-mass black hole at 8.28 kiloparsecs.
Practical Visual Observing Guide
- Although 30 magnitudes of dust block Sgr A* itself in visible light, visual observers can point directly at the Galactic Center's exact sky position: look 4.5° west-northwest of Alnasl (γ² Sagittarii, the spout tip of the Sagittarius Teapot).
- Sweeping the surrounding 5° field with binoculars or a wide-field telescope reveals the Great Sagittarius Star Cloud (Baade's Window), globular clusters M22, M28, NGC 6522, and NGC 6528, and the Lagoon Nebula (M8)—a direct window toward the Milky Way's central bulge.
Catalog Cross-Identifications & Aliases
Common Aliases: Sgr A* · Galactic Center Black Hole
| Catalog Authority | Identifier |
|---|---|
| SIMBAD | NAME Sgr A* |
| WIKIDATA | Q237235 |
Related Objects, Comparisons & Guides
Messier 87 (Virgo A Giant Elliptical Galaxy)
Coordinates, distance (16.4 Mpc / 53.5 Mly), EHT 6.5-billion M☉ black hole M87*, relativistic jet, and observing guide for Messier 87.
Cygnus X-1 (HDE 226868 X-Ray Black Hole)
Coordinates, VLBA/Gaia distance (2.22 kpc), 21.2-M☉ stellar black hole mass, 5.60-day O-supergiant binary, and observing guide for Cygnus X-1.
Lagoon Nebula (Messier 8 / NGC 6523)
Coordinates, Gaia DR3 distance (1.25 kpc), NGC 6530 cluster, Hourglass core, and observing guide for the Lagoon Nebula (M8) in Sagittarius.
3C 273 (Brightest Quasar in Virgo)
Coordinates, redshift z = 0.1583 (~2.4 billion ly), magnitude 12.85, supermassive black hole accretion, and observing guide for quasar 3C 273.
Sagittarius A* vs M87* Black Hole Comparison
Compare the Event Horizon Telescope supermassive black holes Sagittarius A* (Milky Way) and M87* (Virgo Cluster) in mass, distance, and shadow size.
Northern Summer Milky Way Observing Tour
Summer observing guide from the Summer Triangle (Vega, Altair, Deneb) and Hercules Keystone down the Milky Way rift to the Sagittarius Galactic Center.
Measurement Uncertainty & Optics in Astronomy
How random vs systematic errors, asymmetric error bars, model dependencies, and optical aperture/exit-pupil limits shape astronomical measurements.
Angular Size, Separation & Field of View
How angular diameter (degrees, arcminutes, arcseconds), spherical angular separation, and small-angle physical diameter relate to telescope field of view.
Scientific Sources & Dataset Provenance
- Event Horizon Telescope Collaboration Horizon-Scale Results (2019 ApJL 875 L1 & 2022 ApJL 930 L12): Event Horizon Telescope Collaboration 2019, ApJL, 875, L1; 2022, ApJL, 930, L12 — Event Horizon Telescope Collaboration
- SIMBAD Astronomical Database (CDS) (2026-10 release): Wenger, M., et al. 2000, A&AS, 143, 9 (The SIMBAD astronomical database) — Centre de Données astronomiques de Strasbourg