Sagittarius A* is the compact radio source identified as the Milky Way’s central supermassive black hole, located about 26,700 light-years from Earth. Its mass is roughly four million times that of the Sun, and in May 2022 it became the second black hole ever imaged directly by the Event Horizon Telescope. In 2024, horizon-scale polarimetric imaging revealed strong, organized magnetic fields near its event horizon.
Last updated November 10, 2025
Sagittarius A* is the compact radio source at the dynamical center of the Milky Way and is identified with a Supermassive black hole of roughly four million solar masses at a distance of about 8.178 kiloparsecs (≈26,700 light-years). The distance has been measured geometrically from the orbit of the star S2 using interferometric astrometry, and the black hole interpretation is supported by horizon-scale imaging and dynamical constraints. According to the GRAVITY Collaboration’s 2019 measurement, R0 = 8178 ± 25 pc, enabling percent-level mass and distance estimates; these results were published in Astronomy & Astrophysics. Astronomy & Astrophysics. Direct imaging at 1.3 mm in The Astrophysical Journal Letters by the Event Horizon Telescope (EHT) in 2022 revealed a bright, thick ring with a diameter of 51.8 ± 2.3 microarcseconds consistent with the shadow of a Kerr black hole, tying these measurements to the central mass inferred from stellar orbits. The Astrophysical Journal Letters.
Discovery and naming
The compact source now known as Sagittarius A* was first isolated as a distinct, intense radio emitter at the Galactic Center in February 1974 by Bruce Balick and Robert L. Brown using NRAO’s Green Bank interferometer; Brown introduced the “*” notation in 1982, by analogy with excited atomic states. A historical account of the discovery and naming appears in a 2003 review by Goss, Brown, and Lo. arXiv. Early very-long-baseline measurements in the late 1970s constrained the source to sub-astronomical-unit scales and showed no resolvable expansion or contraction, strengthening the case for an extremely compact object. Astrophysical Journal (NTRS record).
Long-term monitoring of short-period “S-stars,” especially S2, established a central point mass of about four million solar masses confined within a region comparable to the Solar System’s scale, ruling out clusters of dark remnants. The GRAVITY Collaboration’s 2019 study reported a purely geometric distance R0 = 8178 ± 25 pc from S2’s 16-year orbit, which underpins precise mass inferences for Sagittarius A*. Astronomy & Astrophysics. Near S2’s 2018 pericenter, the same program detected the combined gravitational redshift and transverse Doppler effect at the level predicted by general relativity, rejecting Newtonian dynamics at high significance. Astronomy & Astrophysics. Follow-up analysis reported the in-plane prograde Schwarzschild precession of S2’s orbit, further validating relativistic predictions in the Galactic Center potential. Astronomy & Astrophysics. Complementing these stellar tests, EHT imaging in 2017—analyzed in 2022—found the observed ring size to be within about ten percent of Kerr-metric expectations when anchored to independent mass-to-distance constraints, providing a horizon-scale consistency check with general relativity. The Astrophysical Journal Letters.
Event Horizon Telescope imaging and polarimetry
Using Earth-sized Very Long Baseline Interferometry at 230 GHz, the EHT observed Sagittarius A* in April 2017 and, after specialized calibration to handle rapid intrahour variability, reconstructed a ring-dominated image released on 12 May 2022. The preferred image diameter is 51.8 ± 2.3 μas; analysis across several methods favored ring morphologies and quantified strong but intrinsic short-timescale variability. The Astrophysical Journal Letters; The Astrophysical Journal Letters; The Astrophysical Journal Letters. In March 2024, the EHT collaboration published the first resolved polarized images of Sagittarius A*, revealing a highly polarized ring with a spiral electric-vector pattern and peak linear polarization of order 40% in portions of the ring—evidence for strong, organized magnetic fields near the event horizon. The Astrophysical Journal Letters; The Astrophysical Journal Letters.
Accretion flow and variability across the spectrum
X-ray observations show that Sagittarius A* accretes very inefficiently: a 2012 Chandra campaign inferred that less than about one percent of gas within the capture radius reaches the event horizon, consistent with its low quiescent luminosity and intermittent flaring. NASA/Chandra. On February 18, 2025, NASA reported James Webb observations capturing rapid, stochastic near-infrared flares from material in the inner accretion flow, expanding the time-domain view of variability on timescales from seconds to months. James Webb Space Telescope – NASA Science. Together with contemporaneous millimeter and X-ray monitoring, these data contextualize the horizon-scale radio structure seen by the EHT. The Astrophysical Journal Letters.
Recognition and scientific context
In 2020, the Nobel Prize in Physics honored Reinhard Genzel and Andrea Ghez for the discovery of a supermassive compact object at the center of our Galaxy using stellar dynamics, sharing the prize with Roger Penrose for foundational work on black hole theory. NobelPrize.org. These results, together with the EHT’s direct imaging of Sagittarius A*, establish a coherent, multi-scale picture of the Galactic Center black hole that anchors studies of black hole demographics and accretion physics in other galaxies. The Astrophysical Journal Letters; American Physical Society.
Facilities and methods
EHT observations synthesize an Earth-sized aperture from widely separated radio observatories using Very Long Baseline Interferometry, enabling angular resolutions of tens of microarcseconds; the 2017 campaign employed eight telescopes at six sites with specialized calibration for Sagittarius A*’s rapid structural variability. The Astrophysical Journal Letters. Near-infrared interferometry with the GRAVITY instrument at ESO’s VLTI provided astrometric precision at the tens of microarcsecond level, critical for measuring S2’s orbit and R0, while space- and ground-based facilities—including the James Webb Space Telescope and Chandra—trace the variable accretion flow at shorter wavelengths. Astronomy & Astrophysics; NASA/Chandra; James Webb Space Telescope – NASA Science.
Sagittarius A* is the compact radio source identified as the Milky Way’s central supermassive black hole, located about 26,700 light-years from Earth. Its mass is roughly four million times that of the Sun, and in May 2022 it became the second black hole ever imaged directly by the Event Horizon Telescope. In 2024, horizon-scale polarimetric imaging revealed strong, organized magnetic fields near its event horizon.
Last updated November 10, 2025
Sagittarius A* is the compact radio source at the dynamical center of the Milky Way and is identified with a Supermassive black hole of roughly four million solar masses at a distance of about 8.178 kiloparsecs (≈26,700 light-years). The distance has been measured geometrically from the orbit of the star S2 using interferometric astrometry, and the black hole interpretation is supported by horizon-scale imaging and dynamical constraints. According to the GRAVITY Collaboration’s 2019 measurement, R0 = 8178 ± 25 pc, enabling percent-level mass and distance estimates; these results were published in Astronomy & Astrophysics. Astronomy & Astrophysics. Direct imaging at 1.3 mm in The Astrophysical Journal Letters by the Event Horizon Telescope (EHT) in 2022 revealed a bright, thick ring with a diameter of 51.8 ± 2.3 microarcseconds consistent with the shadow of a Kerr black hole, tying these measurements to the central mass inferred from stellar orbits. The Astrophysical Journal Letters.
Discovery and naming
The compact source now known as Sagittarius A* was first isolated as a distinct, intense radio emitter at the Galactic Center in February 1974 by Bruce Balick and Robert L. Brown using NRAO’s Green Bank interferometer; Brown introduced the “*” notation in 1982, by analogy with excited atomic states. A historical account of the discovery and naming appears in a 2003 review by Goss, Brown, and Lo. arXiv. Early very-long-baseline measurements in the late 1970s constrained the source to sub-astronomical-unit scales and showed no resolvable expansion or contraction, strengthening the case for an extremely compact object. Astrophysical Journal (NTRS record).
Long-term monitoring of short-period “S-stars,” especially S2, established a central point mass of about four million solar masses confined within a region comparable to the Solar System’s scale, ruling out clusters of dark remnants. The GRAVITY Collaboration’s 2019 study reported a purely geometric distance R0 = 8178 ± 25 pc from S2’s 16-year orbit, which underpins precise mass inferences for Sagittarius A*. Astronomy & Astrophysics. Near S2’s 2018 pericenter, the same program detected the combined gravitational redshift and transverse Doppler effect at the level predicted by general relativity, rejecting Newtonian dynamics at high significance. Astronomy & Astrophysics. Follow-up analysis reported the in-plane prograde Schwarzschild precession of S2’s orbit, further validating relativistic predictions in the Galactic Center potential. Astronomy & Astrophysics. Complementing these stellar tests, EHT imaging in 2017—analyzed in 2022—found the observed ring size to be within about ten percent of Kerr-metric expectations when anchored to independent mass-to-distance constraints, providing a horizon-scale consistency check with general relativity. The Astrophysical Journal Letters.
Event Horizon Telescope imaging and polarimetry
Using Earth-sized Very Long Baseline Interferometry at 230 GHz, the EHT observed Sagittarius A* in April 2017 and, after specialized calibration to handle rapid intrahour variability, reconstructed a ring-dominated image released on 12 May 2022. The preferred image diameter is 51.8 ± 2.3 μas; analysis across several methods favored ring morphologies and quantified strong but intrinsic short-timescale variability. The Astrophysical Journal Letters; The Astrophysical Journal Letters; The Astrophysical Journal Letters. In March 2024, the EHT collaboration published the first resolved polarized images of Sagittarius A*, revealing a highly polarized ring with a spiral electric-vector pattern and peak linear polarization of order 40% in portions of the ring—evidence for strong, organized magnetic fields near the event horizon. The Astrophysical Journal Letters; The Astrophysical Journal Letters.
Accretion flow and variability across the spectrum
X-ray observations show that Sagittarius A* accretes very inefficiently: a 2012 Chandra campaign inferred that less than about one percent of gas within the capture radius reaches the event horizon, consistent with its low quiescent luminosity and intermittent flaring. NASA/Chandra. On February 18, 2025, NASA reported James Webb observations capturing rapid, stochastic near-infrared flares from material in the inner accretion flow, expanding the time-domain view of variability on timescales from seconds to months. James Webb Space Telescope – NASA Science. Together with contemporaneous millimeter and X-ray monitoring, these data contextualize the horizon-scale radio structure seen by the EHT. The Astrophysical Journal Letters.
Recognition and scientific context
In 2020, the Nobel Prize in Physics honored Reinhard Genzel and Andrea Ghez for the discovery of a supermassive compact object at the center of our Galaxy using stellar dynamics, sharing the prize with Roger Penrose for foundational work on black hole theory. NobelPrize.org. These results, together with the EHT’s direct imaging of Sagittarius A*, establish a coherent, multi-scale picture of the Galactic Center black hole that anchors studies of black hole demographics and accretion physics in other galaxies. The Astrophysical Journal Letters; American Physical Society.
Facilities and methods
EHT observations synthesize an Earth-sized aperture from widely separated radio observatories using Very Long Baseline Interferometry, enabling angular resolutions of tens of microarcseconds; the 2017 campaign employed eight telescopes at six sites with specialized calibration for Sagittarius A*’s rapid structural variability. The Astrophysical Journal Letters. Near-infrared interferometry with the GRAVITY instrument at ESO’s VLTI provided astrometric precision at the tens of microarcsecond level, critical for measuring S2’s orbit and R0, while space- and ground-based facilities—including the James Webb Space Telescope and Chandra—trace the variable accretion flow at shorter wavelengths. Astronomy & Astrophysics; NASA/Chandra; James Webb Space Telescope – NASA Science.
