Sagittarius A*

Sagittarius A* (abbreviated Sgr A*, pronounced "Sagittarius A-star") is the supermassive black hole at the center of the Milky Way galaxy, approximately 4 million times the mass of the Sun and about 26,000 light-years from Earth. It is the closest known supermassive black hole and the only one whose surrounding stellar orbits can be resolved star by star, which has made it the primary test bed for the black-hole hypothesis.^{1 • 2}

Identification and mass

The radio source at the Galactic Center was first detected in 1931, and astronomers spent decades debating whether it was a cluster of dark stars, an undiscovered particle, or a single gigantic black hole before the name Sagittarius A* was settled on.¹⁰ The decisive evidence came from tracking individual stars near the radio source. Beginning in the mid-1990s, two independent groups, one led by Andrea Ghez of UCLA using the Keck Telescope in Hawaii and one led by Reinhard Genzel of the Max Planck Institute using the European Southern Observatory in Chile, followed thousands of stars circling the galactic center and precisely plotted the orbits of 30 of them. The orbits showed the stars racing around a compact, invisible mass of about 4 million solar masses.¹¹

Later work sharpened the number. Interferometric astrometry of some 50 stellar orbits over nearly three decades shows the gravitational potential is dominated by a compact central mass of about 4.3 × 10⁶ solar masses concentrated within 120 AU of the center, 1,400 times the Schwarzschild radius of a non-rotating black hole of that mass.¹² The orbit-fitting analysis of the star S301 uses a fixed central mass of 4.297 × 10⁶ solar masses at a distance of 8,277 parsecs.¹³

The stellar orbits and tests of relativity

A few dozen stars, the so-called S-stars, orbit Sgr A* on nearly Keplerian paths with randomly oriented, roughly relaxed eccentricities, reaching speeds as high as 30,000 km/s, about 10 percent of the speed of light.^{13 • 14} The most prominent, S2, completes a 16-year orbit and passes within 1,400 Schwarzschild radii of the black hole. During its 2018 pericentre passage, the gravitational redshift of Sgr A* shifted the star's measured spectral lines by about 200 km/s, and by 2020 its orbit was shown to have precessed by around 12 arcminutes, consistent with motion in the Schwarzschild metric. These measurements relied on the GRAVITY near-infrared interferometer at the European Southern Observatory's Very Large Telescope, which reaches astrometric precision of about 30 microarcseconds, comparable to one Schwarzschild radius of Sgr A*.¹³

A faster test particle. In 2023, GRAVITY observations revealed S301, a faint main-sequence star on an 8.7-year orbit with a semi-major axis 33 percent smaller than S2's and an eccentricity of about 0.983, bringing its pericentre to only about 136 Schwarzschild radii, roughly ten times closer than S2. At pericentre it moves at about 25,600 km/s, 8.5 percent of the speed of light, and its orbit precesses by about 2 degrees per revolution. Its motion is expected to be directly sensitive to the spin of Sgr A*, a Kerr-metric effect of order (v/c)³ that S2's orbit cannot yet resolve.¹³

The Event Horizon Telescope image

On 12 May 2022, a collaboration of 300 astronomers from 80 institutes released the first image of Sagittarius A*, produced by the Event Horizon Telescope (EHT), a network of radio telescopes around the world operating as a single Earth-sized interferometer. The image, resembling a fuzzy orange ring, shows superheated matter swirling around the black hole; it was only the second direct image of a black hole, after M87* in 2019.^{5 • 15}

The observations were taken in April 2017 at a wavelength of 1.3 mm with eight observatories at six locations, including ALMA and APEX in Chile, the Large Millimeter Telescope in Mexico, the James Clerk Maxwell Telescope and Submillimeter Array in Hawaii, and telescopes in Spain and Arizona.¹⁶ Imaging Sgr A* was harder than imaging M87*: although Sgr A* is far closer, it is about 4 million solar masses against M87*'s 6.5 billion, so its structure changes on timescales of minutes rather than years. Sgr A* exhibits intraday variability that violates the static-source assumptions of ordinary Earth-rotation aperture synthesis (combining telescope signals to simulate one huge telescope), so the collaboration developed calibration methods and a "variability noise budget" that allowed imaging algorithms to reconstruct time-averaged structure.^{17 • 18 • 15}

The resulting image is dominated by a bright, thick emission ring with a diameter of 51.8 ± 2.3 microarcseconds and a central brightness depression, matching the expected shadow of a 4 × 10⁶ solar-mass black hole at 8 kpc. Model comparisons disfavor viewing the black hole at high inclination (above 50 degrees), non-spinning black holes, and retrograde accretion disks. The result connected, for the first time, dynamical mass measurements from stellar orbits on scales of 10³ to 10⁵ gravitational radii with an event-horizon-scale image.^{6 • 19}

Variability and flares

Sgr A* is variable across the spectrum. The Chandra X-ray Observatory has detected X-ray flares rising above a quiescent thermal background about once per day over many years of monitoring; one proposed mechanism is the tidal shredding of asteroids passing within about 100 million miles of the black hole, roughly the Earth–Sun distance.^{8 • 20} Simultaneous X-ray and infrared campaigns over roughly two decades aim to measure time lags between flares at the two wavelengths, which constrain models of the accretion flow and the flare emission mechanism.²⁰

Webb's continuous watch. In 2023 and 2024, the James Webb Space Telescope's NIRCam instrument observed Sgr A* for a total of 48 hours in 8- to 10-hour increments across a year, the longest continuous infrared record yet. The accretion disk emitted five to six large flares per day, interspersed with smaller sub-flares and fainter brightness changes surging over months, with no periods of rest and no discernible pattern. Study leader Farhad Yusef-Zadeh of Northwestern University proposed that two separate processes are at work: turbulent fluctuations compressing plasma in the disk produce the faint flickers, while collisions of magnetic fields produce the larger eruptions.⁹

A quiet giant

Compared with the supermassive black holes powering active galactic nuclei in other galaxies, Sgr A* is relatively calm, consuming very little matter, though its temperature and magnetic fields are high. Astronomers do not yet know why it is so quiet, and Webb observations are planned to investigate this and to refine the mass measurement.^{1 • 15} The black hole's mass, about 4 million solar masses, is on the low end for galaxies the size of the Milky Way.¹

The black-hole interpretation is not the only model in play. A research group has proposed that the central object could be a compact clump of fermionic dark matter whose dense core mimics a supermassive black hole of about 4.6 million solar masses while matching the S-star and G-source orbits; the team argues its model can also accommodate the EHT image. This remains a minority proposal against the converging dynamical, relativistic, and imaging evidence for a black hole.¹⁴

Recognition

The 2020 Nobel Prize in Physics was divided among three black-hole researchers: Roger Penrose received half "for the discovery that black hole formation is a robust prediction of the general theory of relativity," and Reinhard Genzel and Andrea Ghez jointly shared the other half "for the discovery of a supermassive compact object at the centre of our galaxy."⁷ Ghez, of UCLA, was the fourth woman to win the physics Nobel and the first honored for astrophysics.¹⁰

References

1. What Is the Center of Our Galaxy Like? NASA Science
2. How Scientists Captured the First Image of a Black Hole, NASA JPL Education
3. NASA Animation Sizes Up the Universe's Biggest Black Holes, NASA
4. Telescopes Get Extraordinary View of Milky Way's Black Hole, NASA JPL Education
5. Black hole at the centre of our Galaxy imaged for the first time, Nature news (2022)
6. First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way, arXiv
7. Nobel Prize in Physics: 1901–Present, Live Science
8. Chandra Finds Milky Way's Black Hole Grazing on Asteroids, NASA
9. Webb Reveals Rapid-Fire Light Show From Milky Way's Central Black Hole, NASA Science (2025)
10. A Search for Knowledge That Led Andrea Ghez to a Nobel Prize, UCLA Newsroom
11. The 2020 Physics Nobel Winners Helped Us Better Understand the Universe's Most Mysterious Phenomenon, Time
12. Mass distribution in the Galactic Center based on interferometric astrometry of multiple stellar orbits, Astronomy & Astrophysics
13. Discovery of a star sensitive to the spin of Sagittarius A*, Nature
14. Could the Milky Way galaxy's supermassive black hole actually be a clump of dark matter? Space.com
15. Astronomers Capture First Image of Supermassive Black Hole at the Center of our Galaxy, Time
16. First Sagittarius A* Event Horizon Telescope Results. I., The Astrophysical Journal Letters 930 L12
17. Characterizing and Mitigating Intraday Variability in Accreting Black Holes with mm-VLBI, Event Horizon Telescope
18. First Sagittarius A* Event Horizon Telescope Results. IV. Variability, Morphology, and Black Hole Mass, IOPscience
19. First Sagittarius A* Event Horizon Telescope Results. III. Imaging of the Galactic Center Supermassive Black Hole, arXiv
20. Simultaneous X-Ray and Infrared Observations of Sagittarius A*'s Variability, NASA NTRS

Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026

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