Sagittarius A*

From stellar orbits to a black hole’s shadow

Sagittarius A* is the supermassive black hole at the heart of the Milky Way. To identify an object that cannot be seen directly, astronomers learned to read the orbits of nearby stars, radio waves from hot gas, and light bent by intense gravity. Its history is not the story of a single “black hole photograph,” but of independent observations converging on the same conclusion.

• Distance
  About 26,700 light-years from Earth
• Mass
  About 4.15 million times the Sun’s mass
• S2 orbital period
  About 16 years
• EHT ring diameter
  51.8 ± 2.3 microarcseconds

Why was the Galactic Center hidden?

Looking roughly 26,700 light-years toward Sagittarius leads to the center of our galaxy. Thick interstellar dust along that sightline, however, blocks visible light. Astronomers therefore observed the region in infrared and radio wavelengths, which pass through the dust more readily. In 1974, they found an extremely compact, powerful radio source at the Galactic Center. It later received the name Sagittarius A*, abbreviated Sgr A*.

A bright radio source alone was not enough to prove that it was a black hole. The decisive clues came from the stars around it. If stars move rapidly around an unseen point, the size and speed of their orbits reveal the mass concentrated at the center.

S2 traced the invisible mass

The best-known of these stars, S2, completes an orbit around Sgr A* about once every 16 years. At closest approach it comes within roughly 120 AU—about 120 times the Earth–Sun distance—and reaches about 7,700 km/s, nearly 3% of the speed of light. Decades of precise tracking of S2 and other stars showed that roughly four million solar masses are confined to a very small region inside S2’s orbit.

During S2’s 2018 closest approach, astronomers measured gravitational redshift. They subsequently confirmed Schwarzschild precession, in which the orbit’s closest point advances slightly on each circuit. Instead of the closed ellipse of Newtonian mechanics, the orbit draws the rosette predicted by general relativity. This long observing campaign helped establish the compact supermassive object at the Galactic Center; Reinhard Genzel and Andrea Ghez shared the 2020 Nobel Prize in Physics for related work.

Sgr A* was not established by a single image. Stellar orbits first weighed it, relativistic effects tested its gravity, and the EHT finally resolved structure on the scale of the event horizon.

What did the EHT image?

The Event Horizon Telescope is not one telescope. In April 2017, radio telescopes at eight observatories around the world observed the same target at a wavelength of 1.3 mm. Very-long-baseline interferometry combined them into an Earth-sized virtual telescope. The enormous, atomic-clock-synchronized data set produced the result released in 2022.

The orange ring is not the black hole’s surface or the event horizon itself. Radio waves emitted by hot plasma around the black hole are bent by intense gravity and reach us along multiple paths, forming a bright ring. The dark central area is the black hole shadow, which appears larger than the event horizon and results from captured light and a deficit of light around that region. The measured thick-ring diameter of 51.8 ± 2.3 microarcseconds agreed with general relativity’s prediction for a roughly four-million-solar-mass Kerr black hole.

Why was it harder than M87* despite being closer?

Sgr A* is far closer than the black hole in galaxy M87, yet it was harder to image. Gas around both black holes moves near the speed of light, but Sgr A* is much less massive and smaller, so gas circles its ring in only minutes. While telescopes across Earth collected data for hours, the target’s brightness and structure kept changing.

The published result is therefore not an ordinary photograph of one frozen instant. Researchers generated many reconstructions using models with different time variability, then distilled the ring and central brightness depression that persisted across independent algorithms into representative images. The EHT data and analysis procedures were also released for independent reanalysis.

Quiet, but not inactive

Compared with active galactic nuclei in other galaxies, Sgr A* is remarkably dim and quiet. It is not a cosmic vacuum cleaner that indiscriminately swallows its surroundings; it accretes only a small amount of gas that comes close enough. Stars at a safe distance maintain stable orbits just as they would around any object of the same mass. Plasma that moves inward, however, heats up, changes rapidly, and sometimes produces infrared and X-ray flares.

In 2024, the EHT’s polarized image revealed strong, twisted, relatively ordered magnetic fields close to the shadow. Their resemblance to the fields around the much larger M87* suggests that the physics by which supermassive black holes accrete matter and launch outflows may be similar across very different scales. A jet from Sgr A*, however, has not yet been confirmed and remains a hypothesis to be tested observationally.

How to read the observational evidence

The values are approximate and include model-dependent observational uncertainties. In particular, “shadow,” “event horizon,” and “bright ring” do not refer to the same boundary.

Sources

• Event Horizon Telescope Collaboration — First Sagittarius A* EHT Results I
• Event Horizon Telescope — First image of the Milky Way black hole
• ESO — S2 Schwarzschild precession around Sagittarius A*
• NASA — What is the center of our galaxy like?
• Event Horizon Telescope — Polarized view of Sagittarius A*