A black hole four million times the Sun’s mass sits at the center of our galaxy

At the heart of the Milky Way, about 26,000 light-years from Earth, stars race around an object that emits no light of its own. Their orbits reveal a compact mass roughly four million times that of the Sun.

Astronomers call the object Sagittarius A*, and observations from stellar tracking to horizon-scale radio imaging identify it as the galaxy’s central supermassive black hole.

Stars orbit an invisible central mass

NASA’s guide to the center of the Milky Way places Sagittarius A* at the dominant center of a dense stellar environment and gives its mass as approximately four million Suns. Nearby stars complete tight, fast orbits around a point that remains dark.

Applying gravity to those measured paths yields the central mass and constrains its size. Packing millions of solar masses inside the tiny volume allowed by the closest stellar orbit rules out an ordinary star cluster; it would collapse or disperse.

Supermassive describes a category, not its appetite

NASA’s black-hole classification places Sagittarius A* among supermassive black holes, which range from hundreds of thousands to billions of solar masses. Four million is enormous beside a black hole made by one star, though modest beside the giants in some other galaxies.

The event horizon scales with mass. For Sagittarius A*, it spans a region comparable to Mercury’s orbit, while the mass influencing surrounding stars extends far beyond. The black hole does not pull the whole galaxy inward like a drain; stars orbit according to gravity and their motion.

Radio telescopes imaged its shadow

In 2022, the Event Horizon Telescope collaboration released a ring-like image of Sagittarius A*. The European Southern Observatory explained that the image combines synchronized radio observatories across Earth to achieve the resolution of a planet-sized telescope.

The bright ring comes from hot plasma bent by intense gravity, while the central dark region is a shadow larger than the event horizon itself. Rapid changes in the orbiting gas made reconstruction harder than for the more distant but steadier black hole in galaxy M87.

Sagittarius A* is comparatively quiet

Some galactic centers blaze as matter falls into their black holes, converting gravitational energy into radiation. Sagittarius A* currently consumes relatively little material. It produces flares in X-ray and infrared wavelengths but lacks the brilliant sustained disk and jets associated with an active galactic nucleus.

That quiet state lets astronomers study low-rate accretion. Gas from stellar winds approaches the center, yet only a fraction reaches the event horizon. Magnetic fields, turbulence and outflows redirect much of it.

The black hole is central without dominating every orbit

Four million Suns sound capable of controlling an entire galaxy, but the Milky Way contains hundreds of billions of stars plus dark matter. Sagittarius A* dominates motion only in the innermost region. Farther out, the combined mass of the bulge, disk and halo determines stellar orbits, including the Sun’s.

The title’s compact statement therefore carries two separately measured facts: location and mass. Stellar dynamics locate the object at the galaxy’s gravitational center, while multiple orbital measurements converge on roughly four million solar masses. Radio imaging supplies an independent view of the black-hole environment.

One star traced nearly a full orbit

The star known as S2 passes close to Sagittarius A* on an orbit lasting about 16 years. Tracking its position and speed through repeated infrared observations gives astronomers a gravitational scale: the tighter and faster the orbit, the more mass must lie inside it. Multiple completed passes reduce dependence on a short arc or a single instrument.

Spectroscopy adds motion along the line of sight while imaging measures motion across the sky. Combining both produces a three-dimensional orbit and allows tests of relativistic effects near the central mass. Other stars follow different paths around the same focus, creating a network of independent constraints rather than one spectacular trajectory.

The Event Horizon Telescope then tests a different scale. Its ring size agrees with the mass and distance inferred from stellar orbits, while the rapid variability fits a compact source. The methods do not merely repeat one assumption: stars respond to gravity over years, and radio emission traces plasma near the horizon over minutes. Their agreement makes the four-million-Sun conclusion unusually robust.

Distance to the galactic center enters the mass calculation, so teams fit those quantities together and compare results from different telescopes. Dust blocks visible light, making adaptive-optics infrared observations essential for separating crowded stars. Improved instruments have reduced uncertainty without moving the answer away from roughly four million solar masses. “Sits at the center” likewise reflects the shared orbital focus, not a photographed black sphere pinned to a map. The orbital center is the measurement, and independent stars repeatedly return to it. Their common focus excludes a wandering object.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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