A supermassive black hole is a black hole with a mass ranging from millions to billions of times the mass of our Sun, sitting at the center of nearly every large galaxy in the universe. Our own Milky Way hosts one called Sagittarius A*, which weighs about 4 million solar masses. That sounds enormous, but it’s actually modest compared to the largest known supermassive black holes, which can exceed 40 billion solar masses.
These objects are fundamentally different from the smaller black holes that form when massive stars collapse. Supermassive black holes shape entire galaxies, influence how stars form across vast distances, and can outshine everything around them when they actively consume matter.
How They Differ From Other Black Holes
Black holes come in distinct size classes. Stellar-mass black holes, the most common type, form from the death of large stars and typically weigh between 5 and 100 times the mass of the Sun. Intermediate-mass black holes occupy a middle ground, ranging from hundreds to hundreds of thousands of solar masses, though confirmed examples remain rare. Supermassive black holes start at roughly a million solar masses and scale up from there.
The size difference isn’t just academic. A stellar-mass black hole has an event horizon (the boundary beyond which nothing escapes) only a few kilometers across. Sagittarius A*’s event horizon spans about 12 million kilometers, roughly 17 times the diameter of the Sun. The supermassive black hole in the galaxy M87, which was the first black hole ever directly imaged in 2019, has an event horizon large enough to swallow our entire solar system several times over.
One counterintuitive consequence of that scale: the density at the event horizon of a supermassive black hole is actually lower than that of a smaller one. If you could somehow approach the event horizon of a sufficiently massive black hole, the tidal forces pulling your body apart would be far gentler than near a stellar-mass black hole. You could theoretically cross the boundary without being torn apart, at least initially.
Where They’re Found
Nearly every galaxy with a central bulge of stars appears to harbor a supermassive black hole. This isn’t a coincidence. The mass of the black hole correlates tightly with properties of the galaxy around it, particularly the mass and speed of stars in the central bulge. A galaxy with a more massive central region hosts a more massive black hole, following a remarkably consistent ratio. This relationship suggests the black hole and its galaxy grow together over cosmic time, each influencing the other’s development.
Sagittarius A* sits about 26,000 light-years from Earth, at the gravitational center of the Milky Way. Astronomers confirmed its existence by tracking individual stars orbiting an invisible point at tremendous speeds. One star, known as S2, completes an orbit every 16 years at velocities reaching 3% the speed of light. Only an extraordinarily compact, massive object could hold a star in that kind of orbit. The researchers who mapped these stellar paths, Andrea Ghez and Reinhard Genzel, shared the 2020 Nobel Prize in Physics for that work.
How They Form
The origin of supermassive black holes is one of the biggest open questions in astrophysics. The challenge is timing. Astronomers have detected supermassive black holes that already existed when the universe was less than a billion years old. Building something that massive that quickly is difficult to explain with straightforward growth models.
Several competing theories attempt to account for this. One proposes that massive “seed” black holes formed directly from the collapse of enormous clouds of primordial gas in the early universe, skipping the star phase entirely. These seeds could have started at tens of thousands of solar masses, giving them a significant head start. Another scenario involves dense clusters of early stars merging rapidly, with their remnant black holes combining into progressively larger objects. A third possibility is that stellar-mass black holes grew extremely quickly by consuming surrounding matter at rates exceeding what physicists once thought possible.
None of these explanations is fully settled, and the real answer may involve a combination. What’s clear is that supermassive black holes were already in place remarkably early in cosmic history, which constrains how they could have formed.
What Happens When They Feed
A supermassive black hole that is actively pulling in surrounding gas and dust becomes what astronomers call an active galactic nucleus. The infalling material doesn’t drop straight in. Instead, it spirals inward, forming a flat, rotating disk of superheated matter called an accretion disk. Friction and magnetic forces within this disk heat the gas to millions of degrees, causing it to radiate intensely across the electromagnetic spectrum.
The most extreme version of this process powers quasars, which are among the brightest objects in the observable universe. A single quasar can outshine its entire host galaxy by a factor of a hundred or more, all from a region not much larger than our solar system. Quasars were far more common in the early universe, when galaxies had more available gas to feed their central black holes. Most supermassive black holes today, including Sagittarius A*, are relatively quiet, consuming very little material compared to their peak activity billions of years ago.
When a supermassive black hole feeds actively, it can also launch jets of charged particles at nearly the speed of light, extending thousands or even millions of light-years into intergalactic space. These jets carry enormous energy and can heat surrounding gas to the point that it can no longer cool and collapse to form new stars. This feedback mechanism is one of the primary ways supermassive black holes regulate the growth of their host galaxies.
How Astronomers Observe Them
You can’t see a black hole directly, since light cannot escape one. But the environment around a supermassive black hole is among the most observable in the universe. X-ray telescopes detect the intense radiation from accretion disks. Radio telescopes pick up emissions from jets. Infrared observations can peer through the dust that often surrounds galactic centers.
The Event Horizon Telescope collaboration produced the first direct image of a supermassive black hole’s shadow in 2019, showing the black hole at the center of galaxy M87. That image, a bright ring of glowing gas surrounding a dark central region, required linking radio telescopes across the globe to function as a single Earth-sized instrument. In 2022, the same collaboration released an image of Sagittarius A*, which was technically harder to capture because its smaller size means the gas around it changes appearance within minutes rather than days.
Gravitational wave detectors represent another frontier. Current ground-based detectors can pick up mergers of stellar-mass black holes, but supermassive black hole mergers produce gravitational waves at much lower frequencies. Detecting those requires either space-based instruments, which are in development, or pulsar timing arrays that use the precise signals of distant spinning stars as a galaxy-scale detector. In 2023, several pulsar timing collaborations reported the first evidence of a background hum of gravitational waves that likely originates from supermassive black hole pairs slowly spiraling toward each other across the universe.
What Happens When Two Merge
When galaxies collide, their central supermassive black holes eventually sink toward the center of the merged galaxy through gravitational friction. Over hundreds of millions of years, the two black holes spiral closer together, forming a binary pair. In the final stages, they merge into a single, larger supermassive black hole, releasing a burst of gravitational waves that briefly carries more power than all the stars in the observable universe combined.
The Milky Way is on a collision course with the Andromeda galaxy, expected to begin in about 4.5 billion years. When the two galaxies eventually merge, Sagittarius A* and Andromeda’s central black hole (which is estimated at 100 to 200 million solar masses) will likely form a binary and ultimately combine. The merger won’t pose any direct danger to individual star systems, since the distances between stars are so vast that physical collisions between them are extremely unlikely even during a galactic merger.