At the center of most large galaxies sits a supermassive black hole, an object millions to billions of times the mass of our Sun compressed into a surprisingly small space. The Milky Way’s own central black hole, called Sagittarius A*, packs about 4 million solar masses into a region smaller than our solar system. But black holes aren’t the only structures found at galactic centers. Smaller galaxies often harbor dense clusters of stars instead, and some galaxies appear to have both.
The Supermassive Black Hole
Nearly every large galaxy astronomers have examined contains a supermassive black hole at its core. These objects range from a few million solar masses in galaxies like ours to billions of solar masses in the largest elliptical galaxies. Despite their enormous mass, they’re remarkably compact. Sagittarius A* is roughly one astronomical unit across, about 93 million miles, which is tiny compared to the 2.8-billion-mile distance from the Sun to Neptune.
What makes these black holes “supermassive” rather than ordinary is scale. A regular black hole forms when a single massive star collapses and typically weighs 5 to 30 times the Sun’s mass. Supermassive black holes are in an entirely different category, and how they grew so large remains one of the bigger open questions in astrophysics. One leading idea is that they formed from the direct collapse of enormous gas clouds in the early universe, starting out already thousands of times heavier than the Sun. The alternative, that they grew from the merging of smaller black holes over time, has trouble explaining why many small galaxies lack black holes entirely.
How We Know It’s There
You can’t see a black hole directly, so astronomers proved the existence of the Milky Way’s central black hole by watching stars orbit it. Teams led by Reinhard Genzel and Andrea Ghez tracked individual stars near the galactic center for years, measuring their paths and speeds. One star, called S2, swings within 17 light hours of Sagittarius A*, roughly four times the orbit of Neptune, confirming that millions of solar masses are packed into a planetary-scale space. That rules out essentially every alternative to a black hole.
More recently, a fainter star called S29 set a speed record during its closest approach in May 2021, reaching 8,740 kilometers per second, nearly 3% the speed of light. These orbital measurements, combined with the fact that no other known object could concentrate so much mass so tightly, make the case ironclad. In 2022, the Event Horizon Telescope collaboration released the first image of the radio glow surrounding Sagittarius A* itself, captured using a network of radio dishes spanning the globe.
When the Black Hole Is “On”
Not all central black holes behave the same way. Our galaxy’s black hole is quiet. It produces surprisingly little radiation for its size, with a mysterious absence of the X-rays and ultraviolet light that astronomers would expect. It’s essentially dormant, not actively consuming much material.
Other galaxies aren’t so calm. When gas, dust, and stars fall toward a central black hole, the material spirals inward and forms a superheated disk. Friction and gravitational forces heat this disk to extreme temperatures, causing it to radiate enormous amounts of energy. In the most extreme cases, these “active” galactic centers, called quasars, can outshine the entire rest of their host galaxy. The energy output is so intense that radiation pressure alone can push gas outward against the pull of gravity. This is how a black hole weighing a tiny fraction of its galaxy’s total mass can influence the evolution of the entire galaxy around it.
There’s a tight relationship between how massive a central black hole is and how fast stars move in the surrounding galactic bulge. Black hole mass scales with the fourth power of that stellar velocity, meaning a galaxy where stars orbit twice as fast in the bulge will tend to have a central black hole roughly 16 times more massive. This pattern holds across spiral galaxies, elliptical galaxies, and the giant ellipticals found at the centers of galaxy clusters, though with different baselines for each type. The connection suggests that the black hole and its host galaxy regulate each other’s growth over cosmic time.
Nuclear Star Clusters
Supermassive black holes aren’t the only objects occupying galactic centers. Many galaxies, particularly smaller ones, contain nuclear star clusters: extremely dense collections of stars packed into a compact region at the galaxy’s core. These clusters are among the densest stellar environments in the universe.
How these clusters form depends on the galaxy’s size. In galaxies below about a billion solar masses, the clusters appear to be built mainly from smaller star clusters that spiraled inward over time due to gravitational friction. In larger galaxies, gas funneled to the center fuels new star formation directly within the nucleus. Some galaxies have both a nuclear star cluster and a supermassive black hole coexisting at the center, and their growth may be intertwined. The extreme density of these clusters creates conditions for rare events, including stars being torn apart by the black hole’s gravity and close encounters between compact objects that produce gravitational waves.
Galaxies Without a Central Black Hole
Not every galaxy has a supermassive black hole at its center. A study using NASA’s Chandra X-ray Observatory found that many smaller galaxies simply lack one. The researchers ruled out the possibility that these black holes were merely too faint to detect. The drop in X-ray sources in low-mass galaxies was steeper than could be explained by reduced gas falling inward alone, pointing to a genuine absence of black holes rather than just quieter ones.
This finding supports the idea that supermassive black holes are born large, forming preferentially in the most massive galaxies. If they instead grew from the merging of small black holes left behind by dying stars, you’d expect smaller galaxies to host them at roughly the same rate as larger ones. As Anil Seth, a physicist at the University of Utah and co-author of the study, put it: the formation of big black holes is expected to be rarer, occurring preferentially in the most massive galaxies being formed.
The Milky Way’s Center Up Close
Our own galactic center lies about 26,000 light-years from Earth in the direction of the constellation Sagittarius. It’s one of the closest supermassive black holes in the universe, which makes it a unique laboratory. At radio wavelengths, the brightest feature of the region is the compact radio source Sagittarius A*. Surrounding it is a rich and chaotic environment: wispy magnetic filaments, dense clusters of massive and mysterious stars, and streams of gas spiraling inward toward the central dark mass.
Because our galactic center is relatively close, astronomers can watch the flow of matter near the black hole in real time, something that’s impossible for more distant galaxies. This proximity has made Sagittarius A* the best-studied supermassive black hole in existence, even though, paradoxically, it’s one of the quietest.