Black holes are among the most extreme objects in the universe, but they are not all the same. Astronomers sort them into four broad categories based primarily on mass: stellar-mass, intermediate-mass, supermassive, and the still-theoretical primordial black holes. Each type tells a different story about how the universe builds structure across cosmic time.
Stellar-mass black holes: The most common kind
Stellar-mass black holes form when a star with more than about 20 times the mass of the Sun runs out of nuclear fuel and its core collapses under its own gravity. The collapse triggers a supernova explosion, and what remains is an object so dense that its gravitational pull prevents even light from escaping.
These black holes typically range from a few times the mass of the Sun up to roughly 100 solar masses. They are scattered throughout galaxies, often found in binary systems where they pull gas from a companion star. The infalling gas heats up and emits X-rays, which is how astronomers detect most stellar-mass black holes. Cygnus X-1, discovered in the 1970s, was the first widely accepted black hole candidate and remains a textbook example.
Astronomers estimate that the Milky Way alone may contain up to 100 million stellar-mass black holes, though only about 50 have been confirmed. Gravitational wave observatories like LIGO and Virgo regularly detect mergers between pairs of these objects, providing precise mass and spin measurements that help map the population.
Supermassive black holes: The galactic giants
At the other end of the scale sit supermassive black holes, with masses ranging from hundreds of thousands to billions of times the mass of the Sun. These objects reside at the centers of nearly every large galaxy, including the Milky Way.
Sagittarius A-star, the supermassive black hole at our galaxy’s core, has a mass of about 4 million Suns. The black hole at the center of the galaxy Holmberg 15A is at least 40 billion solar masses, placing it among the heaviest known.
How supermassive black holes grow so large remains an open question in astrophysics. Some may have formed from the direct collapse of enormous gas clouds in the early universe. Others may have grown through repeated mergers with smaller black holes and through steady feeding on surrounding gas and stars. Observations show that some supermassive black holes were already in place less than a billion years after the Big Bang, pushing formation theories to account for rapid early growth.
When supermassive black holes are actively consuming large amounts of material, they power some of the brightest objects in the universe: quasars and active galactic nuclei. The Event Horizon Telescope produced the first direct image of a black hole’s shadow by targeting the supermassive black holes in M87 and the Milky Way.
Intermediate-mass black holes: The missing link
Between the stellar-mass and supermassive categories lies a population that has long frustrated astronomers. Intermediate-mass black holes, ranging from roughly 100 to 100,000 solar masses, are expected to exist in large numbers. Collisions between stellar-mass black holes over cosmic time should have produced a continuum of sizes. Yet confirmed examples remain scarce.
Several promising candidates have been identified. The Hubble Space Telescope imaged one, designated 3XMM J215022.4-055108, with an estimated mass of about 50,000 Suns. In 2019, LIGO detected GW190521, a gravitational wave signal from the merger of two black holes that produced a final object of 142 solar masses, squarely in the intermediate range.
Scientists think intermediate-mass black holes may be the seeds from which supermassive black holes grow. Finding enough of them would fill a critical gap in the evolutionary picture. They are thought to lurk in dense stellar environments such as globular clusters and dwarf galaxies, where repeated mergers and gas accretion could gradually build up their mass.
Primordial black holes: A cosmic fossil
The fourth category remains hypothetical. Primordial black holes could have formed not from collapsing stars but from density fluctuations in the first second after the Big Bang, when the universe was still hot and dense enough for pockets of material to collapse directly into black holes.
These objects could span an enormous range of masses, from less than a paperclip’s worth of material up to 100,000 solar masses. The smallest primordial black holes would have evaporated through Hawking radiation long ago, but larger ones could still exist today.
No primordial black hole has ever been confirmed, which makes them one of the most active areas of theoretical research. Some cosmologists have proposed that they could make up part or all of the universe’s dark matter. Others suggest they might be the seeds from which the earliest supermassive black holes grew. Upcoming surveys and gravitational wave observations may help settle the question.
The boundaries between these categories are not rigid. Scientists continue to reassess where one type ends and another begins. What unites them all is the same basic physics: a region of spacetime where gravity has won, and the rules of the ordinary universe no longer apply.
Images:
- Illustration of a stellar-mass black hole pulling material from a companion star (NASA)
- Supermassive black hole Sagittarius A-star multiwavelength composite (NASA/ESA/CXC/STScI)
- Hubble image of candidate intermediate-mass black hole 3XMM J215022.4-055108 (NASA/ESA/D. Lin)
- Primordial black hole formation concept illustration (NASA)
