How Can Black Holes Have Different Sizes?
Black holes are not all the same mass, and that difference comes from how they form, how they grow, and the limits set by gravity and cosmic history.
Understanding how can black holes have different sizes reveals why some are only a few times the Sun’s mass while others contain billions of solar masses.
What determines a black hole’s size?
In astronomy, a black hole’s “size” usually means its mass, because mass determines the strength of gravity and the size of the event horizon.
The event horizon is the boundary beyond which nothing, not even light, can escape.
A black hole with more mass has a larger event horizon and a larger Schwarzschild radius if it is not spinning.
For non-rotating black holes, the radius grows in direct proportion to mass, so a black hole ten times heavier has an event horizon ten times larger.
- Mass: the main factor that defines a black hole’s gravitational scale.
- Spin: rotating black holes have a more complex horizon structure.
- Formation history: the original star or gas cloud sets the starting point.
- Growth over time: accretion and mergers can greatly increase mass.
What are the main types of black holes?
Astronomers generally classify black holes by mass.
Each type forms in a different environment and follows a different growth path.
Stellar-mass black holes
Stellar-mass black holes usually contain about 3 to 100 times the mass of the Sun, though some may be larger.
They form when massive stars collapse after exhausting their nuclear fuel and exploding as supernovae or undergoing direct collapse.
These black holes are often found in binary systems, where they pull matter from a companion star.
X-ray telescopes such as NASA’s Chandra X-ray Observatory detect the hot gas that heats up as it falls in.
Intermediate-mass black holes
Intermediate-mass black holes are thought to contain hundreds to hundreds of thousands of solar masses.
They are harder to find because they are less common and often do not produce the strong signals seen in smaller or larger systems.
Astronomers have identified candidates in dense star clusters and through gravitational-wave events detected by LIGO and Virgo.
These objects may represent a missing link between stellar-mass and supermassive black holes.
Supermassive black holes
Supermassive black holes contain millions to tens of billions of solar masses and sit at the centers of most large galaxies, including the Milky Way.
Sagittarius A*, the black hole at the center of our galaxy, has a mass of about 4 million Suns.
These giants likely grew through long periods of gas accretion, repeated mergers, and rapid early growth in the young universe.
The Event Horizon Telescope produced the first images of the black holes in M87* and Sagittarius A*, showing how astronomers can study their shadows indirectly.
Why can black holes grow so large?
Black holes grow when they accrete matter from nearby gas, dust, stars, or even other black holes.
As material falls inward, it forms an accretion disk that heats to extreme temperatures and can radiate powerful X-rays and other energy.
Growth can also happen through mergers.
When two black holes collide, they combine into a single object with more mass than either original black hole.
Gravitational waves, first directly detected by LIGO in 2015, provide evidence for these cosmic collisions.
In the early universe, some black holes may have experienced unusually fast growth because gas was abundant and galaxies were still forming.
This helps explain how supermassive black holes existed less than a billion years after the Big Bang.
Can a black hole shrink?
Yes, in theory black holes can lose mass through Hawking radiation, a quantum mechanical process proposed by Stephen Hawking.
For astrophysical black holes, this effect is incredibly tiny and far weaker than normal growth through accretion or mergers.
That means most black holes in the universe are expected to get larger over time rather than smaller.
The exception is when they are isolated and extremely tiny, which is not the case for the black holes astronomers usually observe.
Does spin change the apparent size?
Spin does not change the fact that mass is the main reason black holes differ in size, but it does affect the geometry near the event horizon.
Rotating black holes, described by the Kerr solution in general relativity, have an ergosphere outside the horizon where space-time is dragged around by rotation.
This can influence how matter falls in, how jets form, and how the shadow appears in telescope images.
So while spin does not explain the mass range, it does shape the black hole’s observable behavior.
How do scientists measure black hole sizes?
Because black holes emit no light from inside the event horizon, astronomers measure them indirectly using the motion of nearby stars, gas, and radiation.
Kepler’s laws and general relativity help scientists infer the mass from orbital speeds and timing.
- Stellar orbits: used for Sagittarius A* by tracking stars near the galactic center.
- X-ray binaries: used to estimate mass from companion star motion and accretion signals.
- Gravitational waves: used to calculate masses from merging compact objects.
- Event Horizon Telescope imaging: used to compare the shadow size with relativistic models.
Why are some black holes much larger than others?
The answer lies in both starting conditions and cosmic environment.
A black hole formed from a massive star begins with only a few solar masses, while a black hole in a galaxy center can feed on surrounding gas for billions of years.
In dense regions with frequent mergers and plenty of fuel, black holes can become enormous.
In quieter regions, growth slows because there is less matter available to consume.
Key reasons black holes differ in size
- Progenitor star mass: more massive stars can leave behind more massive remnants.
- Metallicity: low-metallicity stars lose less mass in winds, which can lead to heavier black hole remnants.
- Galactic environment: galaxy centers provide more material for long-term growth.
- Mergers: repeated collisions increase mass rapidly.
- Age of the system: older black holes have had more time to grow.
What does size mean for black hole behavior?
Black hole size affects the tidal forces near the horizon, the temperature of the accretion flow, and the kinds of objects that can survive nearby.
Smaller black holes create stronger tidal gradients close to the horizon, while supermassive black holes can have horizons large enough that a falling astronaut would not notice the horizon crossing immediately.
Mass also influences how long a black hole takes to evolve and how visible it is to astronomers.
Stellar-mass black holes are often revealed by bright X-ray emission, while supermassive black holes are studied through galaxy dynamics and high-resolution radio observations.
Why this matters in modern astronomy
Explaining how can black holes have different sizes helps astronomers connect stellar evolution, galaxy formation, and general relativity into one picture.
It also guides searches for intermediate-mass black holes and improves models of black hole mergers observed with gravitational-wave detectors.
As instruments improve, including the Event Horizon Telescope, LIGO, Virgo, and next-generation observatories, researchers are building a more complete map of black hole mass across the universe.