Black holes are regions of spacetime where gravity is so intense that nothing, not even light, can escape their pull. They form when extremely massive stars collapse under their own weight. While some black holes are only a few times the mass of our Sun, others are giants residing at the centers of galaxies, with masses millions to billions of times greater. Understanding the largest of these objects requires defining size based on mass and examining the current cosmic record holders. The most massive black holes challenge our understanding of how quickly such enormous structures could have formed in the relatively young universe.
Defining “Biggest”: Mass Versus Size
When astronomers refer to the “biggest” black hole, they are primarily talking about its mass, not its physical diameter. The physical boundary, known as the event horizon, is directly proportional to its mass and marks the point of no return. The radius of the event horizon, called the Schwarzschild radius, means that a more massive black hole will always have a larger event horizon. The largest black holes are classified as Supermassive Black Holes (SMBHs). The most extreme examples are Ultramassive Black Holes (UMBHs), typically exceeding ten billion solar masses.
The Current Record Holders
The most massive black hole candidates known today are truly staggering in their scale, reaching tens of billions of times the mass of the Sun. One long-standing contender is the black hole powering the quasar TON 618, estimated at about 66 billion solar masses. This object is so distant that its light has traveled over ten billion years to reach Earth. Its event horizon has a diameter of roughly 2,600 astronomical units, making it about 40 times wider than the orbit of Neptune. Another candidate, often cited as potentially the most massive, is the black hole at the center of the Phoenix Cluster (Phoenix A). Estimates for Phoenix A range up to 100 billion solar masses, though this relies on indirect modeling of the surrounding galaxy’s dynamics rather than direct light emission. A more recent discovery, an ultramassive black hole in the galaxy hosting the Cosmic Horseshoe gravitational lens, was measured at 36 billion solar masses.
How Black Holes Grow to Ultramassive Sizes
Ultramassive black holes grow to their colossal sizes through two primary astrophysical processes: accretion and mergers. Accretion involves the slow, steady consumption of gas, dust, and stars from the black hole’s immediate galactic environment. As matter spirals inward, it forms a superheated accretion disk that emits intense radiation, which is what we observe as a quasar. This process of “feeding” is limited by the Eddington limit, where the outward pressure of the emitted radiation counteracts the inward gravitational pull.
The other dominant growth mechanism is the collision and coalescence of two or more smaller supermassive black holes. This occurs when two galaxies merge, dragging their central black holes toward the center of the newly formed, larger galaxy. These black hole mergers are thought to be the fastest way for a black hole to gain billions of solar masses. Evidence suggests that the largest UMBHs, like the one in the Phoenix Cluster, may have formed through a series of such extreme merger events in the early history of the cosmos.
Methods Used to Measure Giant Black Holes
Measuring the mass of a black hole billions of light-years away is an indirect process requiring sophisticated techniques.
Reverberation Mapping
One widely used method is reverberation mapping, which is effective for active black holes like TON 618. This technique measures the time delay between light variations from the black hole’s accretion disk and the “echo” response from surrounding gas clouds. This time delay allows scientists to calculate the distance and velocity of the gas, which is then used to determine the black hole’s mass.
Stellar and Gas Dynamics
For black holes that are dormant, astronomers use stellar and gas dynamics. This involves observing the speed and orbit of stars or cold gas disks in the black hole’s immediate vicinity. By applying Kepler’s laws of motion, scientists can calculate the gravitational force required to maintain those orbits, thereby estimating the black hole’s mass.
Gravitational Lensing
Finally, gravitational lensing offers a unique way to measure mass by observing how the black hole’s immense gravity warps the light from a background object. The degree of distortion in the background light provides a direct measure of the total mass of the lensing object.