Black holes are regions of spacetime defined by the intense gravitational field they generate. Their gravity is so powerful that nothing, not even light, can escape their pull. Since they are invisible and lack a physical surface like a planet or star, determining their size is complex. Astronomers rely on indirect measurements, using the laws of physics to determine their true dimensions.
Defining the Boundary: The Event Horizon
The size of a black hole is measured by its event horizon, the boundary of no return in space. This spherical surface is the point where the escape velocity exceeds the speed of light. Anything that crosses this threshold is permanently captured, which is why the black hole appears “black.”
The size of the event horizon is directly proportional to the black hole’s mass. A more massive black hole will have a proportionally larger event horizon radius, sometimes called the Schwarzschild radius. The mass dictates the extent of the gravitational influence, and thus the physical size we associate with the black hole. Determining mass directly gives the size of the boundary.
Stellar-Mass Black Holes: Relatives of the Sun
The smallest and most common black holes are the stellar-mass variety, formed from the gravitational collapse of massive stars. These objects typically have masses ranging from three to a few dozen times the mass of our Sun.
Despite their enormous mass, these black holes are incredibly compact, with event horizons spanning only a few tens of miles across. For example, the black hole in the binary system Cygnus X-1 has a mass estimated to be around 21 times that of the Sun. This mass is compressed into a sphere with an event horizon radius of only about 63 kilometers (39 miles).
If the Sun were to collapse into a black hole, its event horizon would only be about 3 kilometers in radius. A stellar black hole is a massive object the size of a small city. Their small size makes them difficult to spot directly, but their presence is inferred by observing the powerful X-rays emitted as they siphon material from a companion star.
Supermassive Black Holes: Anchors of Galaxies
At the opposite end of the size spectrum are supermassive black holes (SMBHs), found at the centers of nearly all large galaxies, including our own Milky Way. These giants possess masses millions to billions of times greater than the Sun. Their event horizons are consequently immense, stretching across distances that dwarf the scale of our solar system.
The SMBH at the heart of the Milky Way, known as Sagittarius A, has a mass of about 4.3 million Suns. Its event horizon spans approximately 12 million kilometers, which is large enough to contain the orbit of the planet Mercury. Other SMBHs are far larger; the one at the center of the galaxy M87 has a mass of over 6.5 billion solar masses, with an event horizon that could encompass the orbit of Pluto.
The largest known black holes, such as TON 618, have estimated masses exceeding 60 billion solar masses. The event horizon of such an object would have a diameter of hundreds of billions of kilometers. This boundary is so vast it would extend multiple times beyond the orbit of Neptune, swallowing entire planetary systems whole.
Scientists measure the size of these galactic anchors by observing the gravitational influence they exert on nearby stars and gas. By tracking the high-speed, tight orbits of stars closest to the galactic center, astronomers calculate the central object’s mass with high precision. This calculated mass then provides the measurement for the radius of the event horizon.