A black hole is defined not by a solid surface but by a boundary in spacetime where gravity is so intense that nothing, not even light, can escape. This immense gravitational pull results from compressing an enormous amount of mass into an unbelievably small area. Comparing a black hole’s size to a familiar planet like Earth is complex, as the two objects are measured by completely different standards. This comparison requires understanding a spectrum of sizes that stretches from fractions of an inch to distances wider than our entire solar system.
What Determines the Size of a Black Hole?
Unlike a planet or a star, which have measurable physical volumes, a black hole is best described as a region of space. The actual core is theorized to be a singularity, a point of infinite density and zero volume where all the object’s mass is compressed. Astronomers do not measure this infinitesimal point when determining a black hole’s size.
The measurable size is defined by the event horizon, the point of no return surrounding the singularity. This spherical boundary is the precise distance from the center where the gravitational escape velocity equals the speed of light. Anything crossing this line is inevitably drawn toward the center.
The diameter of the event horizon is directly proportional to the black hole’s mass, a relationship determined by the Schwarzschild radius formula. If a black hole doubles its mass, the diameter of its event horizon will also double. Therefore, a black hole’s mass dictates its apparent size and is the primary physical property used for classification.
Comparing Earth to Stellar-Mass Black Holes
The most common type is the stellar-mass black hole, which forms from the collapse of a star 5 to 20 times more massive than the Sun. These objects represent the lower end of the black hole size spectrum. For example, a stellar black hole with ten times the mass of the Sun would have an event horizon diameter of approximately 60 kilometers (37 miles).
Earth’s diameter is about 12,742 kilometers. The event horizon of a typical stellar black hole, despite having ten times the mass of the Sun, is only wide enough to span a small city. This demonstrates the extreme density of these objects, which crush stellar mass into a volume thousands of times smaller than Earth’s.
The contrast is striking when considering the hypothetical size of Earth if it were compressed into a black hole. For Earth’s mass to form an event horizon, its entire volume would need to be crushed down to a sphere with a diameter of only about nine millimeters. This tiny size, roughly the size of a marble, showcases the immense compaction required to turn any mass into a black hole.
Visualizing Extreme Scale: Supermassive Black Holes
Moving beyond stellar black holes are the supermassive black holes (SMBHs), which reside at the center of nearly every large galaxy. These colossal objects range from millions to billions of times the mass of the Sun. The size of their event horizons illustrates the upper limits of black hole scale.
The supermassive black hole at the center of the Milky Way, Sagittarius A\ (Sgr A\), has a mass roughly 4.3 million times that of the Sun. Its event horizon measures about 44 million kilometers across. If Sgr A\ were placed at the center of our solar system, its event horizon would extend about one-third of the way to the orbit of Mercury.
Even larger black holes dwarf Sgr A\, such as the one at the center of the galaxy M87, which is billions of solar masses. M87’s event horizon is so vast that it is larger than the orbit of Pluto, stretching across a significant fraction of our solar system. Black hole size is a massive spectrum, ranging from the size of a marble to larger than an entire planetary system.