The question of whether a black hole is bigger than the Sun depends entirely on the measure of “size” used. The Sun is a physical body with a defined volume and surface, while a black hole is a region of spacetime defined by an extreme gravitational field. To understand the scale difference, it is necessary to compare their mass, physical radius, and density. This reveals a complex picture where black holes can be both vastly more massive and yet physically smaller than the Sun.
Defining the Sun’s Scale
The Sun serves as a conventional baseline for measuring mass and size in our solar system. Its mass is about 1.989 x 10^30 kilograms, which is defined as one solar mass (M☉). The Sun has a mean radius of approximately 696,000 kilometers (435,000 miles). This size is determined by its visible surface, the photosphere, which is the boundary of the hot plasma that constitutes the Sun.
The Sun’s average density is about 1,410 kilograms per cubic meter, which is only about 1.4 times the density of water. This density is not uniform, as the core is extremely dense at about 150 grams per cubic centimeter, while the outer layers are much less dense. The Sun is a stable object whose size is maintained by the outward pressure from nuclear fusion balancing the inward pull of gravity.
Understanding Black Hole Structure and Scale
A black hole is not a physical object with a conventional surface, but rather a point in space where gravity is so intense that nothing, not even light, can escape. All the mass of a black hole is compressed into an infinitely small, dense point at its center called the singularity. This means the black hole itself has essentially zero volume.
The apparent “size” of a black hole is defined by its boundary, the Event Horizon. This boundary is the point of no return, where the escape velocity exactly equals the speed of light. The radius of the Event Horizon is known as the Schwarzschild Radius, and its size is determined solely by the black hole’s mass. The formula for the Schwarzschild Radius shows it is directly proportional to the mass, meaning a more massive black hole has a proportionally larger Event Horizon.
Mass vs. Radius: The Direct Comparison
The comparison between a black hole and the Sun depends entirely on the black hole’s mass. For a hypothetical object with the same mass as the Sun, its Schwarzschild Radius would be only about three kilometers. This means that if the Sun were to collapse into a black hole, the resulting Event Horizon would be dramatically smaller than the Sun’s current 696,000-kilometer radius.
Stellar-mass black holes are far more massive than the Sun, ranging from a few to hundreds of solar masses. A stellar-mass black hole with ten times the mass of the Sun would have a Schwarzschild Radius of approximately 29.5 kilometers. While this is ten times the mass of the Sun, the resulting black hole’s radius is still over 23,500 times smaller than the Sun’s physical radius.
The comparison shifts completely when considering supermassive black holes, which reside at the centers of most galaxies. These giants range from millions to billions of solar masses. For instance, a supermassive black hole with a mass of 10 million Suns would have an Event Horizon radius of about 30 million kilometers. This size is physically larger than the Sun’s radius and would nearly engulf the planet Mercury’s orbit.
How Black Holes Form and Grow
Stellar-mass black holes, the smallest type, are the remnants of a single, massive star that has exhausted its fuel. The star’s core collapses under its own gravity, forming a black hole with a mass typically between a few and a few dozen solar masses. This process, called stellar collapse, is violent and sheds much of the original star’s outer material in a supernova explosion.
Supermassive black holes are thought to begin as smaller “seed” black holes in the early universe. Their growth to millions or billions of solar masses is primarily driven by two processes: accretion and mergers. Accretion involves the black hole slowly feeding on surrounding gas, dust, and stars that spiral into its gravitational field, often forming a bright accretion disk. They also grow through merging with other black holes when their host galaxies collide, adding their masses together.