The question of “how many suns can fit in a black hole” asks about capacity, but black hole physics is not governed by volume in the traditional sense. These cosmic objects are defined entirely by their mass and the resulting gravitational influence they exert on spacetime. The mass of a black hole is conventionally measured using the Sun’s mass as a standard unit, referred to as Solar Mass. The amount of stellar material a black hole can contain is a function of the total mass it has accumulated, not a physical volume it needs to fill. A black hole’s size is a direct consequence of its gravity, which establishes a boundary for all matter and light.
Understanding Solar Mass Versus Volume
The size of a black hole is not its internal volume but the radius of its event horizon, which marks the boundary of no return. This radius is known as the Schwarzschild Radius, and its size is directly proportional to the black hole’s mass. If a black hole doubles its mass, its Schwarzschild Radius also doubles, meaning the size of the event horizon expands linearly.
The interior of a black hole does not contain a cavernous space to be filled, but rather a singularity, a point of infinite density where all the mass is theoretically compressed. For a non-rotating black hole, every Solar Mass added increases the size of its event horizon by approximately three kilometers. The Sun itself would need to be squeezed down to a sphere with a radius of about three kilometers to become a black hole.
This relationship between mass and size leads to a counterintuitive fact about density. While the singularity is infinitely dense, the average density of the matter contained within the event horizon decreases as the black hole grows. A small, stellar-mass black hole is incredibly dense, but a supermassive one containing billions of Solar Masses has a vastly larger event horizon. The volume contained by this immense boundary can be so large that its average density is less than that of water, illustrating that the concept of “fitting” is meaningless.
The Stellar Classification of Black Holes
The most common type of black hole forms from the gravitational collapse of a single, massive star at the end of its life cycle. These are called stellar-mass black holes, and they represent the lower end of the mass spectrum. They possess between five and a few tens of Solar Masses, though some can reach nearly 100 Solar Masses.
For a star to collapse into a black hole, it must have started with a mass significantly greater than the Sun, generally exceeding 20 times the Sun’s mass. When the star exhausts its nuclear fuel, its core collapses, triggering a supernova explosion that blows off the outer layers. The remaining core must have a residual mass of at least three Solar Masses to overcome the outward pressure of neutrons and form a black hole.
Examples of these compact objects have been identified throughout the Milky Way, often in binary systems where they pull material from a companion star. The smallest black holes identified are still substantial, with some measuring around 3.3 Solar Masses. This means even the smallest known black holes have accumulated the mass of several Suns.
The Supermassive Classification of Black Holes
On the opposite end of the scale are the supermassive black holes, which anchor the centers of almost all large galaxies, including our own Milky Way. These objects dwarf their stellar-mass cousins, possessing hundreds of thousands to billions of Solar Masses. Their formation mechanism is still an active area of research, likely involving the collapse of colossal gas clouds and the subsequent merging of smaller black holes.
The supermassive black hole at the center of the Milky Way, known as Sagittarius A, contains about four million Solar Masses. Despite holding this immense quantity of mass, its event horizon is only about 17 times the radius of the Sun.
Even larger examples exist in other galaxies, such as the one at the core of Holmberg 15A, which is estimated to contain up to 40 billion Solar Masses. This single cosmic entity contains the mass equivalent of tens of billions of stars like our Sun. The growth of these giants occurs over cosmic time through two primary mechanisms: the slow, continuous accretion of surrounding gas and dust, and the occasional catastrophic merger with other black holes during galactic collisions.
When a Star Crosses the Boundary
When a star strays too close to a black hole, the immense difference in gravitational pull between the star’s near and far sides creates powerful tidal forces. These forces stretch and tear the star apart in a process sometimes called “spaghettification.” The process by which a star contributes its mass is less like pouring water into a glass and more like a violent, destructive encounter.
This event, known as a Tidal Disruption Event (TDE), does not result in the star being swallowed whole. Instead, the star’s material is drawn out into a long, thin stream of gas that wraps around the black hole. Roughly half of this stellar material escapes the black hole’s gravity and is flung out into space, while the other half remains bound to the black hole.
The bound material settles into a rapidly rotating structure called an accretion disk, which spirals inward toward the event horizon. This material heats up to extreme temperatures, emitting brilliant flares of X-rays and other radiation that astronomers can observe. The mass of the star is added to the black hole gradually, as the material in the accretion disk slowly crosses the event horizon.