Black holes are regions of spacetime where gravity is so intense that nothing, not even light, can escape. Comparing a black hole’s size to the Sun requires focusing on mass and density rather than simple volume. The Sun is a massive, luminous sphere of gas, but a black hole is defined by an invisible boundary that changes how we measure its physical dimensions.
Defining the “Size” of a Black Hole
Unlike a star, which has a distinct physical surface, a black hole is defined by the event horizon. This boundary is a mathematical limit, often called the black hole’s “surface,” representing the point of no return. Here, the escape velocity exceeds the speed of light, and anything crossing this membrane is drawn toward the singularity at the center.
The measure of this boundary is the Schwarzschild Radius, which is the distance from the black hole’s center to the event horizon. For a non-rotating black hole, this radius is directly proportional to its mass. If a black hole doubles its mass, its radius also doubles.
This concept is illustrated by gravitational collapse. Every object, including the Sun, has a theoretical Schwarzschild Radius. If the Sun were compressed to a radius of approximately 3 kilometers (about 1.8 miles), its gravity would become too strong for light to escape, forming a black hole. This shows that a black hole’s size is defined by the extreme density achieved by its mass, not the volume of matter.
Stellar-Mass Black Holes Compared to the Sun
Stellar-mass black holes highlight a dramatic difference between density and volume compared to the Sun. The Sun has a physical radius of about 696,000 kilometers (432,000 miles) and is defined as one solar mass. It is not massive enough to collapse into a black hole; it will eventually expand into a red giant and fade into a white dwarf.
Stellar-mass black holes are remnants of stars that began with at least 20 to 30 times the Sun’s mass, typically ranging from 5 to several tens of solar masses. Despite their high mass, their event horizons are incredibly compact. For example, a black hole five times the mass of the Sun would have a Schwarzschild Radius of roughly 15 kilometers, resulting in a diameter of just 30 kilometers.
The Sun’s physical diameter is nearly 1.4 million kilometers. A stellar-mass black hole ten times the Sun’s mass would have an event horizon diameter of only about 60 kilometers, making it vastly smaller than our star. This contrast emphasizes that a black hole can be far heavier than the Sun while occupying only a tiny fraction of its volume.
Supermassive Black Holes: The Ultimate Scale Difference
The scale difference changes completely when comparing the Sun to supermassive black holes (SMBHs), which are found at the centers of nearly all large galaxies. These objects possess masses ranging from millions to billions of times that of the Sun. This immense mass translates to an enormous event horizon, making them physically larger than entire solar systems.
Our galaxy’s central black hole, Sagittarius A (Sgr A), is a relatively small SMBH with a mass of about 4 million solar masses. Its event horizon has a diameter of roughly 24 million kilometers, slightly more than half the distance between the Sun and the orbit of Mercury. While Sgr A is small compared to the Sun’s total volume, its event horizon is larger than the entire orbit of our solar system’s innermost planet.
The most massive known black holes present a staggering scale. The supermassive black hole at the core of the M87 galaxy, for instance, has a mass of about 6.5 billion solar masses. Its event horizon stretches across a diameter of roughly 38 billion kilometers, large enough to encompass the orbits of all the major planets in our solar system. The largest examples, estimated to be tens of billions of solar masses, have event horizons so vast that light would take weeks to cross them.