Black holes are regions of spacetime where gravity is so strong that nothing, not even light, can escape. The Sun is a familiar celestial body. Understanding the true “size” of a black hole and comparing it to a star like our Sun reveals a fascinating complexity in astrophysics.
What Are Black Holes and the Sun?
A black hole is an incredibly dense concentration of mass within a region of spacetime. Its defining characteristic is the “event horizon,” a boundary where the gravitational pull becomes inescapable. This event horizon is often referred to as the black hole’s “size,” representing the point of no return.
The Sun is a star, a massive sphere of hot plasma primarily composed of hydrogen and helium. It generates energy through nuclear fusion in its core. The Sun has a diameter of approximately 1.39 million kilometers (865,000 miles) and a mass about 333,000 times that of Earth, containing over 99.8% of the mass in our solar system.
How Their Sizes Compare
A black hole’s size, defined by its event horizon (or Schwarzschild radius), is directly proportional to its mass. More massive black holes have larger event horizons.
Stellar-mass black holes, formed from the collapse of massive stars, can have event horizons much smaller than the Sun. For example, a black hole with the same mass as the Sun would have an event horizon only about 3 kilometers (1.9 miles) across. This is significantly smaller than the Sun’s diameter of 1.39 million kilometers.
Supermassive black holes, found at the centers of galaxies, are colossal. Their event horizons can be millions or even billions of times larger than the Sun’s diameter. Sagittarius A, the supermassive black hole at the center of our Milky Way galaxy, has a mass equivalent to about 4 million Suns and an event horizon with a diameter of approximately 12.7 million kilometers (7.9 million miles). This size is large enough to encompass several planets in our solar system, though still smaller than Mercury’s orbit. Some supermassive black holes are even larger, with event horizons that can extend for billions of kilometers, comparable to the size of entire solar systems.
Why Their Sizes Differ
The difference in size between black holes and stars like the Sun stems from their fundamental nature and formation processes. A black hole’s size is determined solely by the amount of mass compressed within its event horizon. The more mass an object has, the larger its Schwarzschild radius will be.
Stellar-mass black holes form when very massive stars, typically those 20 to 30 times the Sun’s mass or more, exhaust their nuclear fuel and undergo gravitational collapse. Even though they originate from enormous stars, the stellar material is crushed into an incredibly dense point, forming a black hole with a relatively small event horizon.
Supermassive black holes are thought to grow over billions of years through two primary mechanisms. They can accumulate vast amounts of gas and dust from their surroundings, a process called accretion. Additionally, they can grow by merging with other black holes, especially during galactic collisions. These continuous growth processes allow supermassive black holes to reach their immense sizes. A star’s physical size is maintained by the outward pressure from nuclear fusion balancing the inward pull of gravity, whereas a black hole’s “size” is simply the boundary beyond which gravity dominates completely.
Can Our Sun Become a Black Hole?
Our Sun does not possess sufficient mass to ever become a black hole. Only stars significantly more massive than the Sun, at least 8 to 10 times its mass, have the gravitational force necessary to collapse into a black hole at the end of their lives.
The Sun’s expected life cycle will follow a different path. In about 5 billion years, it will expand into a red giant, potentially engulfing Mercury, Venus, and possibly Earth. After this phase, the Sun will shed its outer layers, forming a planetary nebula. What remains will be a white dwarf, a very dense, compact stellar remnant roughly the size of Earth, which will slowly cool and fade over trillions of years.