Black hole weight classes
The standard black hole, known as a stellar-mass black hole, forms when a massive star (greater than about 8 solar masses) reaches the end of its life. After depleting the last of its remaining nuclear fuel, the star’s uncontested gravity causes it to rapidly collapse before rebounding outward in an epic blast known as a supernova. What remains, depending on the mass of the star, will either be a neutron star or a black hole. These stellar-mass black holes can range from a couple to several dozen times the mass of the Sun.
However, the origins of supermassive black holes like Sagittarius A*, which can range from millions to billions of times the mass of the Sun, remain unknown. Astronomers do know their extreme size and mass seems to be related to the galaxies they call home, with the biggest supermassive black holes found in the centers of the biggest galaxies.
This line of evidence — as well as recent evidence for a theorized class of mid-sized black holes called intermediate-mass black holes (which range from hundreds to a million solar masses) — seems to suggest that supermassive black holes might get their heft after countless stellar-mass and intermediate-mass black holes merge together over the eons.
And although it’s clear that different types of black holes can dramatically vary in mass, it’s less obvious how much they vary in size.
What if Earth and the Sun were black holes?
To explore the sizes of black holes, let’s first start by looking at two objects that we’re more familiar with: Earth and the Sun.
Earth has a mass of some 6x10
24 kilograms. And though that’s more massive than any human can truly comprehend, when it comes to black holes, Earth is puny.
To create a black hole, you need enough mass that the object’s gravity overcomes any outward forces preventing it from total collapse. This is why there are no known black holes as light as Earth — they just wouldn’t have enough mass to completely collapse. (However, some scientists think there could be a class of ancient
primordial black holes forged in the first few moments after the big bang. These theoretical black holes could range from less massive than a paperclip to tens of thousands of times the mass of the Sun.)
At the center of a black hole is believed to be an infinite gravitational well in the fabric of spacetime, called a gravitational singularity. This singularity is infinitely dense, and anything that reaches it is stuck there for good. The outer edge of the black hole, called the event horizon, is the boundary beyond which nothing can escape the gravitational pull of the black hole, including light. Where this event horizon starts depends on the mass of the black hole and was first calculated by German astronomer Karl Schwarzschild in 1916.
Using the Schwarzschild radius calculation, a black hole the size of Earth would have a radius of less than one inch, making it about as big as a ping pong ball. The Sun, on the other hand, would have a radius of just under two miles.