The short answer

A black hole is an extremely dense region in space where gravity is so strong that not even light can escape.

The long answer

When you jump up on Earth, gravity pulls you back down. Earth's gravity, while strong enough to keep us on the ground, is not so powerful as to prevent light from escaping. This is why Earth is visible from space.

Now imagine gravity so strong that not even light – the fastest thing in the universe – can escape. That is a black hole.

A black hole is a place in space where gravity is so powerful that not even light can get out, which is why it appears black. In more technical terms, the velocity needed to escape a black hole exceeds the speed of light.

One way to conceptualize this effect is to consider the velocity a rocket needs to achieve in order to escape Earth's gravity and enter space (about 11km/second). Comparatively, if you wanted to escape a black hole, you would need to travel faster than the speed of light, or about 300,000 km/second.

You might be wondering how we have even discovered black holes if they are essentially invisible. But black holes can be detected by observing the behavior of light and matter around them. For instance, stars orbiting an unseen mass or gas heating up as it spirals into a black hole can be telltale signs.

How is a black hole formed?

Generally speaking, most black holes form when a large star "dies." When stars with 8-10x the mass of our Sun run out of fuel (hydrogen), they explode in a supernova. The core left behind is so massive that it collapses under its own gravity, squeezing the mass into an incredibly small space. This is what is known as a stellar black hole.

What is inside a black hole?

It's a fantastic mystery. The core of a black hole, known as the "singularity," is a point where matter is thought to be infinitely dense. Everything that enters a black hole, including light and matter, is believed to converge at the singularity, but the laws of physics as we understand them cannot describe what happens at this point.

What would happen if you went inside a black hole?

You would die. How you would die is more up for debate.

It has long been theorized that gravity would fatally stretch you out as you entered a black hole. The astrophysics term for this effect is colorfully called "spaghettification."

Even passing stars are at risk of being torn apart by black holes. I highly recommend checking out this animation of a star being ripped apart by the gravity of a black hole.

Another theory suggests that the outer boundary of a black hole (known as "event horizon") would act much like a wall of fire. As you passed the event horizon, it's thought that you would instantly burn to death. But you'd probably be spaghettified first.

Check out some other curious questions:

Sources

Choi, C. Q. (2012, May 2). Monster Black Hole Caught Swallowing Unlucky Star. Space.com. https://www.space.com/15506-giant-black-hole-swallows-star.html

Haynes, K. (2020, February 24). How do black holes form?. Astronomy Magazine. https://www.astronomy.com/science/how-do-black-holes-form/

Merali, Z. (2013, April 3). Astrophysics: Fire in the hole!. Nature News. https://www.nature.com/articles/496020a

NASA. (n.d.). Black Holes. NASA. https://science.nasa.gov/astrophysics/focus-areas/black-holes

NASA. (2022, October 12). Anatomy of a Black Hole. NASA Science. https://universe.nasa.gov/black-holes/anatomy/

Reddy, F. (2020, September 8). What Are Black Holes?. NASA. https://www.nasa.gov/universe/what-are-black-holes/

Tillman, N. T., & Dobrijevic, D. (2023, May 19). Black holes: Everything you need to know. Space.com. https://www.space.com/15421-black-holes-facts-formation-discovery-sdcmp.html