A black hole is a region in space where the gravitational field is so strong that nothing, not even light, can escape its pull. It is formed when a massive star runs out of fuel and collapses under its own gravity, compressing its mass into a single point called a singularity. The boundary around the singularity is called the event horizon, beyond which nothing can escape.

Black holes come in different sizes, ranging from a few times the mass of the Sun to billions of times its mass. The smaller ones are thought to be more common and are called stellar black holes, while the larger ones are found at the centers of galaxies and are known as supermassive black holes.

The first black hole was discovered in 1964 by physicist John Wheeler, who coined the term "black hole" to describe the phenomenon. Since then, numerous observations and experiments have confirmed the existence of black holes and their properties.

One of the most famous examples of a black hole is the one at the center of our Milky Way galaxy, called Sagittarius A*. It has a mass of about 4 million times that of the Sun and is located about 26,000 light-years away from Earth. Another notable example is the supermassive black hole in the galaxy Messier 87, which was imaged by the Event Horizon Telescope in 2019.

Black holes have a profound impact on their surroundings, affecting the orbits of nearby stars and gas clouds, and emitting powerful jets of radiation and particles. They also play a crucial role in the evolution of galaxies, as their gravitational pull can shape the distribution of stars and gas.

In summary, a black hole is a region in space where the gravitational pull is so strong that nothing can escape, formed by the collapse of a massive star. They come in different sizes and have a significant impact on their surroundings.