In stars, nuclear fusion reactions continuously occur, ensuring that they emit light and heat, and maintain a stable high temperature.
A star is essentially a ball of gas, and due to its high temperature, it exerts significant radiation pressure outward. This radiation pressure balances the gravitational pull of the star's material, resulting in a stable star. Currently, the Sun is in such a balanced state, which is why it remains stable.
However, it is important to note that the star's nuclear fuel will eventually be depleted. When that day arrives, it marks the star's death. Without a replenishment of energy, the temperature will begin to drop. As the temperature falls, the radiation pressure can no longer counteract the gravitational force, leading the star to continuously contract. This contraction, driven by its own gravity, is known as collapse.
Newton's law of universal gravitation tells us that gravity is proportional to mass and inversely proportional to the square of the distance. As the star collapses, its radius decreases, causing the gravitational force to increase, which in turn accelerates the collapse. The star becomes increasingly smaller and denser, and the rate of collapse intensifies. In the final moments, the temperature of the entire star can soar to a staggering one hundred million degrees Celsius, resulting in a series of violent explosions known as a supernova. The star is torn apart, and billions of tons of particles are ejected into space, emitting an exceptionally bright light.
In the rapid demise of massive stars, those remnants greater than three times the mass of the Sun will continue to collapse infinitely. Under immense gravitational pressure, the star's diameter shrinks further and further. Eventually, a star with a diameter of millions of kilometers can be compressed into a "point," referred to as a "singularity." Within a certain range surrounding this point, gravity becomes infinitely strong, and anything that approaches it will be consumed, including light itself. This means that within this range, gravity is so powerful that light is bent and cannot escape, or in other words, cannot break free. The star ultimately disappears from our view—this is how a black hole forms. The region from which light cannot escape is known as the "event horizon" of the black hole.
Black holes are incredibly small; if a black hole had the mass of Earth, its diameter would be less than 2 centimeters. Astronomers also believe that, in addition to black holes formed by gravitational collapse, there exist supermassive black holes that may lurk in many galaxies. Thousands of stars collide and compress each other, forming these enormous collapsed bodies—supermassive black holes.
