Everything in the world has a life cycle, and even something as magnificent as the sun is no exception. Like many stars, the sun also has a birth and a death.
The sun originated from a huge gas cloud mainly composed of hydrogen molecules. About 5 billion years ago, a previously very sparse nebula began to contract under the influence of some external factors. Once contraction begins, gravity will accelerate it continuously. During the contraction process, kinetic energy is converted into thermal energy, causing the gas temperature to gradually rise. As the density of the gas increases, the central temperature also increases. Once the core reaches a high enough temperature (about 7 million Kelvin), a thermonuclear fusion reaction will be triggered. After the core's thermonuclear reaction is "ignited", the continuous generation of "radiation pressure" from light radiation can effectively resist the action of gravity, so that the gas will no longer continue to contract, and a stable sun is born.
Once a star undergoes thermonuclear fusion, it enters a long and relatively stable period, known as the "main sequence star" period. For a medium-mass star like the sun, the "main sequence star" stage lasts for 10 billion years. Therefore, the sun is currently in its "middle age" and can still remain stable for another 50 billion years.
After a long 50 billion years, the sun will also enter its "old age". By that time, most of the hydrogen in the sun's core will have been consumed, and the sun's outer layer will expand, becoming a huge red giant star, whose radius can exceed 1 astronomical unit, that is, it can "swallow up" the entire Earth's current orbit. What will happen to humans on Earth? Of course, we need to consider moving elsewhere early, but that will happen several billion years later, so there's no need to worry about it now.
After the red giant stage, the sun will enter a very unstable state, constantly producing strong pulsations, like a patient constantly gasping for breath. The outer layer of the sun will be ejected outward, forming a beautiful planetary nebula. The sun's core will become a very dense white dwarf star, which will slowly cool down and fade over the long years.
The ultimate fate of a star depends on its mass. The sun is just a medium-mass star in the universe, and its ultimate fate is to evolve into a white dwarf star, with a mass of about 0.6 times the current mass of the sun and a volume similar to that of the Earth. If the original star has a larger mass, its late evolutionary stage will end with a supernova explosion. The remaining mass in the center of the star will exceed about 1.4 times the mass of the sun, evolving into a neutron star with a density even greater than that of a white dwarf star. If the remaining star mass exceeds about 3 times the mass of the sun, it will eventually evolve into a black hole, from which no matter can escape its grasp.