The question of why the sun shines has been the subject of various speculations throughout history. The most intuitive explanation is that the sun is just a burning coal ball. However, a coal ball the size of the sun would only burn for a maximum of 3,000 years. In fact, the recorded history of humanity spans over 5,000 years, and this contradiction is clearly impossible to explain. Some people suggest that the sun's energy comes from meteorite impacts, which convert kinetic energy into heat and light. However, the accumulation of meteorites would increase the sun's mass, gradually strengthening its gravity, and causing the Earth's orbit to speed up. If the sun's energy really came from meteorite impacts, the Earth would have become shorter by one or two seconds each year. But in reality, this has not happened. In the 19th century, scientists Kelvin and Helmholtz proposed that the sun's energy comes from the contraction of its own matter under the force of gravity, converting potential energy into thermal energy. However, calculations showed that the energy generated by such contraction would only be enough to power the sun for 18 million years.
Since the 20th century, with the development of atomic physics, people have finally solved the problem of the sun's energy. Einstein discovered the relationship between an object's mass and energy, which is the famous mass-energy equation E=mc². According to this relationship, a small amount of mass can be converted into a huge amount of energy. For example, if 1 gram of matter were completely converted into energy, it would be equivalent to the heat released by the combustion of 10,000 tons of coal. The sun's composition is 71% hydrogen, 26% helium, and a small amount of other elements. Under the extreme high temperature and high pressure (about 250 billion atmospheric pressures) inside the sun, hydrogen atoms undergo thermonuclear reactions, specifically including the "proton-proton chain reaction" (pp chain) and the "carbon-nitrogen-oxygen cycle" (CNO cycle). However, the net effect of these processes is always the fusion of 4 hydrogen nuclei into 1 helium nucleus. During this process of material transformation, a small portion of mass is lost. However, it is this small "lost" mass that is converted into enormous energy. This energy generation principle is similar to the explosion of hydrogen bombs on Earth. The sun consumes 630 million tons of hydrogen per second, and the lost 4.2 million tons of mass are converted into 3.8×10^26 joules of energy, equivalent to the explosion of 100 billion million-ton TNT-equivalent hydrogen bombs. Therefore, we can metaphorically say that it is the continuous occurrence of countless large-scale "hydrogen bomb explosions" in the sun's core that provides the sun with a continuous source of energy and brings us long-lasting warmth and light.