Why Does the Sun Keep Shining

The Sun we see every day brings us warmth, light, and vitality. We take it for granted, but have you ever wondered why the Sun can continuously provide energy? Where does its seemingly endless fuel come from?

The Sun is mainly composed of hydrogen and helium—about 74% hydrogen, 25% helium, and small amounts of oxygen, carbon, nitrogen, silicon, sulfur, and other elements. In essence, the Sun is a gigantic ball of gas. Its size is enormous—about 1.3 million times that of Earth—almost beyond imagination. At its core, the Sun has extremely high temperature and pressure, conditions that favor nuclear fusion. In the oxygen-free environment of space, the Sun burns and shines steadily thanks entirely to nuclear fusion.

Nuclear fusion is a type of nuclear reaction. Simply put, it is the process in which two or more atomic nuclei combine into a heavier nucleus, requiring extremely high temperature and pressure. Under these conditions, nuclei collide repeatedly until they successfully fuse. Not all mass is converted or combined; the portion lost is released as energy. In the Sun’s core, hydrogen nuclei are constantly fusing into helium nuclei. This means that every second, the Sun undergoes about 360,000,000,000,000,000,000,000,000,000,000,000,000 fusion reactions. Each second, about 4.3 million tons of mass is converted into energy. This is the key to the Sun’s continuous energy output. In other words, the Sun shines and radiates heat because it is ceaselessly undergoing countless “explosions.”

But why can the Sun provide energy seemingly without limit? In fact, its energy is finite. As explained, the Sun generates energy by fusing hydrogen into helium, but its hydrogen supply, though vast, will eventually run out. The Sun has already been shining for about 4.6 billion years and is expected to continue for another 5 billion years. What happens after that?

About 4.6 billion years ago, the Sun formed from the collapse of part of a molecular cloud—a dense mass of hydrogen and dust. After its formation, favorable conditions in the core gradually produced heat, eventually triggering nuclear fusion and beginning the main sequence stage, which is the stage the Sun is in now. During this stage, nuclear reactions occur steadily in the core, while the Sun grows hotter, larger, and brighter. In other words, since entering the main sequence, the Sun has already become significantly hotter, bigger, and brighter.

After another 5 billion years, the hydrogen in the core will be exhausted, leaving only helium. Fusion of hydrogen will stop. The core will contract, raising its temperature, and begin fusing helium into carbon. Meanwhile, hydrogen still exists in the outer layers. As the core contracts and heats up, fusion will also occur in the outer layers, making the Sun brighter and hotter. As a result, the Sun will expand dramatically, entering the red giant stage. At this point, its energy and mass will be immense, possibly engulfing nearby planets—Venus, Mercury, and even Earth. However, scientists believe that by then, life on Earth may already have disappeared, since the Sun’s increasing brightness during the main sequence stage would have long made radiation too intense for survival.

When the helium in the core is finally depleted, helium fusion will stop. By then, the Sun’s mass will have diminished greatly, leaving it unable to fuse carbon for further energy. Its gases will disperse, and it will ultimately become a white dwarf. At that stage, the Sun will be a nuclear remnant, no longer undergoing fusion, just a carbon core. It may still emit light, but only from residual heat radiation left over from its earlier intense fusion reactions.