The Sun, the star at the center of our solar system, is a massive body of hot plasma held together by its own gravity. It constantly generates immense amounts of energy, radiating it outward as light and heat. The Sun is continuously shedding its mass into space, a process that has occurred since the star first ignited billions of years ago. This mass loss happens through two distinct physical mechanisms.
The Primary Loss: Mass Converting to Energy
The largest source of the Sun’s mass loss is nuclear fusion occurring deep within its core. High temperatures and pressures force hydrogen nuclei to fuse, primarily forming helium nuclei through the proton-proton chain.
During this reaction, the mass of the resulting helium nucleus is slightly less than the combined mass of the four hydrogen nuclei that initiated the process. This deficit in mass is converted directly into energy, following the principle of mass-energy equivalence (\(E=mc^2\)). This conversion releases vast quantities of electromagnetic radiation, which we perceive as sunlight.
The energy released is carried away from the Sun as photons, which effectively carry away mass from the star. The rate of this mass-to-energy conversion is enormous. Every second, approximately 4 million metric tons of the Sun’s mass is converted into radiant energy that streams out into space. This process is the dominant factor in the Sun’s overall mass loss.
The Secondary Loss: Physical Matter Ejection
The Sun also loses mass through the physical ejection of subatomic particles and plasma into the surrounding solar system. This mechanism is separate from the mass-energy conversion in the core.
The most consistent form of this physical loss is the solar wind, a continuous flow of charged particles, primarily protons and electrons, streaming away from the Sun’s upper atmosphere, the corona. The solar wind is driven by the corona’s extreme heat, allowing particles to reach escape velocity and overcome the Sun’s gravitational pull.
This constant outflow results in a steady drain on the Sun’s mass, carrying away about 1.5 million metric tons of physical matter every second. A less frequent, but more dramatic, form of physical mass loss occurs during Coronal Mass Ejections (CMEs). While CMEs involve the eruption of billions of tons of solar plasma, the cumulative mass loss from the steady solar wind is significantly greater than the loss from these sporadic ejections.
Quantifying the Loss and Its Significance
Combining the two primary mechanisms, the Sun sheds a total of roughly 5.5 million metric tons of mass every second. This total rate is a combination of the mass converted into energy and the mass physically expelled as particles.
Although the number is staggeringly large by terrestrial standards, it must be viewed in the context of the Sun’s overall size, approximately \(2 \times 10^{30}\) kilograms. Compared to this immense figure, the continuous mass loss is relatively small. Over the Sun’s 4.5-billion-year lifespan, it has lost only about 0.05 percent of its initial mass.
This slow decrease in mass has a long-term effect on the solar system. The loss of mass results in a weakening of the Sun’s gravitational influence on the orbiting planets. As the central gravitational anchor weakens, the planets’ orbits slowly expand to compensate.
Scientific observations of Mercury’s orbit confirm this phenomenon. The Earth’s orbital distance from the Sun increases by about 1.5 centimeters each year due to this effect.