The Sun appears to be a constant source of light and heat, yet this massive star is an extremely active engine, constantly undergoing colossal physical and energetic processes. These continuous reactions at the Sun’s core and surface result in an unavoidable consequence: the star is relentlessly shedding mass into space. While the Sun’s size remains stable over human timescales, its immense energy output means it is not a static object but one that is steadily losing weight.
Mass Loss Through Energy Generation
The primary way the Sun loses mass is not by flinging particles away, but through the very act of generating the energy that makes it shine. Deep within the core, intense pressure and temperature allow hydrogen atoms to fuse together, forming helium in a process known as the proton-proton chain. This nuclear fusion reaction results in the formation of a helium nucleus that has slightly less mass than the four hydrogen nuclei that went into creating it.
This small difference in mass, often called the mass defect, is converted into a tremendous amount of energy. This conversion is governed by Albert Einstein’s famous equation, \(E=mc^2\), which states that mass (\(m\)) and energy (\(E\)) are interchangeable. The light and heat that escape the Sun are energy, and this energy has an equivalent mass value.
As this radiated energy leaves the Sun, it takes its corresponding mass with it, effectively reducing the star’s overall mass. This mass-to-energy conversion process is responsible for the vast majority of the Sun’s weight loss. Since the Sun’s energy output is measured precisely, the mass equivalent lost through radiation can be calculated directly.
Mass Loss Through Particle Ejection
A second, less significant mechanism for solar mass loss involves the physical expulsion of matter from its outer layers. The Sun’s outermost atmosphere, the corona, is so hot that the Sun’s gravity cannot hold onto all of its particles. This superheated plasma escapes in a continuous outflow of charged particles, primarily protons and electrons, which is known as the solar wind.
The solar wind streams outward in all directions, carrying away a small fraction of the Sun’s material mass. While this is a steady contributor to mass loss, more violent, transient events also contribute to particle ejection. Coronal Mass Ejections (CMEs) are colossal bursts of plasma and magnetic field that erupt from the Sun, suddenly ejecting billions of tons of matter at once. These irregular, explosive events add significantly to the total physical mass lost.
The Current Rate of Solar Mass Loss
The total mass loss rate of the Sun is the sum of the mass converted to energy and the mass physically ejected as particles. The Sun loses approximately 5.5 million tons of mass every single second. This number is difficult to grasp, but it is a combination of two distinct figures.
The mass loss due to the conversion of matter into energy via nuclear fusion accounts for about 4 million tons every second, representing the dominant portion of the total loss. The remaining 1.5 million tons per second is attributed to the solar wind and occasional CMEs—the actual particles physically escaping the Sun’s gravity. To put this magnitude into perspective, the Sun sheds material roughly equivalent to the mass of ten Great Pyramids of Giza every second.
The Long-Term Impact on the Solar System
Though 5.5 million tons per second sounds like an immense amount, the Sun’s total mass is so overwhelming that this loss is negligible over short periods. Over its 4.5-billion-year lifespan so far, the Sun has only lost about 0.05 percent of its total original mass. This tiny fraction ensures the Sun will continue to burn steadily for billions of years.
The continuous mass loss does, however, have a subtle but measurable effect on the orbits of the planets. According to the laws of gravity, a planet’s orbit is determined by the mass of the star it circles. As the Sun loses mass, its gravitational pull slightly weakens over time, which allows the planets to slowly spiral outward into slightly larger orbits. For Earth, this effect is minuscule, causing its orbit to expand by about 1.5 centimeters per year.