The Sun converts roughly 4.3 million metric tons of its own mass into energy every second through nuclear fusion. That number comes from a straightforward application of Einstein’s famous mass-energy relationship to the Sun’s measured luminosity of about 3.84 × 10²⁶ watts. But fusion-related mass loss is only part of the story, and the details of how the Sun sheds that mass, what it tells us about the solar interior, and how the rate will change over billions of years are worth unpacking.
Where the Number Comes From
The Sun radiates energy in all directions at a rate that astronomers have measured with increasing precision for decades, using space-based instruments that track the total solar irradiance reaching Earth and then scaling up to the full sphere of radiation at the Sun’s distance.1PubMed Central. Solar irradiance measurements That measured power output is about 3.84 × 10²⁶ watts, a figure sometimes expressed in older literature as 3.84 × 10³³ ergs per second.2Nature. Neutrinos from the primary proton–proton fusion process in the Sun Einstein’s mass-energy equivalence tells us that for any process converting mass to energy, the mass lost equals the energy produced divided by the speed of light squared. Plug in the Sun’s luminosity and you get about 4.3 billion kilograms per second, or roughly 4.3 million metric tons vanishing as pure radiant energy every second of every day.
To put that in perspective, you could line up about 20,000 fully loaded aircraft carriers and vaporize them each second, and you’d still fall a bit short of the mass the Sun is converting. Yet compared to the Sun’s total mass of about two billion billion billion metric tons, this loss is almost laughably tiny. Over the Sun’s roughly 4.6-billion-year life so far, fusion has consumed only a few hundredths of a percent of its total mass.
What Happens Inside the Core
The Sun does not burn fuel in any chemical sense. Deep in its core, where temperatures exceed 15 million degrees and the pressure is crushing, hydrogen nuclei (protons) are forced close enough together for the strong nuclear force to overcome their electrical repulsion and fuse. The dominant process is called the proton-proton chain, which accounts for about 99 percent of the Sun’s energy output.2Nature. Neutrinos from the primary proton–proton fusion process in the Sun Through a series of steps, four protons are ultimately combined into one helium-4 nucleus. The helium nucleus weighs slightly less than the four protons that went into it, and that missing mass is what gets released as energy, mostly in the form of gamma-ray photons and fast-moving particles called neutrinos.
The photons do not zip straight out of the Sun. They bounce around inside the dense solar interior for tens of thousands to hundreds of thousands of years, being absorbed and re-emitted so many times that by the time they reach the surface, they have degraded from gamma rays to visible light and infrared. The neutrinos, by contrast, barely interact with matter at all. They fly straight out of the core at nearly the speed of light, passing through the entire Sun in about two seconds. This difference is what makes neutrinos so valuable for understanding solar fusion, and we’ll come back to that.
The Minor Fusion Pathway
The proton-proton chain is not the only game in town. A secondary set of reactions called the CNO cycle (for carbon, nitrogen, and oxygen) also fuses hydrogen into helium, but it uses carbon, nitrogen, and oxygen nuclei as catalysts rather than relying solely on proton-proton collisions. The standard solar model predicts that the CNO cycle accounts for about 1.5 percent of the Sun’s energy.3PubMed. Does the sun shine by pp or CNO fusion reactions? That’s a small slice, but it matters enormously in astrophysics because the CNO cycle becomes the dominant energy source in stars more massive than the Sun, where core temperatures are higher.
For years, the evidence that the CNO cycle actually operates in the Sun was indirect. Neutrino experiments had already confirmed the proton-proton chain in detail, but detecting the neutrinos from CNO reactions required extraordinary sensitivity. In 2020, the Borexino experiment, a massive liquid-scintillator detector buried deep underground in Italy, reported the first direct observation of CNO neutrinos from the Sun.4Nature. Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun The detection confirmed that both fusion pathways are indeed running simultaneously in the solar core, exactly as models predicted. Before that measurement, neutrino experiments had only been able to set an upper limit of about 7.3 percent on the CNO contribution, an order of magnitude improvement over earlier bounds.3PubMed. Does the sun shine by pp or CNO fusion reactions?
From the perspective of mass loss, both pathways contribute to the same bottom line. Whether four protons fuse via the proton-proton chain or the CNO cycle, the net reaction is the same: four hydrogen nuclei become one helium nucleus, and the mass deficit becomes energy. The 4.3 million tons per second figure already includes both pathways, because it is derived from the total luminosity regardless of which reactions produced it.
Why Neutrinos Are the Only Direct Window
Almost everything we know about the Sun’s surface comes from light. But light tells us nothing directly about conditions in the core, because photons take that long, tortuous random walk through the solar interior before escaping. Neutrinos are the only particles that travel straight from the fusion reactions to our detectors without being scattered or absorbed along the way. They are, in the words of the Borexino team, “the only direct probe of the deep interior of the star.”4Nature. Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun
This is why decades of effort have gone into building increasingly sensitive neutrino detectors underground. The measurement of proton-proton neutrinos, which directly correspond to the first and most fundamental step in the fusion chain, confirmed that the energy we see radiating from the Sun’s surface today matches what the core was producing hundreds of thousands of years ago, when those photons began their journey outward.2Nature. Neutrinos from the primary proton–proton fusion process in the Sun In principle, if the fusion rate had changed dramatically in the interim, the neutrino flux and the luminosity would not match. They do, which tells us the Sun’s core has been remarkably stable over that timescale.
Fusion Mass Loss Is Not the Only Mass the Sun Sheds
When people ask how much mass the Sun loses per second, they usually mean the fusion number. But the Sun also loses mass the old-fashioned way: by literally blowing particles off its surface. The solar wind is a continuous stream of charged particles (mostly protons and electrons) flowing outward from the Sun’s upper atmosphere at speeds of a few hundred kilometers per second. This wind carries away roughly one to two million metric tons of material per second, depending on where we are in the solar cycle.
On top of the steady wind, the Sun periodically erupts with coronal mass ejections, or CMEs, enormous bubbles of magnetized plasma hurled into space. A single CME can carry billions of metric tons of material, but CMEs are sporadic. Near solar minimum, they might happen a few times a month. Near solar maximum, they can occur several times a day. Research tracking CME occurrence rates over full 11-year solar cycles has found that near solar maximum, CMEs can account for roughly 15 percent of the average mass flux in the near-ecliptic solar wind.5Journal of Geophysical Research: Space Physics. The solar cycle variation of coronal mass ejections and the solar wind mass flux Studies using data from solar cycles 23 and 24 have investigated how the mass loss rate via solar wind and CMEs varies with solar magnetic activity, tracking the relationship between sunspot number, X-ray background luminosity, and mass ejection rates.6Monthly Notices of the Royal Astronomical Society. Mass loss via solar wind and coronal mass ejections during solar cycles 23 and 24 More recent analyses using the latest CME catalog data have continued to refine the picture of how CME-related mass loss varies across solar cycles.7The Astrophysical Journal Supplement Series. Solar Cycle Variation of the Mass-loss Rate of Coronal Mass Ejections
Adding it all up, the Sun’s total mass loss from all sources combined runs in the neighborhood of five to six million metric tons per second. Fusion-related radiation accounts for the majority, with the solar wind and CMEs making up most of the remainder. There is also a small contribution from the neutrinos themselves, which carry away about two percent of the fusion energy and therefore represent a tiny slice of the mass-energy conversion. Because neutrinos escape the Sun entirely and almost never interact with anything, that energy is genuinely lost to the Sun’s gravitational budget in real time, unlike the photons that linger inside for millennia before departing.
Does the Mass Loss Rate Change?
Over the 11-year solar cycle, the fusion-driven mass loss barely budges. The Sun’s total luminosity varies by only about 0.1 percent between solar minimum and solar maximum, which translates to a negligible change in the fusion mass conversion rate. The solar wind mass loss, on the other hand, can vary more noticeably. CME frequency ramps up dramatically during active phases of the cycle, and the background solar wind itself changes character, with faster and slower streams shifting in prevalence.
Over much longer timescales, the fusion rate does change. As the Sun gradually converts hydrogen to helium in its core, the core slowly contracts and heats up, causing the fusion rate to increase. The Sun today is roughly 30 percent more luminous than it was when it first settled onto the main sequence about 4.6 billion years ago. That means the fusion mass loss rate has been creeping upward over geological time, and it will continue to do so. But the change is glacially slow by human standards, amounting to a few percent increase per billion years.
How This Compares to Other Stars
The Sun’s mass loss rate through fusion is modest by stellar standards. More massive stars burn through their fuel far faster, not just because they have higher luminosities but because luminosity scales steeply with mass. A star ten times the Sun’s mass can be thousands of times more luminous, converting proportionally more mass to energy each second. These stars also drive much more powerful stellar winds. Massive hot stars can lose a significant fraction of their total mass through winds alone over their lifetimes, something the Sun’s gentle breeze will never accomplish.
At the other end of the scale, red dwarf stars with a fraction of the Sun’s mass are so dim that their fusion mass loss rates are negligible. A star with one-tenth the Sun’s mass might convert only a few hundred metric tons per second. The trade-off is longevity: these stars burn so slowly that they will outlast the Sun by trillions of years.
Metallicity, the abundance of elements heavier than helium in a star, also affects wind-driven mass loss. For very metal-poor massive stars, research has shown that mass-loss rates through stellar winds can drop dramatically, because the heavier elements that normally absorb radiation and help drive the wind are scarce. Stars hotter than about 50,000 Kelvin with very low metallicity may have no wind at all, since the high-ionization forms of carbon, nitrogen, and oxygen have too few strong spectral lines to push material outward.8Astronomy & Astrophysics. Mass-loss predictions for evolved very metal-poor massive stars The Sun, with its moderate metallicity and moderate temperature, sits comfortably in the regime where it maintains a steady but unspectacular wind.
The Long Road to Solving the Solar Energy Problem
It’s easy to take for granted that we know what powers the Sun, but the question tormented physicists for nearly a century. In the mid-1800s, the leading hypothesis was gravitational contraction: the Sun was slowly shrinking, converting gravitational potential energy into heat. The problem was that this mechanism could only sustain the Sun’s luminosity for about 20 to 30 million years, far too short once geologists began producing evidence that the Earth was billions of years old.
The discovery of radioactivity in the 1890s hinted that subatomic energy sources existed, and by the early twentieth century, scientists like Arthur Eddington were arguing that some kind of atomic process must be responsible. But the specific mechanism remained elusive. The path from Eddington’s intuition to a real theory passed through the work of Robert Atkinson, George Gamow, and others, who gradually developed the nuclear physics needed to describe fusion reactions in stellar interiors. The breakthrough came in the late 1930s, when Hans Bethe worked out the details of both the proton-proton chain and the CNO cycle, providing a quantitative explanation for solar energy that matched observations.9arXiv. Before Bethe: Early Ideas of the Sun’s Generation of Energy Bethe won the Nobel Prize in Physics in 1967 for this work, among other contributions to nuclear theory.
What Happens When the Hydrogen Runs Out
The Sun has enough hydrogen fuel in its core to sustain its current fusion rate for another five billion years or so. After that, the story gets dramatic. When the core’s hydrogen is exhausted, the core will contract and heat up until hydrogen fusion ignites in a shell surrounding the now-helium core. This shell burning will cause the Sun’s outer layers to expand enormously, turning it into a red giant hundreds of times its current diameter. Later still, helium fusion will ignite in the core, and eventually the Sun will pass through a second giant phase called the asymptotic giant branch.
During these giant phases, the Sun’s mass loss rate will skyrocket, not through fusion but through enormously enhanced stellar winds. The bloated outer envelope becomes loosely bound by gravity, and radiation pressure and pulsations drive material outward at rates millions of times greater than the current solar wind. By the time the Sun exhausts its nuclear fuel and sheds its outer layers as a planetary nebula, it will have lost half or more of its original mass.10arXiv. Giant branch planetary systems: Dynamical and radiative evolution What remains will be a white dwarf, a dense remnant roughly the mass of a small star but only about the size of Earth, slowly cooling over billions of years with no fusion left to sustain it.
The gravitational consequences of all that mass loss will be real. As the Sun sheds mass, its gravitational pull on the planets will weaken, and planetary orbits will expand outward. Whether Earth will be engulfed by the expanding red giant or will migrate outward fast enough to survive remains an active area of research, and the answer depends sensitively on exactly how much mass the Sun loses and when. Either way, Earth’s surface will be sterilized long before the Sun reaches the giant branch, as the gradually increasing luminosity will boil away the oceans within a billion or two years.