Every second the Sun burns through roughly 600 million tonnes of hydrogen, fusing it into helium in a core hotter than 15 million degrees Celsius. About 4 million tonnes of that mass — the difference between the ingredients and the product — vanishes outright, converted into pure energy by the arithmetic of Einstein’s E=mc². That is the mass of a small mountain, deleted from existence every tick of the clock, reappearing as light.
The sunlight warming your face right now is the tail end of that transaction. And it has been in transit for a very long time.
The 4-million-tonne receipt
Fusion in the Sun’s core follows the proton-proton chain: four hydrogen nuclei end up welded into one helium-4 nucleus, and the helium weighs about 0.7 per cent less than the four protons that went in. That missing 0.7 per cent leaves the reaction as gamma-ray photons and neutrinos.
Multiply that by the scale of the Sun’s core, and you get a luminosity of roughly 3.8 × 10²⁶ watts. Four million tonnes of matter, per second, becoming light. For comparison, humanity’s entire annual energy consumption is delivered by the Sun to Earth’s upper atmosphere in about 90 minutes.
The Sun has been running this reaction for about 4.6 billion years and has enough hydrogen in its core to keep going for roughly another five billion. It has burned through only a small fraction of its fuel. The rate is stupendous but the tank is stupendous-er.
Where the light is actually born
The photons that end up as sunlight are not born as sunlight. They are born as gamma rays — the most energetic form of electromagnetic radiation, the kind produced by nuclear reactions and hospital radiotherapy machines. A single gamma-ray photon from the core carries enough energy to shatter chemical bonds.
By the time that same energy reaches your skin as visible light and gentle infrared warmth, it has been degraded, scattered, absorbed, and re-emitted so many times that a single original gamma-ray photon’s worth of energy has been diluted into roughly a million lower-energy photons of visible light.
The place where this diluting happens is the radiative zone, a layer of the Sun’s interior that starts at the edge of the core and extends outward for about 500,000 kilometres. It is astonishingly dense — closer to the density of lead near the bottom, thinning to something like water at the top — and it is opaque. A photon cannot travel in a straight line through it. It bounces.
The 170,000-year commute
Physicists call the journey a random walk. A photon is created by a fusion event, travels a tiny distance — millimetres to centimetres — before slamming into a plasma ion, gets absorbed, and is re-emitted in a random direction. Then it does it again. And again. Trillions of times.
Because each step is random, the net outward progress is punishingly slow. Estimates for how long it takes a unit of energy to random-walk from the core to the surface range from roughly 10,000 years at the fast end to around 170,000 years at the slower end, depending on assumptions about density, mean free path, and where in the radiative zone the photon starts.
To put that in human terms: the sunlight on your hand today began its journey around the time Homo sapiens was first spreading out of Africa. When the energy that becomes this afternoon’s sunshine was released deep inside the Sun, mammoths still walked Europe. The Sahara was green and forested at various points during that window, and the light finishing its outbound trip now is older than nearly every human civilisation combined.
The last eight minutes are the easy part
Once the energy escapes the radiative zone, it hits the convective zone, the outer third of the Sun by radius. Here the plasma is cool enough — a relative term, still around two million degrees at the base — that it can churn like a pot of boiling water. Huge convection cells carry hot material to the surface, dump their heat as radiation, and sink back down. That layer takes weeks to months to traverse.
The surface the energy finally reaches is the photosphere, a layer only about 500 kilometres thick where the plasma becomes transparent and light can, at last, fly free. Photosphere temperature: about 5,500 degrees Celsius. That is where sunlight, as anyone would recognise it, comes into being.
From there it is a straight sprint. Light crosses the 149.6 million kilometres to Earth in about 8 minutes and 20 seconds. The final leg is the fastest leg by a factor of roughly a hundred billion.
Neutrinos: the honest witnesses
The other product of fusion — neutrinos — do not random-walk. They barely interact with matter at all. A neutrino created in the Sun’s core exits the entire Sun in about two seconds and arrives at Earth eight minutes later, essentially unchanged. Roughly 65 billion solar neutrinos pass through every square centimetre of your body every second, and almost none of them notice you are there.
This gives astrophysicists a peculiar advantage. Neutrino observatories like Super-Kamiokande in Japan and SNO in Canada see the Sun’s core in real time. The light hitting the telescope alongside them is up to 170,000 years out of date. If the Sun’s core stopped fusing tomorrow, the neutrino count would drop within eight minutes. Sunlight would keep pouring out for tens of thousands of years before anyone noticed the difference.
Why the number matters for what happens on Earth
Only a sliver of that 3.8 × 10²⁶ watts reaches Earth — about one two-billionth — but it is enough to drive every weather system, every ocean current, every photosynthetic organism, and, increasingly, the technology humans build to run on it.
The dream of using sunlight directly to make chemical fuel — rather than converting it first to electricity and then to something else — has attracted researchers for decades because it mimics what plants have been doing for over three billion years. A German KIT spin-off called Photreon showed a working one-square-metre photoreactor prototype at Hannover Messe in April that takes in sunlight and water and produces hydrogen directly, with no electrolyser in the loop. The physics is a single step: light excites a catalyst, the catalyst splits water into hydrogen and oxygen right there on the panel.
The catch is efficiency. A landmark 2021 Nature paper documented Japan’s 100-square-metre outdoor photocatalytic array running for a year and peaking at just 0.76 per cent solar-to-hydrogen conversion, while paired solar-cell-plus-electrolyser systems have hit 30 per cent. The best lab result published anywhere is 9.2 per cent, achieved in 2023 with an indium gallium nitride catalyst under concentrated light. Researchers usually cite 10 per cent as the commercial threshold.
A Korean team led by Professor In Sun Cho at Ajou University pushed a hematite-based photoanode to a record 8.7 per cent solar-to-hydrogen efficiency — the highest reported for an iron-oxide electrode in a bias-free tandem device — by using hydrazine, a toxic industrial waste, as a sacrificial fuel to accelerate the oxidation half of the reaction. Their electrode ran for over 100 hours.
None of these approaches come close to the Sun’s own trick, which converts hydrogen to helium with an energy density seven million times greater than burning coal. But the whole enterprise of harvesting sunlight — as electricity, heat, or fuel — is essentially an attempt to intercept a fraction of that 4-million-tonnes-per-second matter-to-energy transaction after it has finished its 170,000-year commute.
The Sun is getting brighter
The fusion rate is not constant on geological timescales. As hydrogen in the core is converted to helium, the core slowly contracts and heats up, which makes fusion run faster. The Sun today is about 30 per cent brighter than it was when Earth formed 4.5 billion years ago — a fact known as the faint young Sun paradox, because early Earth should have been frozen solid under such a dim star, and geologically it clearly was not.
The brightening continues. In roughly one billion years, the Sun will be luminous enough to boil Earth’s oceans, well before it swells into a red giant five billion years from now. The 600-million-tonnes-per-second figure will climb steadily along the way.
Fusion research on Earth has spent seventy years trying to reproduce even a whisper of what the Sun does effortlessly. The best terrestrial fusion experiments, including the National Ignition Facility’s December 2022 breakthrough, produce net-positive energy for fractions of a second at temperatures higher than the solar core, because Earth cannot supply the pressure of 200 billion atmospheres that the Sun’s own weight provides for free.
The receipt in your hand
Hold your palm up to a window on a clear day. The warmth you feel is roughly 1,000 watts per square metre at sea level — about the power of a bar heater — delivered by photons that finished a journey most of which was spent bouncing pointlessly around a plasma denser than lead.
Somewhere in the Sun’s core, 4 million tonnes of matter has just ceased to exist in the time it took to read that sentence. The energy is on its way. If nothing goes wrong, it will arrive here in about 100,000 years, give or take, as a photon so ordinary you will not notice it landing.
The equivalent of a small mountain, delivered as sunshine, one silent instalment at a time.