How Does the Sun Make Its Energy? What Underground Neutrino Detectors Confirmed
Short answer: the Sun shines because its core, at about 15 million °C, fuses hydrogen into helium. That idea is decades old, but it was only tested directly in the last twelve years. An underground detector in Italy called Borexino caught the neutrinos released by the Sun's main fusion reaction in 2014, measured the whole main fusion chain in 2018, and in 2020 found neutrinos from a second, rarer fusion route called the CNO cycle. Together those results show that about 99% of the Sun's power comes from one route, the proton–proton chain, and about 1% from the other.
This page explains how the Sun's fusion works, why scientists use neutrinos rather than sunlight to check it, what each Borexino measurement found, and which questions are still open. Numbers come from NASA and the papers listed at the end.
What the Sun is made of and how hot it is
NASA describes the Sun as a yellow dwarf star, a huge ball of hydrogen and helium held together by its own gravity. It holds 99.8% of the solar system's mass and formed about 4.6 billion years ago from a collapsing cloud of gas and dust. NASA's figures for its layers:
| Region | What NASA reports |
|---|---|
| Core | About 15 million °C; about 138,000 km thick; where hydrogen is fused into helium |
| Radiative zone | Energy carried outward by radiation; it takes about 170,000 years to get from the core to the top of the convection zone |
| Convection zone | Below about 2 million °C; bubbles of hot plasma carry energy upward |
| Photosphere (visible surface) | About 5,500 °C; most of the Sun's radiation escapes into space from here |
| Corona (outer atmosphere) | Up to about 2 million °C, hotter than the surface below it |
Source: NASA Science, "Sun: Facts" (science.nasa.gov/sun/facts), checked 22 October 2026.
The heat of the core creates outward pressure that holds up the Sun's enormous mass and stops it collapsing, NASA explains. NASA also estimates the Sun is a little less than halfway through its life, with roughly another 5 billion years before it swells into a red giant and eventually becomes a white dwarf.
Why sunlight cannot show what the core is doing now
The light we see comes from the photosphere, not the core. Energy made in the core spends about 170,000 years working its way through the radiative zone before convection carries it the rest of the way up, according to NASA. So sunlight tells us how much energy the Sun produces on average, but not directly what the core is doing at this moment, or exactly which reactions produce it.
Neutrinos are different. They are tiny particles released by some of the fusion reactions, and they pass through matter almost without interacting. The 2020 Borexino paper calls neutrinos from these reactions "the only direct probe of the deep interior" of the Sun. The catch is that the same property makes them extremely hard to catch: for the rarest signal, Borexino recorded only a few counts per day per 100 tonnes of detector material.
The two fusion routes
The 2020 paper describes two well-understood ways stars fuse hydrogen into helium.
- The proton–proton (pp) chain. It starts with two protons (hydrogen nuclei) fusing. A series of further reactions follows, ending in helium. Several steps release neutrinos with different energies, named after the reaction that makes them: pp, beryllium-7, pep, boron-8 and hep.
- The CNO cycle. Here carbon, nitrogen and oxygen act as catalysts: they help four protons become a helium nucleus and are released again at the end. Because the catalysts are needed, the rate of this route depends on how much carbon, nitrogen and oxygen the core contains.
What Borexino measured, step by step
Borexino is a large liquid-scintillator detector, built to be extremely free of radioactive contamination, at the underground Laboratori Nazionali del Gran Sasso in Italy. Its results came in stages.
| Year | Result | Paper |
|---|---|---|
| 2014 | First spectral observation of pp neutrinos from the very first fusion step; shows about 99% of the Sun's power, 3.84 × 1033 ergs per second, comes from the proton–proton process | Nature 512, 383 |
| 2018 | Complete study of the pp chain: rates for pp, beryllium-7, pep and boron-8 neutrinos and a limit on hep; a direct determination of how often the chain ends by each of its two main branches | Nature 562, 505 |
| 2019 | Phase-II precision results from 1,291.51 days of data (December 2011 to May 2016); beryllium-7 rate measured to 2.7% | Phys. Rev. D 100, 082004 |
| 2020 | Direct observation of CNO-cycle neutrinos with high statistical significance; CNO supplies about 1% of the Sun's energy | Nature 587, 577 |
Sources: the four Borexino Collaboration papers listed at the end of this article.
2014: the first fusion step
The 2014 paper explains that the pp neutrinos from the first proton–proton fusion "constitute nearly the entirety of the solar neutrino flux", yet they had never been detected directly because their energy is so low. Neutrinos from later steps had already been seen, which had established the nuclear origin of the Sun's energy and contributed to the discovery that neutrinos change type as they travel. Seeing the pp neutrinos themselves closed the gap at the very start of the chain.
2018: the whole chain
Four years later the collaboration measured four of the five pp-chain neutrino sources in one study and set a limit on the fifth. That let them work out how often the chain ends through each of its two main branches. The paper also reports "an indication" that the temperature profile inside the Sun fits better with solar models that assume a higher proportion of heavier elements, which astronomers call high metallicity.
2020: the second route
The CNO signal was tiny: the paper describes an excess of only a few counts per day per 100 tonnes of detector material above the background. The hardest background to pin down came from bismuth-210, a radioactive contaminant in the scintillator. The team constrained it after five years of work keeping the detector's temperature stable. The result put the CNO share of the Sun's energy at about 1%, but the authors stress why it still matters: the same process is the main energy source in stars more massive than the Sun, and their abstract states that the occurrence of this mechanism "has been proven".
What is still open
- How much heavy material is in the Sun. The 2018 paper found an indication favouring high-metallicity models, but the 2019 Phase-II paper notes that telling the high and low models apart "is now largely dominated by theoretical uncertainties", not by the measurement. Because the CNO rate depends directly on carbon, nitrogen and oxygen in the core, the 2020 authors write that their result "paves the way" to measuring the Sun's metallicity with CNO neutrinos. That measurement is a next step, not a finished one.
- Precision of the CNO result. The CNO signal sits only slightly above the background and depends on how well the bismuth-210 contamination is constrained. The share of about 1% is an order-of-magnitude figure, as the paper puts it.
- The hot corona. Fusion explains where the Sun's energy comes from, but NASA lists the corona, which reaches up to about 2 million °C above a surface of about 5,500 °C, as one of the Sun's biggest mysteries. That puzzle is about the Sun's magnetic atmosphere, not its core.
Common questions
Is the Sun burning, like a fire?
No. A fire is a chemical reaction. The Sun's energy comes from nuclear fusion in its core, where hydrogen nuclei combine into helium at about 15 million °C, according to NASA.
How much power does the Sun produce?
The 2014 Borexino paper gives the Sun's power as 3.84 × 1033 ergs per second, about 3.84 × 1026 watts (our unit conversion), and shows that about 99% of it comes from the proton–proton process.
How long will the Sun keep shining?
NASA estimates the Sun is a little less than halfway through its lifetime and will last another 5 billion years or so before becoming a white dwarf, after first expanding into a red giant.
Why build a neutrino detector underground?
Borexino sits at the underground Gran Sasso laboratory. The papers describe the main challenge as separating a very small neutrino signal from background radioactivity, which is why the detector is built to be extremely radio-pure.
Do bigger stars work the same way?
They fuse hydrogen into helium too, but the 2020 paper notes that in massive stars the CNO cycle, which supplies only about 1% of the Sun's energy, is the dominant process.
Sources and how to check
- NASA Science. "Sun: Facts." science.nasa.gov/sun/facts (checked 22 October 2026).
- Borexino Collaboration. "Neutrinos from the primary proton–proton fusion process in the Sun." Nature 512, 383–386 (2014). DOI: 10.1038/nature13702 · PMID 25164748
- Borexino Collaboration. "Comprehensive measurement of pp-chain solar neutrinos." Nature 562, 505–510 (2018). DOI: 10.1038/s41586-018-0624-y · PMID 30356186
- Borexino Collaboration. "Simultaneous precision spectroscopy of pp, 7Be, and pep solar neutrinos with Borexino Phase-II." Physical Review D 100, 082004 (2019). DOI: 10.1103/PhysRevD.100.082004 · arXiv: 1707.09279
- Borexino Collaboration. "Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun." Nature 587, 577–582 (2020). DOI: 10.1038/s41586-020-2934-0 · arXiv: 2006.15115
- Abstracts checked on arXiv and Europe PMC, and NASA's page read, on 22 October 2026. The conversion from ergs per second to watts (1 W = 107 erg/s) is ours.