In Parts 1-3, we met the neutrino as a particle, learned how we know it exists, and discovered that it changes identity in flight. In this part we look in more detail at where neutrinos actually come from — every major source that produces the constant flux passing through us every moment.
The Sun
The Sun produces about 2 × 10³⁸ neutrinos per second through nuclear fusion in its core. By the time they reach Earth, about 8.3 minutes later, they are flying through every square centimeter of every surface at a rate of roughly 60 billion per second.
The dominant solar fusion process is the pp chain — a series of reactions that fuses four protons into helium-4, releasing energy. Each step in the chain produces neutrinos of characteristic energies:
- pp neutrinos — the lowest energy and most abundant. Energies up to about 423 keV.
- ⁷Be neutrinos — produced when beryllium-7 captures an electron. Energies of 384 keV and 862 keV.
- pep neutrinos — rare reactions producing 1.44 MeV neutrinos.
- ⁸B neutrinos — the highest-energy solar species, up to about 15 MeV. Rare but easy to detect.
Detectors have probed each component over six decades. Borexino completed the full picture by detecting CNO-cycle neutrinos in 2020 — closing out the experimental verification of solar fusion theory. For the full historical arc, see our solar neutrino experiments timeline.
Supernovae
When a massive star runs out of fuel, its core collapses on a timescale of milliseconds. The gravitational energy released — about 3 × 10⁵³ erg — is emitted almost entirely as neutrinos in roughly 10 seconds. Supernovae are, fundamentally, neutrino bombs. About 99% of the energy goes into neutrinos. The optical brightness we associate with a supernova is the tiny remaining fraction.
SN 1987A, the 1987 supernova in the Large Magellanic Cloud, is the only supernova whose neutrinos have been directly detected. Three underground detectors (Kamiokande, IMB, Baksan) registered 24 events within 23 seconds. The result confirmed decades of theoretical work on stellar collapse and set the first direct laboratory bound on neutrino mass from the brief travel-time spread.
A galactic supernova would produce a much bigger signal — modern detectors would catch tens of thousands of events from a single event. The Supernova Early Warning System connects multiple detectors to provide immediate alerts to optical observatories when the next galactic supernova occurs (which it will, on average, every 30-50 years).
Cosmic-ray atmospheric neutrinos
Cosmic rays — high-energy particles from outside the solar system — constantly hit Earth’s upper atmosphere. The collisions produce showers of secondary particles including muons and muon neutrinos. The resulting atmospheric neutrino flux is constant, isotropic on average, and well-measured.
Atmospheric neutrinos were the source that revealed neutrino oscillation in 1998. Super-Kamiokande’s discovery was based on the observation that muon neutrinos coming up through the Earth — having traveled the planet’s diameter — were missing relative to those coming down from the sky overhead. The deficit’s specific pattern proved oscillation.
The flux is also the dominant background for many other neutrino searches. Underground detectors carefully model the atmospheric flux to subtract it out. See the atmospheric neutrino flux article for the production chain and the Honda flux model.
Cosmic neutrinos from distant sources
Some of the highest-energy neutrinos detected come from far outside our galaxy. IceCube — a cubic kilometer of instrumented Antarctic ice — caught the first astrophysical neutrinos in 2013. Three identified extragalactic sources have followed:
- TXS 0506+056 — a blazar 4 billion light-years away, identified in 2017 via a coincident gamma-ray flare.
- NGC 1068 — a Seyfert galaxy 47 million light-years away, identified in 2022 from steady-state emission.
- The Milky Way galactic plane — diffuse glow from cosmic-ray interactions in our own galaxy, confirmed in 2023.
These are the first messengers of high-energy neutrino astronomy — a field that will mature substantially over the next decade with IceCube-Gen2, KM3NeT, and other observatories now under construction.
The Earth’s interior
Even the planet beneath us produces neutrinos. Radioactive decay of uranium, thorium, and potassium-40 deep inside the Earth produces antineutrinos called geo-neutrinos. The total flux is small but measurable.
Detecting geo-neutrinos lets us directly measure how much of Earth’s internal heat — about 47 terawatts total — comes from radioactive decay versus primordial heat left over from planetary formation. The current best estimate is that radioactivity accounts for roughly half of the heat flow. The remainder must come from primordial cooling.
KamLAND in Japan and Borexino in Italy have produced the leading geo-neutrino measurements. Future programs will refine the picture and probe Earth’s deep composition through neutrino measurements alone.
Nuclear reactors
A typical commercial nuclear reactor produces about 6 × 10²⁰ electron antineutrinos per second as a byproduct of fission. Reactor antineutrinos have been at the heart of many of the most important measurements in neutrino physics.
The original 1956 detection by Cowan and Reines used reactor antineutrinos. Daya Bay and RENO used them to measure the mixing angle θ₁₃ in 2012. KamLAND used them to pin down Δm²₂₁. JUNO, currently taking data, will use them to settle the mass ordering question.
There is also active research on using reactor antineutrino detectors for nuclear non-proliferation monitoring — verifying what fuel a reactor is burning without intrusive inspection.
The Big Bang
About one second after the Big Bang, the universe was hot enough that neutrinos were in equilibrium with all other particles. As the universe expanded and cooled, neutrinos “decoupled” — stopped interacting frequently enough to stay in equilibrium — and have been streaming freely through space ever since.
The result is the cosmic neutrino background (CνB) — about 336 relic neutrinos per cubic centimeter, everywhere in space. It is by far the largest reservoir of neutrinos in the observable universe.
The CνB has not been directly detected. The relic neutrinos have very low energies (less than a milli-eV), making them extraordinarily hard to catch. The PTOLEMY experiment and others are trying. Direct detection would open a window onto the universe at one second of age — much earlier than the cosmic microwave background, which lets us see about 380,000 years post-Big-Bang.
What comes next
In Part 5, we’ll look at the major open questions in neutrino physics. Why are neutrinos so light? Are they their own antiparticles? Is there a fourth, sterile neutrino? Why does the universe contain matter rather than antimatter, and could CP violation in neutrinos be the answer?
Frequently asked
What are the major natural sources of neutrinos?
The Sun (fusion in the core), supernovae (core collapse), cosmic rays (atmospheric showers), the Earth's interior (radioactive decay), distant active galaxies (cosmic neutrino astronomy), and the Big Bang (relic cosmic neutrino background). Nuclear reactors are the dominant artificial source.
How many solar neutrinos pass through me?
About 60 billion solar neutrinos pass through every square centimeter of Earth's surface every second. Through a typical human cross-section, that is roughly 100 trillion neutrinos per second. Most are unaffected by the body's atomic structure — they simply pass through.
Have we caught a supernova in neutrinos?
Only one: SN 1987A in the Large Magellanic Cloud, on February 23, 1987. Three underground detectors caught 24 neutrinos within 23 seconds. No galactic supernova has been observed since — modern detectors would catch tens of thousands of events from a galactic event.
Where can I see all neutrino sources at a glance?
Our interactive detector map at /map shows every major neutrino detector in the world, organized by location and target physics. Our reference timelines collect solar and mass-measurement experiments chronologically. The topic hubs at /topics organize coverage by source and physics.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2026, January 19). Neutrinos 101 — Part 4: Neutrinos from everywhere. Neutrino Times. https://neutrino-times.com/articles/neutrinos-101-part-4-neutrinos-from-everywhere/
Chicago
Neutrino Times Editorial Team. "Neutrinos 101 — Part 4: Neutrinos from everywhere." Neutrino Times, January 19, 2026. https://neutrino-times.com/articles/neutrinos-101-part-4-neutrinos-from-everywhere/.
MLA
Neutrino Times Editorial Team. "Neutrinos 101 — Part 4: Neutrinos from everywhere." Neutrino Times, 19 Jan. 2026, https://neutrino-times.com/articles/neutrinos-101-part-4-neutrinos-from-everywhere/.
BibTeX
@misc{neutrino-times-neutrinos-101-part-4-neutrinos-from-everywhere,
author = {Neutrino Times Editorial Team},
title = {Neutrinos 101 — Part 4: Neutrinos from everywhere},
howpublished = {Neutrino Times},
year = {2026},
month = {jan},
url = {https://neutrino-times.com/articles/neutrinos-101-part-4-neutrinos-from-everywhere/},
note = {Accessed: 2026-01-19}
} RIS
TY - GEN TI - Neutrinos 101 — Part 4: Neutrinos from everywhere AU - Neutrino Times Editorial Team PY - 2026 DA - 2026-01-19 PB - Neutrino Times UR - https://neutrino-times.com/articles/neutrinos-101-part-4-neutrinos-from-everywhere/ ER -