8 Facts About The Sun’s Most Ghostly Particle: The Neutrino
By: Ethan Siegel
Tags: Space, Sun, Physics, Astronomy, Neutrinos

The active Sun produces much more than our eyes perceive.

Although we typically think of the Sun as emitting radiation (in the form of photons) and particles (from flares, coronal mass ejections, and the solar wind), it also emits neutrinos that are produced in the core from nuclear reactions. These ghostly, almost-invisible particles carry a tremendous amount of information about our Universe. (NASA’S SOLAR DYNAMICS OBSERVATORY / GSFC)

Its core reactions copiously create energetic neutrinos, not merely radiation.

The most straightforward and lowest-energy version of the proton-proton chain, which produces helium-4 from initial hydrogen fuel. Note that neutrinos are copiously produced during the first fusion step: this is the overwhelming means of production of neutrinos in the Sun. (SARANG / WIKIMEDIA COMMONS)

Here are 8 surprising facts about the Sun’s most ghostly particle.

Deep inside the Sun’s core, where temperatures rise above ~4 million K, nuclear fusion occurs between subatomic particles. This produces photons, particles and antiparticles, and neutrinos, the last of which carries a little more than 1% of the Sun’s total energy output away. (JAMES JOSEPHIDES, CAS SWINBURNE UNIVERSITY OF TECHNOLOGY)

1.) Neutrinos carry ~1% of the Sun’s total energy output.

It isn’t just photons and charged particles that carry energy away from the Sun, but also solar neutrinos, which are produced in the Sun’s core and hardly interact at all with other particles. A total of approximately ~1% of the Sun’s energy is emitted in the form of these solar neutrinos. (ALAN STONEBRAKER/APS)

The Sun produces ~10³⁸ neutrinos every second, carrying 4 × 10²⁴ W of continuous power.

Whereas photons scatter off of the particles inside the Sun repeatedly, preventing them from reaching the Sun’s photosphere and being emitted into the outward Universe for about 100,000–200,000 years after production, neutrinos stream outward at nearly the speed of light, exiting the Sun 2–3 seconds after being produced. (OPENSTAX CNX, CREATIVE COMMONS, LUMEN LEARNING)

2.) They exit the Sun less than 3 seconds after being generated.

This cutaway showcases the various regions of the surface and interior of the Sun, including the core, which is where nuclear fusion occurs. With a radius of approximately 432,000 miles (~700,000 km), neutrinos take less than three seconds to exit the Sun from the time they are produced. (WIKIMEDIA COMMONS USER KELVINSONG)

Despite moving slower than photons, neutrinos barely interact with matter, streaming out at approximately c.

The assembly of the PROSPECT detector at Oak Ridge National Laboratory was designed to measure reactor neutrinos, but is also sensitive to the solar neutrino background. Each second, approximately 70 billion neutrinos pass through each square centimeter of area here on Earth: about the size of a human thumbnail. (PROSPECT COLLABORATION/M LAVITT)

3.) They’re abundant on Earth.

The way a neutrino detector typically works on Earth is that a large tank of material, designed to interact with neutrinos, is surrounded by photomultiplier tubes sensitive to the secondary signals produced by a neutrino interaction. The detector must be pristine and well-shielded for the signal to rise above the background noise. (ROY KALTSCHMIDT, LBNL, US DEPT. OF ENERGY)

70 billion solar neutrinos pass through your thumbnail, undetected, every second.

A neutrino event, identifiable by the rings of Cherenkov radiation that show up along the photomultiplier tubes lining the detector walls, showcase the successful methodology of neutrino astronomy. This image shows multiple events, and is part of the suite of experiments paving our way to a greater understanding of neutrinos. However, the total flux of neutrinos detected is only about 1/3 of the naive expectation for their flux rate; it would take a detector made of lead a light-year thick to capture about ~50% of the Sun’s neutrinos. (SUPER KAMIOKANDE COLLABORATION)

4.) We only observe ⅓ of the Sun’s predicted neutrino rate.

If you begin with an electron neutrino (black) and allow it to travel through either empty space or matter, it will have a certain probability of oscillating, something that can only happen if neutrinos have very small but non-zero masses. The solar and atmospheric neutrino experiment results are consistent with one another, indicating a massive nature for neutrinos. (WIKIMEDIA COMMONS USER STRAIT)

The Sun produces electron neutrinos, which oscillate into two other flavors, demonstrating the neutrino’s massive nature.

A variety of nuclear reactions occur in the Sun’s interior, emitting electron neutrinos of a variety of energies through a variety of processes. Measuring these neutrinos and their energy spectra reveal which nuclear processes are occurring in the Sun’s interior. (DOROTTYA SZAM/SZDÓRI OF WIKIMEDIA COMMONS)

5.) Neutrinos arrive with specific, discrete energy spectra.

While the overwhelming majority of neutrinos are produced at low energies through the proton-proton chain in the Sun, other nuclear processes leave specific signatures in the neutrino energy spectrum, enabling us to reconstruct, from measurements, what must be occurring in the Sun’s core. (BAHCALL, JOHN; SERENELLI, ALDO (2005), ASTROPHYS. J. 621: L85–L88)

Measuring neutrino energies reveal explicit, rare reactions occurring inside the Sun.

This is an image of the Sun as produced from detected neutrinos by the Super-K collaboration. Neutrinos arrive day-and-night, enabling us to not only measure properties of the Sun from the neutrinos produced in the core, but to measure the differences in neutrinos received owing to their interactions with the Earth that they pass through. (SUPER KAMIOKANDE COLLABORATION)

6.) Solar neutrinos have imaged the Sun.

Although the total neutrino flux from the Sun doesn’t change much from day to night, the fraction of electron vs. mu/tau flavors does change, as the Earth’s interior induces oscillations. These day/night differences increase as a function of the neutrino energy. (ICRR/UNIV. TOKYO (L); A. FRIEDLAND, C. LUNARDINI, C. PEÑA GARAY (2004), PHYS. LETT. B, 594(3–4) (R))

Passing freely through Earth, neutrinos reveal the Sun continuously: day or night.

The anatomy of the Sun, including the inner core, which is the only place where fusion occurs. Even at the incredible temperatures of 15 million K, the maximum achieved in the Sun, the Sun produces less energy-per-unit-volume than a typical human body. Neutrinos are only produced in the core, enabling us to reconstruct the dynamics of the core’s interior from neutrino measurements. (NASA/JENNY MOTTAR)

7.) They constrain the size of the Sun’s core.

Various reactions occur inside the Sun at a variety of temperatures/densities. By measuring the neutrino flux at a variety of energies, we can reconstruct not only which reactions are occurring where in the Sun’s interior, but we can infer the size and temperature of the Sun’s core. (KELVIN MA/KELVIN13 OF WIKIMEDIA COMMONS (L); JOHN BAHCALL/NEUTRINO ASTROPHYSICS (R))

Based on electron-neutrino scattering, nuclear reactions occur only in the Sun’s innermost 20–25%.

If the Sun were to suddenly cease its nuclear fusion, its gravity and its emitted light would be largely unaffected for long periods of time: hundreds of millennia. However, the neutrino flux would change immediately, providing us with a noticeable signal that would arrive on Earth after 8–9 minutes, depending on the Earth-Sun distance at that particular moment. (SHUTTERSTOCK/PUBLIC DOMAIN)

8.) They’re our first “solar apocalypse” warning.

This logarithmic chart of Solar System distances shows how far away various objects are from the Sun in Astronomical Units, where the Earth-Sun distance is defined as one Astronomical Unit. It takes light, and neutrinos (which travel at speeds indistinguishable from the speed of light), about 8 minutes and 20 seconds to traverse the Sun-Earth distance. (NASA / JPL-CALTECH)

If the Sun’s interior changed significantly, altered neutrino fluxes would alert humanity in under ~9 minutes.