How the Sun warms us

The Sun sends about 1 kilowatt of energy to us on every square meter of the earth's surface, which solar panel users know. This is enough to operate 100 10 watt Led bulbs.

Chis energy comes to us in the form of photons. A small part corresponds to the visible range, it is that which is perceived by the retina of the eye; the other photons are distributed between infrared, ultraviolet, X-rays and gammas… So many names which designate different ranges of energy, some of which warm us, others enlighten us, others deteriorate the molecules or cause sunburn … But how are these photons born?

The first solar energy calculations

For a long time, we wondered where the energy of the Sun came from - why is our star burning? The scientist of the XIXe century Hermann von Helmholtz, promoter of principle of energy conservation, first considered the model of a Sun made up of burning coal. Knowing the specific energy of combustion and the total energy emitted by our star, he deduced a life time of about 5000 years. Far too small compared to all the astrophysical ages already known at the time! The ball of glowing charcoal was therefore not the right answer. No chemical reaction being able to explain the luminosity of the Sun, Helmholtz hypothesized a gravitational collapse: it is the contraction of a massive body under the effect of its own attraction. He estimated at 80 meters per year the shrinkage of the Sun necessary to produce its energy. He then estimated that the Sun could be 20 million years old. The account was still not there!

Kelvin in turn tackled the problem. He hypothesized a meteor bombardment. Making a plausible assumption about the energy of objects impacting the Sun, he found this solution to be acceptable, but the process should have affected the Earth's rotation. He rejected it and reverted to the idea of ​​gravitational contraction. Improving Helmholtz's calculations, he obtained an age of 60 million years.

We know today that the age of the Sun is 4,6 billion years, the calculations were therefore very far from the account. The correct solution was to come in the next century, the XXe, in a field totally unknown at the time: nuclear physics.

Nuclear fusion

It was not until Hans Bethe in the 1930s to understand that solar energy results from a nuclear transmutation which takes place at the very heart of our star, where the density of matter is strong enough to "stick" between them protons. The Sun is made up of these primary particles, which can merge in a process starting with the reaction: p + p → d + e+e

Here, d denotes the assembly of a proton and a neutron, called deuteron. The reactions are linked in cascade to lead to a global fusion between 4 protons which form a helium nucleus (composed of 2 protons and 2 neutrons) accompanied by 2 positrons (antielectrons) and 2 neutrinos, denoted ν. At the same time, this reaction releases an energy of 28 MeV (4,5 10 -12 Joules in units of the international system), which will be emitted in the form of photons.

This process implements the mass-energy equivalence according to Einstein's famous formula E = mc2. Indeed, 4 protons “weigh” more than a helium nucleus. So mass is converted into energy, which is released in the process. It is calculated that 5 grams of nuclear fuel gives as much energy as a ton of coal.

Check the theory by detecting neutrinos

How to check what is hidden in the very center of the Sun? With the production of energy, there should be emission of neutrinos - can we detect them? From the received brightness, it is quite easy to calculate the expected flux of these particles. The result is remarkable: 1038 neutrinos are produced every second, resulting in a flux over the Earth of 60 billion neutrinos per second per cm2.

Measuring this neutrino flux would prove that nuclear fusion is indeed the source of solar energy. Problem, it is excessively difficult to capture neutrinos. These are phantom particles that pass through matter without almost leaving a trace. Yet the hunt began, and in the 1960s, Ray Davis built a large swimming pool, full of 600 tons of chlorinated liquid, buried in a gold mine in South Dakota.

An interacting neutrino in the liquid turns a chlorine atom into a radioactive argon atom. Radioactive argon lives an average of 37 days, and can be detected through its radioactive decay. In Dakota, fluid was tested every 10 days or so; one had to look for interesting nuclei drowned in 600 tons of liquid. However, the measurement was of the order of one argon every three days when the detector had been sized to collect one per day. The obstinate research lasted more than 30 years, until the 1990s, but the measurements remained in deficit, two thirds of the neutrinos of the Sun not answering the call.

It was easy not to place great confidence in Ray Davis' result, given the difficulty of the measurement. Technologically more advanced, two other experiments Gallex under the Gran Sasso tunnel in Italy and Legend under the Elbrus mountain in Russia, used a similar method, this time converting gallium into radioactive germanium. These two experiments also measured a deficit of neutrinos compared to the flux predicted by the theory! They thus gave more credibility to the reality of this deficit, but it was not until the gigantic Japanese experience. SuperKamiokande to convince the community that these were not measurement errors.

The interior of the SuperKamiokande detector tank in Japan being filled in April 2006.
Copyright Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo

The detection method is different from the previous ones, because SuperKamiokande detects in real time the bluish light emitted during the passage of neutrinos (theCherenkov effect) in the huge 50 kiloton tank - 7 times the weight of the Eiffel Tower - constituting the detector and buried in an underground gallery. Roughly, neutrinos can interact with electrons in water and release them. These electrons then propagate faster than light which, in water, travels “only” at 220 km / s. They give a light signal which follows the direction of the initial neutrino. The SuperKamiokande experiment therefore makes it possible to ensure that the detected neutrinos indeed come from the Sun which shines at a (known) point in the sky. In fact, a strong peak is detached in the desired direction: this clearly shows a production of neutrinos coming from the interior.

The Sun imaged via its neutrinos. Yellow represents a greater flux of neutrinos, detected by the SuperKamiokande experiment.
Copyright Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo

Thanks to such a study, SuperKamiokande succeeds in the challenge of a real-time “neutrinography” of the Sun: to take “photos” of the Sun while we are buried under the rock, both day and night: very strong!

And let all this make a star in the night!

Victor Hugo, Contemplations

Neutrino detectors show that a large flux of neutrinos does indeed come from the Sun, which proves that the star's energy comes from nuclear fusion. But the enigma then takes a new turn.

Neutrino deficit

Despite the tremendous flux of solar neutrinos received by the gigantic detector, the experiment traps only 15 per day when we expect 40. The deficit was confirmed, measured here at 60%. To explain it, an idea imposed itself, that of oscillations.

We know 3 different types of neutrinos. Those produced in the Sun are of the first type called electron neutrinos, ne. But there are two other types, neutrinos muonics et tauics. Is it possible that these guys mingle on a space trip by swapping their personalities? The detectors used only detected the νe. To prove the idea of ​​oscillations, it was necessary to trap the types other than νe. This was the mission of a Canadian device called SNO (Sudbury Neutrino Observatory). Installed in a mine near Toronto, the detector this time uses a kilotonne of heavy water as a sensitive medium. In heavy water, D2O, the proton is replaced by a deuton. This enables new reaction channels in which all types of neutrinos participate - electronic, muonic, tauic.

Complete information was obtained by experiment. The result concluded that the flux of neutrinos from the Sun agrees well with the theoretical prediction, but the share of νe only explains only a third of the total. This is glaring proof of the oscillation, two thirds of neutrinos have changed flavor between their point of production inside the Sun and their point of detection on Earth. Whatever ?

The apotheosis of neutrinos

Oscillation is a spontaneous change between different types of neutrinos as drastic as the conversion of an apple to a pear as it falls in Newton's orchard; it is a concrete staging of the relations ofHeisenberg uncertainty. Oscillation implements the most subtle properties of quantum mechanics and implies that neutrinos have non-zero mass, which was by no means obvious.

Today and thanks to the combined efforts of several meticulous experiments, the oscillation has made it possible to measure extremely small masses: they are less than one billionth of the mass of a proton. But we also know that neutrinos are billions of times more abundant than other particles of matter, hence the fantastic conclusion: in the balance sheet of the universe, neutrinos, these apparently so humble particles, weigh as much as all the stars. of all galaxies.

The Sun sends an energy of the order of 1 kW / m to Earth2 in the form of photons, but it also sends an additional flux of 600 billion neutrinos. These carry an additional energy of some 000 W / m2. But unlike photons, they are not intercepted by solar panels and therefore their energy cannot be captured. However, these particles may be very discreet, without neutrinos, the Sun would not shine and we would not be here to talk about it.The Conversation

François Vannucci, Professeur émérite, chercheur en physique des particules, specialist des neutrinos, University of Paris

This article is republished from The Conversation under Creative Commons license. Read theoriginal article.

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