What happens when energy is released

Energy production in the sun

In fusion reactions in the sun neutrinos are also created - elementary particles that hardly interact with matter. Because of this property the electrically neutral particles reach Earth in just a few minutes, allowing researchers to gain an insight into the currently taking place inside the sun Processes. How these neutrinos can be detected and what they reveal about energy production in the sun, explains Stefan Schönert from the Technical University of Munich.

The sun has been supplying the earth with energy for over 4.5 billion years: it warms our planet and enables plants to convert sunlight into chemical energy via the process of photosynthesis. The central role of the sun has never been in question in human history. But what exactly is lit up in the sky has been a mystery to researchers for a long time.

Stefan Schönert from the Technical University of Munich

Stefan Schönert: “It used to be thought that chemical reactions played a role - like in an oven in which energy is released through combustion. Another idea was that gravitational forces were converted into thermal energy. And it was only at the beginning of the last century that the idea arose that nuclear fusion processes are actually responsible for this, i.e. processes of nuclear physics. "

With a diameter of almost 1,400,000 kilometers, the sun is by far the largest celestial body in the solar system. In addition, the sun accounts for 99.86 percent of the total available mass. The energy-generating nuclear fusion processes only take place in the center of the sun, which extends from the center of the sun to around a quarter of the sun's radius.

“In the center of the sun it is very hot and very dense, there is a plasma there. This plasma consists essentially of protons and electrons and the temperature is over ten million degrees. "

Under these extreme conditions, the positively charged protons can overcome the electromagnetic repulsive forces and fuse together to form larger chemical elements. During these fusion processes, a tiny fraction of the proton mass is converted into energy and released. In the sun, a total of four protons, i.e. four hydrogen nuclei, fuse to form helium nuclei made up of two protons and two neutrons, known as helium-4. In the sun, this process mainly takes place via the so-called proton-proton chain in several intermediate steps.

Inside the Borexino detector

“And that begins with a proton that fuses with another proton to form a heavy hydrogen nucleus, ie hydrogen-2, also known as deuterium. The deuterium then fuses with a proton to form helium-3. "

If two helium-3 nuclei fuse together, two protons and one helium-4 nucleus result. This completes the reaction cycle. The resulting energy is released in two different ways.

“On the one hand, there are charged particles, namely electrons and positrons, that are emitted. These have kinetic energy, are slowed down in the plasma and thus heat it up. This creates the high temperature inside the sun. And the other part of the energy is carried away by neutrinos. "

A total of 26 megaelectron volts of energy is generated in a single proton-proton chain. Around 1038Such fusion processes take place in the sun times per second - due to the equivalence of mass and energy, it loses more than four million tons of mass per second. The energy that reaches the earth in the form of sunlight comes exclusively from the highly energetic charged elementary particles, i.e. the electrons and their antiparticles, the positrons.

Proton-proton chain

“So the energy is released into heat inside the sun. And that warmth is like a hot oven. The thermal radiation consists of photons that slowly diffuse to the surface and heat it up. So there is a strong gradient, with a high temperature inside the sun and a low temperature on the surface. But the surface is still so hot that it emits electromagnetic radiation. "

Inside the sun, however, the photons cannot propagate in a straight line, but instead interact again and again with the hot plasma. Therefore, around a hundred thousand years pass before they reach the surface of the sun. Not so with neutrinos: These elementary particles are electrically neutral and have an extremely low mass.

“The probability that a neutrino interacts with matter is much smaller than with a charged particle or photon. As a result, a neutrino released in a fusion process inside the sun can then escape from inside the sun without interaction - that is, without reacting. These neutrinos fly almost at the speed of light and arrive here on earth after about eight minutes. "

For particle physicists, the neutrinos offer a unique insight into the energy production processes currently taking place inside the sun. Because of the long journey time, light particles only allow conclusions to be drawn about the nuclear reactions that occurred in the center of the sun over a hundred thousand years ago. In order to investigate the current fusion processes, researchers therefore build special detectors in which they can detect the solar neutrinos through the interaction with matter. One of these detectors is the Borexino experiment, in which Stefan Schönert is also involved. An international research group set up the experiment around 1400 meters deep in the rock of the Italian Gran Sasso massif - well protected from all external influences that could interfere with the sensitive measurements.

The sun generates energy through nuclear fusion

“The detector has an onion structure: there is a very large scintillation detector inside. It's a big ball filled with some kind of mineral oil. This oil generates small flashes of light when a neutrino causes a reaction in it. We then measure these flashes of light with so-called photo multipliers, which are light detectors. We can detect every single photon that is emitted. "

Shortly after it was put into operation in 2007, the Borexino experiment was the first to measure neutrinos that were created during special nuclear fusion processes in the sun. Another success was recently achieved: the team detected what are known as primary pp neutrinos for the first time - those neutrinos that are generated in the very first step of the proton-proton chain when two protons fuse to form heavy hydrogen. The detector registered around 144 of these neutrinos per day. The comparison between the number of neutrinos and the brightness of the sun in the electromagnetic spectrum provided excellent experimental evidence for the theoretical model of the energy production process in the center of the sun.

“With the accuracy with which we can currently measure these pp neutrinos, the two measurements agree wonderfully. That means: The way we imagine the release of energy inside the sun today, i.e. our solar model, is really experimentally confirmed. "