It must be making heat for millions of years, what process could possibly make so much energy for so long?
Understanding the mechanism on how the Sun produces photons explains, how old is the Sun and this question is related to neutrinos?
How Does The Sun Shine?Albert Einstein's famous equation ( E = MC2 ) showed that a tiny amount of mass could in principle be converted into a tremendous amount of energy. Physicists believed that the core of the Sun acts as a nuclear fusion reactor.
The main process is the proton-proton chain reaction, where hydrogen nuclei are fused to form helium producing vast amounts of energy that balances the gravitational collapse of the star. However, the chain also contains other secondary processes that produce other particles and energy. These nuclear reactions are the only feasible way to continuously produce the amount of energy observed for billions of years.
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Hydrogen is the most abundant element in the Sun and other stars, and it also has the highest amount in the universe. If we add four nuclei of hydrogen, one helium-4 nucleus is formed. A helium-4 nucleus has somewhat less mass than four nuclei of hydrogen. Behind them all, E = mc², the famous equation of the special theory of relativity, propounded by Einstein in 1905. Under this equation, the mass is a measure of the quantity of matter, so the significance of this equation is that matter can be converted into energy, and energy can be converted into matter. That is, the mass of helium reduced in the above nuclear fusion will be returned as energy. For this reason, the sun and other stars shine.
The interesting fact is that not all stars spend their hydrogen in the same way. The stars that are larger than our Sun are spending their hydrogen very fast. This means that the larger the stars, the faster they spend hydrogen. Stars that are twice as big as the Sun are spending their hydrogen ten times faster. The stars that are ten times bigger than the Sun are one thousand times faster. Such stars spend their hydrogen in a very short time and reach the verge of death. The stars which are in the shape of our sun, their hydrogen lasts for a long time.
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We can easily see and understand the light coming from the Sun by observing the full spectrum of photons arriving at earth. But they come from the surface and atmosphere of the star, how can we see inside the Sun? How can we confirm the existence of fusion?
Surprisingly, the answer comes in the form of a practically undetectable particle. When a nucleus decays, it emits energetic particles and becomes a more stable isotope.
Role Of Neutrinos In The Sun's ShineIn the early 20th century, accurate measurements of the energy of beta decay products found that if only the nuclei and electron were involved, energy and momentum were lost in the decay. To reconcile this observation with the universal conservation of energy, Wolfgang Pauli, in 1930 felt obliged to invent a particle without mass or electric charge that could participate along with the nuclei and electron in the decays.
Postulating a particle with little evidence for existence caused Pauli some worry. He said, "I've done a terrible thing, I postulated a particle that cannot be detected. This article was later dubbed "Neutrino
(or little neutral one)" by Enrico Fermi in his theory of beta decay.
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Ten years later, Wang Ganchang proposed that the neutrino can be detected in a rare process known as "Beta Capture". In the same, nuclear reaction of the core of the Sun, neutrinos are emitted at a rate of one neutrino for every million photons. 90% of the neutrinos are released in the first proton-proton reaction. The most energetic neutrinos are produced in this so-called "Proton-Proton three chain reactions". But are much less abundant, as this reaction is far less probable accounting for only 0.11% of the energy produced in the Sun.
Photons take up to 500,000 years to get out of the Sun, as they are constantly intercepted, absorbed, and re-emitted. But neutrinos having no electric charge can travel in a straight line from the core of the Sun. They come out at the speed of light as soon as they are created.
Hypothetically, about three percent of the total energy radiated by the Sun is in the form of neutrino. The flux of solar neutrinos at the Earth's surface is on the order of 60 billion per square centimeter per second. Unlike light, neutrinos travel through us so about a trillion solar neutrinos pass through your thumb every second.
How the Sun Shines.pdf - Source: stanford.edu