https://www.forbes.com/sites/startswithabang/2018/11/23/the-sun-wouldnt-shine-without-quantum-physics/#342478ef632c

The Sun Wouldn’t Shine Without Quantum Physics

The Sun is the source of the overwhelming majority of light, heat, and energy on Earth’s surface, and is powered by nuclear fusion. But without the quantum rules that govern the Universe at a fundamental level, fusion wouldn’t be possible at all.PUBLIC DOMAIN

The greatest source of newly-produced energy in the Universe today is starlight. These large, massive, and incredibly common objects emit tremendous amounts of power through the smallest of processes: the nuclear fusion of subatomic particles. If you happen to be on a planet in orbit around such a star, it can provide you with all the energy necessary to facilitate complex chemical reactions, which is exactly what happens here, on the surface of Earth.

How does this happen? Deep inside the hearts of stars — including in our own Sun’s core — light elements are fused together under extreme conditions into heavier ones. At temperatures over about 4 million kelvin and at densities more than ten times that of solid lead, hydrogen nuclei (single protons) can fuse together in a chain reaction to form helium nuclei (two protons and two neutrons), releasing a tremendous amount of energy in the process.

The most straightforward and lowest-energy version of the proton-proton chain, which produces helium-4 from initial hydrogen fuel. Note that only the fusion of deuterium and a proton produces helium from hydrogen; all other reactions either produce hydrogen or make helium from other isotopes of helium.SARANG / WIKIMEDIA COMMONS

At first glance, you might not think energy is released, since neutrons are ever so slightly more massive than protons: by about 0.1%. But when neutrons and protons are bound together into helium, the entire combination of four nucleons winds up being significantly less massive — by about 0.7% — than the individual, unbound constituents. This process enables nuclear fusion to release energy, and it’s this very process that powers the overwhelming majority of stars in the Universe, including our own Sun. It means that every time the Sun winds up fusing four protons into a helium-4 nucleus, it results in the net release of 28 MeV of energy, which comes about through the mass-energy conversion of Einstein’s E = mc2.

A solar flare from our Sun, which ejects matter out away from our parent star and into the Solar System, is dwarfed in terms of ‘mass loss’ by nuclear fusion, which has reduced the Sun’s mass by a total of 0.03% of its starting value: a loss equivalent to the mass of Saturn. E=mc^2, when you think about it, showcases how energetic this is, as the mass of Saturn multiplied by the speed of light (a large constant) squared leads to a tremendous amount of energy produced.NASA’S SOLAR DYNAMICS OBSERVATORY / GSFC

All told, by looking at the power output of the Sun, we measure that it emits a continuous 4 × 1026 Watts. Inside the Sun’s core, on average, a whopping 4 × 1038 protons fuse into helium-4 every second. Although this is a small amount of power-per-unit-volume — a human being metabolizing their food over the course of a day is more energetic than a human-sized volume of the Sun’s core undergoing fusion — the Sun is absolutely enormous.

Adding all of that energy together, and having it be emitted omnidirectionally on a continuous, steady basis, is what enables the Sun to power all the processes that life requires here on Earth.

The brightness distance relationship, and how the flux from a light source falls off as one over the distance squared. The Earth has the temperature that it does because of its distance from the Sun, which determines how much energy-per-unit-area is incident on our planet. The balance between the Sun’s output and the Earth’s distance is what makes life on our world possible.E. SIEGEL / BEYOND THE GALAXY