Temperatures in the sun’s core reach up to 27 million degrees, a huge amount of energy produced by nuclear fusion reactions of primarily hydrogen atoms.
Since the dawn of time, humanity has stood in awe of our sun. Ancient Egyptians venerated it as the god Ra, who sailed across the sky in a celestial boat as one might sail down the Nile; ancient Greeks worshiped it as Helios, who drove a chariot from horizon to horizon pulled by flaming horses. Many religions, ancient and modern, see the radiant, blinding disk in the sky as an icon of divine beings such as Aten, Utu, Tonatiuh, Sol Invictus, Ameratsu, Surya, etc. The sun gives us heat and light, our changing seasons, and makes all life and civilization on Earth possible. You might say, in fact, that our world revolves around the sun.*
*And you would be correct, because it does.
The first person in recorded history to say that our world revolves around the sun, literally and not just metaphorically, was the Greek astronomer Aristarchus of Samos, who lived during the 3rd century BC. Around the same time, another Greek astronomer and philosopher, Anaxagoras, suggested that the sun was not, in fact, the chariot of Helios and was instead a giant ball of flaming metal that orbited the Earth (people did not like being told this). Around the same time, Erastothenes of Cyrene, the Greek mathematician renowned for calculating the circumference of the Earth with astonishing precision, also calculated the distance from the sun to the Earth as being about 150 million kilometers (about 94 million miles). The sun is, in fact, 147 million kilometers away from the Earth at the closest point in our orbit and 153 million kilometers at the farthest point.
Over the next two thousand years or so, scientists and philosophers the world over, in the Mediterranean, in the Middle East, in Asia, and in Europe, learned more and more about the sun, but it wasn’t until the beginning of the modern scientific era in the 19th century AD that we had the tools to start tackling one of the biggest questions in the world—where does all the sun’s energy come from?
It wasn’t until the 20th century, after the discovery of radioactivity, that we figured it out. In 1904, Ernest Rutherford suggested that radioactive decay may be responsible for our sun’s output. Soon after, Albert Einstein developed his theory of mass-energy equivalence, best expressed in his famous formula E=mc2, and in 1920, Sir Arthur Eddington proposed that the sun could be producing energy, as expressed by Einstein’s work, by merging hydrogen atoms to create helium and thus giving out heat and light. Subrahmanyan Chandrasekhar and Hans Bethe developed the theoretical concept of what Eddington had proposed, now known as nuclear fusion, and calculated how the nuclear fusion reactions that power our sun worked.
As soon as we understood the nuclear furnace resting in the heart of our sun, which was in fact a giant ball of incandescent (mostly hydrogen) gas and not, as Anaxagoras had surmised, a fiery metal orb (good guess, though!), we started wondering—“Hey, can we do that here on Earth, too?”