Today, we know the Sun shines due to nuclear fusion. However, stranger theories once prevailed.
Many ancient societies relied on mythology. Often, the Sun was a god travelling across the sky, or a ball of fire moving through divine cosmos. A Greek philosopher suggested that the Sun was a “red-hot stone.”
It wasn’t until the nineteenth century that advances in physics allowed for more testable speculations. One theory suggested the Sun was made of burning coal. But if this were true, then the Sun could only shine for thousands of years before running out of fuel. Discoveries in geology and evolution showed that the Earth was far, far older. The coal theory promptly died.
In the 1840s, a new theory argued that meteors fell into the Sun. As they crashed into the solar body, gravitational energy was released and turned into light and heat. This proposal, too, quickly fell out of fashion, as astronomers realized that an impossible number of meteors would be needed.
Later decades saw the gravitational contraction theory. This idea claimed that the Sun was many millions of years old and slowly contracting, releasing gravitational energy that made it shine. The theory was widely accepted into the twentieth century. As the Earth was proven to be billions of years old, however, the theory was rejected.
In the early nineteen hundreds, as scientists discovered radioactivity, some astrophysicists thought that radioactive elements in the Sun decayed and so produced heat and light. But since no solar radioactive elements were ever observed, this hypothesis quietly disappeared.It wasn’t until the mid-twentieth century that modern physics and nuclear fusion finally illuminated the mystery of the sun.