the source of the Sun's energy

In the 19th century, Lord Kelvin thought that the Sun might be powered by gravity. He suggested that if the Sun was collapsing and gradually getting smaller, the gravitational potential energy might be transferred into the heat and light the Sun emits.

From the size and mass of the Sun, Kelvin calculated that the Sun could not be more than 25 million years old. The problem with Kelvin's theory was that scientists had already shown that the Earth was at least 400 million years old (we now know it to be a lot older than that even). It hardly seemed likely that the Earth could be a lot older than the Sun. Nice try Kelvin!

What is 'gravitational potential energy'?

It is the energy which is stored when work is done against the force of gravity.

A scientist called Bethe finally solved the mystery of the Sun's energy in the 1930s, but he based his ideas on the work Einstein had done many years before.

Einstein had proposed a formula to describe how much energy would be available if matter were transformed into energy.

Einstein’s formula says that a small amount of mass (m) can generate an awful lot of energy (E) because the conversion factor in the formula is c², where c is the speed of light.

E=mc²

The speed of light is 300,000,000 m/s. So c² is one big number...

90,000,000,000,000,000 in fact!

Bethe suggested that if the Sun could change just a small amount of its mass into energy, there would be enough energy to power the Sun for billions of years. He suggested that a way of doing this was to convert hydrogen into helium, and that's exactly what seems to be happening in the Sun.

The process that provides energy in the Sun is the same as that used in hydrogen bombs. You might say that the Sun is one big H-bomb or that H-bombs are mini-Suns!