1. Hydrogen in the Universe and on Earth

Hydrogen is the most abundant element in the Universe, found mainly in stars and interstellar gas. The Sun contains around 75% hydrogen atoms, 24% helium atoms and 1% atoms of heavier elements. Every second, through a nuclear fusion reaction, the Sun consumes 700 MT of hydrogen, which is transformed into 695 MT of helium, while 5 MT of matter is converted into energy. According to Einstein's equation, this transformation of matter represents a power of 4 × 10 ²⁶ W dissipated in the solar system. A tiny fraction – but considerable on our scale – reaches Earth (around 2 × 10 ¹⁷ W). This energy is the basis of life on our planet and is the primary renewable energy source