Complex sentence: The sun is composed of 74% by mass hydrogen, an element whose nucleus is a single proton; energy is released when 4 protons combine into a helium nucleus, a process in which two of them are also converted to neutrons.
Split into simpler sentences: The sun is composed of 74% hydrogen (measured by mass), an element whose nucleus is a single proton. Energy is released in the sun when 4 protons combine into a helium nucleus, a process in which two of them are also converted to neutrons.