The nuclear fusion process in stellar cores where hydrogen nuclei are converted into helium, primarily through the proton-proton chain or CNO cycle. This is the main energy source for main sequence stars and the longest phase of stellar evolution.
From Greek 'hydro' meaning water and 'genes' meaning generator, combined with 'burning' (though no combustion occurs - it's nuclear fusion). The term evolved as astronomers understood stellar energy sources in the mid-20th century.
Hydrogen burning is the cosmic marathon that keeps stars shining for billions of years! Our Sun has been steadily burning hydrogen for 4.6 billion years and has enough fuel to continue for another 5 billion years before moving on to helium burning.
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