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For example , while hydrogen and deuterium are stable ( in nature one finds one atom of deuterium for every six thousand atoms of hydrogen ) , tritium , composed of two neutrons and one proton , is unstable . " However , the ratio of deuterium to hydrogen in cometary water is much higher than in water on our planet . In principle measurements of Deuterium provide extremely powerful constraints because the the predicted BBN yield is highly sensitive to the cosmological density of baryons . So , if we can measure the primordial relative abundance of Deuterium , we have a precise measurement of the density of the universe . Only the slow cooking of the limited amount of deuterium in the stellar interior is possible .