While the helium-3 content can only be determined with substantial uncertainty, the results appear to be the same for all regions, whatever their distance to the galactic center. As the universe cooled, the neutrons either decayed into protons and electrons or combined with protons to make deuterium an isotope of hydrogen.
The two general trends in the remaining stellar-produced elements are: 1 an alternation of abundance of elements according to whether they have even or odd atomic numbers, and 2 a general decrease in abundance, as elements become heavier.
In search of primordial helium, one selects among the dwarf galaxies those that are especially poor in oxygen and nitrogen.
During the s, there were major efforts to find processes that could produce deuterium, but those revealed ways of producing isotopes other than deuterium. For a long time, this meant that to test BBN theory against observations one had to ask: can all of the light element observations be explained with a single value of the baryon-to-photon ratio?
If such a cloud is sufficiently hot, certain atomic reactions involving helium atoms more precisely, helium atoms regaining an electron they had previously lost lead to characteristic emissions of electromagnetic radiation at precisely defined frequencies.
Unsourced material may be challenged and removed. Thuan University of Virginia ] Judging by the oxygen and nitrogen they contain, dwarf galaxies are comparatively primitive.