Questions 12-20
TheelementsotherthanhydrogenandheliumexistInsuchsmallquantitiesthatitisaccuratetosaythattheuniversesomewhatmorethan 25 percentheliumbyweightandsomewhatlessthan 25 percenthydrogen.
Astronomershavemeasuredtheabundanceofheliumthroughoutourgalaxyandinothergalaxiesaswell. HeliumhasbeenfoundInoldstars, inrelativelyyoungones, ininterstellargas, andinthedistantobjectsknownasquasars. Heliumnucleihavealsobeenfoundtobeconstituentsofcosmicraysthatfallontheearth (cosmicraysarenotreallyaformofradiation; theyconsistofrapidlymovingparticlesofnumerousdifferentkinds). Itdoesntseemtomakeverymuchdifferencewheretheheliumisfound. Itsrelativeabundanceneverseemstovarymuch. Insomeplaces, theremaybeslightlymoreofit; Inothers, slightlyless, buttheratioofheliumtohydrogennucleialwaysremainsaboutthesame.
Heliumiscreatedinstars. Infact, nuclearreactionsthatconverthydrogentoheliumareresponsibleformostoftheenergythatstarsproduce. However, theamountofheliumthatcouldhavebeenproducedinthismannercanbecalculated, anditturnsouttobenomorethanafewpercent. Theuniversehasnotexistedlongenoughforthisfiguretohesignificantlygreater. Consequently, iftheuniverseissomewhatmorethan 25 percentheliumnow, thenitmusthavebeenabout 25 percentheliumatatimenearthebeginning..